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Temporal variation in natural and sexual selection of male calling behavior in the field cricket Gryllus Rubens

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Temporal variation in natural and sexual selection of male calling behavior in the field cricket Gryllus Rubens
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Velez, Manuel J
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English
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vi, 89 leaves : ill. ; 29 cm.

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Subjects / Keywords:
Broadcasting ( jstor )
Disease risks ( jstor )
Female animals ( jstor )
Mating behavior ( jstor )
Natural selection ( jstor )
Parasitism ( jstor )
Parasitoids ( jstor )
Seasons ( jstor )
Sexual selection ( jstor )
Spring ( jstor )
Crickets -- Behavior ( lcsh )
Dissertations, Academic -- Zoology -- UF ( lcsh )
Natural selection ( lcsh )
Sexual selection in animals ( lcsh )
Zoology thesis, Ph. D ( lcsh )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 2004.
Bibliography:
Includes bibliographical references.
General Note:
Printout.
General Note:
Vita.
Statement of Responsibility:
by Manuel J. Velez.

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Full Text










TEMPORAL VARIATION IN NATURAL AND SEXUAL SELECTION OF MALE
CALLING BEHAVIOR IN THE FIELD CRICKET GRYLLUS RUBENS













By

MANUEL J. VELEZ


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2004




TEMPORAL VARIATION IN NATURAL AND SEXUAL SELECTION OF MALE
CALLING BEHAVIOR IN THE FIELD CRICKET GRYLLUS RUBENS
By
MANUEL J. VELEZ
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2004


ACKNOWLEDGMENTS
I am indebted to my advisor, H. Jane Brockmann, for her invaluable support
in the design and analysis of this research project. I am also thankful to Colette
M. St. Mary, Benjamin Bolker, Marta L. Wayne, and Thomas J. Walker for their
helpful feedback on my research. My deepest gratitude goes to Laura Sirot,
Suhel Quader, Kavita Isravan, Rebecca Hale, and other colleagues at the
Department of Zoology for their support in the development and analysis of this
project. I am also thankful to George Casella for giving me statistical advice.
My deepest gratitude goes to the University of Florida Beef Teaching Unit
and Environmental Landscape Horticulture Education Lab for allowing me to
conduct field research in their pastures.
This work was funded by an NSF Graduate Predoctoral fellowship. My PhD
committee consisted of Drs. H. Jane Brockmann, Colette M. St. Mary, Benjamin
Bolker, Marta L. Wayne, and Thomas J. Walker.
n


TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS
ABSTRACT v
CHAPTERS
1 GENERAL INTRODUCTION 1
2 VARIABLE SELECTION FOR MALE SONG IN THE
FIELD CRICKET GRYLLUS RUBENS 5
Introduction 5
Methods 9
Study Species 9
Fly Abundance and Activity 10
Field Recordings of Individual Males 10
Night Surveys 12
Results 13
Fly Abundance and Activity 13
Field Recordings 14
Night Surveys 14
Discussion 15
3 SEASONAL VARIATION IN FEMALE PHONOTAXIS
TO MALE CALLING SONG OF THE FIELD CRICKET
GRYLLUS RUBENS 26
Introduction 26
Methods 29
Phonotactic Experiment 29
Source of Male Calls 32
Fly Activity 33
Results 33
Phonotactic Experiment 33
Fly Activity 34
Discussion 35
in


4 VARIATION IN SEXUAL SELECTION FOR MALE
SONG IN THE FIELD CRICKET
GRYLLUS RUBENS 46
Introduction 46
Methods 50
Pitfall Trap Experiment 50
Source of Male Calls 53
Data Analysis 53
Results 54
Discussion 55
5 GENERAL MODEL: A TEST OF THE FACTORS
AFFECTING MALE CALLING DURATION IN THE FIELD
CRICKET GRYLLUS RUBENS 63
Factors Affecting Male Calling Duration 63
Model: Predicting Seasonal Patterns in Male Calling Duration 64
Male Calling Strategies 64
Calculating Fitness of Male Calling Strategies 65
Seasonal Changes in Mating Benefit (M) 66
Seasonal Changes in Probability of Escaping Fly
Parasitism (E) 67
Model Results: Seasonal Changes in Fitness of
Male Calling Strategies 69
Comparison of Optimal Seasonal Strategies with
Observed Calling Durations 69
Variations of the Model 71
Results of Model Variations 71
Relative Importance of Natural versus Sexual Selection 72
Implications of the Model 73
LIST OF REFERENCES 82
BIOGRAPHICAL SKETCH 89
IV


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
TEMPORAL VARIATION IN NATURAL AND SEXUAL SELECTION OF MALE
CALLING BEHAVIOR IN THE FIELD CRICKET GRYLLUS RUBENS
By
Manuel J. Vlez
August 2004
Chair: H. Jane Brockmann
Major Department: Zoology
A primary goal of sexual selection theory is to understand how natural and
sexual selection interact to favor variation in sexually-selected traits. The goal of
this dissertation is to examine variation in male calling behavior of the field cricket
Gryllus rubens and how temporal changes in natural and sexual selection may
favor this variation. The results of this dissertation are important because they
show that temporal variation in natural and sexual selection may favor variation
in male sexually-selected traits.
Male calling song in G. rubens attracts females. It also attracts gravid
females of the parasitoid fly Ormia ochracea. These female flies deposit larvae
on and around calling males, killing infected crickets within 7-10 days. In northern
Florida, O. ochracea is abundant in the fall and rare in the spring, and fly
parasitism rates are higher in the fall than in the spring. This temporal variation in
natural selection could favor different male songs in the fall than in the spring. I
v


evaluated this possibility by examining seasonal variation in male calling
behavior. I found that fewer males sing each night in the fall than in the spring
and that fewer males sing at times when parasitoid flies are most active.
Because seasonal changes in parasitoid fly abundance may change the
costs and benefit of choice, fall and spring females may behave differently
towards calling males. I evaluated this hypothesis by observing the phonotactic
behavior of fall and spring females. I found that fall females are less attracted to
male songs than spring females at times when flies are most active.
Seasonal changes in sexual selection may change the seasonal benefits
males derive from singing. I explored this hypothesis by examining whether the
benefits of singing change seasonally and whether such changes are correlated
with seasonal differences in male calling duration. The results of a playback
experiment show that long duration calls attract more females in the fall, where
call durations are long, than in the spring, where call durations are short.
Temporal changes in natural and sexual selection may favor seasonal
variation in male G. rubens calling behavior. To evaluate how such fluctuating
selection may affect seasonal patterns in calling duration, I developed a model
evaluating the fitness of different calling strategies in the fall and spring given
seasonal changes in natural and sexual selection. By varying these selective
forces independently and comparing the fitness of calling strategies to observed
calling durations, I found that temporal changes in sexual selection may have a
greater effect on calling duration than changes in natural selection.
VI


CHAPTER 1
GENERAL INTRODUCTION
A primary goal of sexual selection theory is to understand how natural and
sexual selection interact to favor variation in male sexually-selected traits
(Andersson 1994). Crickets are ideal subjects for the study of sexually selected
traits for several reasons. First, males produce calling songs to attract females
for mating, and females select mates based on the quality of their songs (for
review see Loher & Dambach 1989). This means that male song is likely to be
influenced by sexual selection. Second, many song characters are highly
variable among males (Cade & Wyatt 1984, Cade 1991, Walker 1998, 2000).
Third, song characters can be manipulated independently from male phenotype
by tape recording male calling songs and editing the recordings. This provides a
means of isolating different song characters and testing for their separate effects
on female choice (Hedrick 1986). Fourth, females choose males overtly: they
approach males and mount them. This female behavior offers the opportunity to
separate the effects of female choice from male-male competition. Additionally,
because females approach speakers broadcasting male songs in the same way
they would approach calling males, female choice can be studied without the
problem of males adjusting their song in response to females, and females
modifying their preference in response to changes in male song. Collectively,
these characteristics make crickets model subjects for studies on sexual
1


2
selection (Cade 1979, Crankshaw 1979, Boake 1984, Hedrick 1986 & 1988,
Simmons 1988).
The goal of this dissertation is to examine variation in male calling
behavior of the field cricket Gryllus rubens and how temporal changes in natural
and sexual selection may favor this variation. The results of this dissertation are
important because they show that temporal variation in natural and sexual
selection may favor variation in male sexually-selected traits.
The field cricket G. rubens is well suited for studying the effects of natural
selection on calling song traits. In addition to attracting females, the calling song
of male G. rubens also attracts gravid females of the parasitoid fly Ormia
ochracea (Walker 1986,1993). These female flies deposit larvae on and around
calling males, killing infected crickets within 7-10 days. In northern Florida, O.
ochracea is abundant in the fall and rare in the spring (Walker 1986), and fly
parasitism rates are higher in the fall than in the spring (Walker & Wineriter
1991). This seasonal variation in the costs of calling suggests that natural
selection on male calling song varies temporally. This temporal variation in
natural selection can result in different calling songs in different seasons. I
examine this hypothesis in the second chapter of this dissertation by exploring
whether male G. rubens exhibit seasonal variation in calling behavior and
whether this variation is associated with seasonal changes in parasitoid fly
abundance.
Gryllus rubens is also well suited for studying the effects of temporal
variation in sexual selection. Even though females do not sing, Walker and


3
Wineriter (1991) have shown that females can also be parasitized. If females are
infected as they approach calling males, then the costs of mate choice may be
higher in the fall when flies are abundant than in the spring when flies are rare.
These seasonal changes in the costs of mate choice may cause females to be
less attracted to male calling song in the fall than in the spring to reduce the risk
of fly parasitism. This temporal variation in female behavior may alter the
strength of sexual selection seasonally. I evaluate this hypothesis in the third
chapter of this dissertation by examining female attraction to male calling song at
different times of the year.
Seasonal variation in sexual selection may alter the seasonal benefits that
males derive from singing. As a result, such variation may favor seasonal
changes in male calling behavior. I evaluate this hypothesis in the fourth chapter
of this dissertation by examining whether the benefits of singing change
seasonally and whether such changes are correlated with seasonal differences in
male calling duration.
I conclude this dissertation with a discussion of how temporal changes in
natural and sexual selection should affect seasonal variation in male G. rubens
calling duration. I test these effects by calculating the fitness of different calling
strategies in the fall and spring given seasonal changes in natural and sexual
selection. By varying natural and sexual selection independently, I estimate the
relative importance of natural versus sexual selection in favoring variation in male
calling duration.


4
By studying male calling behavior and the benefits that males derive from
singing when flies are rare and abundant, this dissertation examines how
temporal variation in natural and sexual selection may interact to favor variation
in male sexually-selected traits.


CHAPTER 2
VARIABLE SELECTION FOR MALE SONG IN THE
FIELD CRICKET GRYLLUS RUBENS
Introduction
Conspicuous male traits such as bright colors, large size, or elaborate
behavior evolve through the joint action of sexual and natural selection
(Andersson 1994). Sexual selection favors males with conspicuous traits since
these traits are advantageous in mate acquisition through either female choice or
male-male competition (Darwin 1871). Natural selection, on the other hand, acts
against males with conspicuous traits if these traits lower male survival.
Conspicuous traits may lower survival by being energetically costly or by making
males more vulnerable to predators. Therefore, conspicuous male traits evolve
depending on the relative strength of sexual and natural selection. This
hypothesis predicts that when the costs of natural selection increase relative to
the benefits of sexual selection, then males should become less conspicuous. In
this study, I evaluate this prediction by examining the male calling behavior of the
field cricket Gryllus rubens in two seasons that differ in the risk of parasitism from
acoustically orienting flies.
Male crickets produce calling songs by rubbing their raised forewings
together, which attracts conspecific females for mating (Loher & Dambach 1989).
In some species, the same calling song also attracts parasitoid flies which
deposit larvae on and around calling males, killing infected crickets within 7-10
5


6
days (Cade 1975, Walker 1986,1993). This fly parasitism risk has been used to
explain variation between species in the nightly calling duration of male field
crickets (Orthoptera; Gryllidae). Cade and Wyatt (1984) found that male G.
texensis, which is parasitized, spend less time calling per night than male G.
veletis, G. pennsylvanicus, and Teleogryllus africanus, which are not parasitized
by phonotactic flies. They argue that fly parasitism has selected against long
nightly calling in species that are parasitized. They also found that the distribution
of male G. texensis nightly calling duration is highly skewed because some
males do not call. It is thought that males that do not call or males that sing less
search for females so there is some expectation of success for these males,
particularly when female densities are high. Cade and Wyatt (1984) attribute the
skew distribution of nightly calling duration in G. texensis to the higher rates of
parasitism in this species.
However, species differences in time spent calling per night may be due to
species differences in the energetic costs of singing. The song of G. texensis is
made up of sound pulses organized into trills. In contrast, the songs of G. veletis
and G. pennsylvanicus are composed of sound pulses grouped into chirps with
quiet periods between them. Because trilling songs may be more energetically
costly than chirping songs (Prestwich & Walker 1981, Prestwich 1994),
differences in time spent calling per night between the trilling G. texensis and the
chirping G. veletis and G. pennsylvanicus may be explained by energetic
differences in song production. Collectively, these findings cannot separate the


7
effects of fly parasitism risk and energetic costs of calling on nightly calling
duration in male crickets.
Fly parasitism risk has also been used to explain variation in the timing of
male calling in crickets. French and Cade (1987) found that male G. texensis call
more around dawn than in the early evening. They hypothesized that dawn
calling is common in G. texensis because parasitoid flies are less active at dawn.
However, G. veletis and G. pennsylvanicus also call more around dawn (French
& Cade 1987), and they are not infected by parasitoid flies. Furthermore, in all
three species, male calling around dawn coincided with increased female
movement and mating activities (French & Cade 1987). Therefore, it is not clear
whether male G. texensis call more around dawn because flies are less active at
that time or because females are more sexually receptive around dawn.
Testing the effect of fly parasitism risk on nightly calling duration and
timing of calling in male crickets under laboratory conditions is difficult because it
is possible that crickets do not respond directly to flies but rather to
environmental cues associated with fly activity. If so, then examining male calling
behavior in an experimental arena with and without flies will not reveal the effect
of parasitoid flies on behavior. One solution to this problem is to study male
calling behavior in populations with and without parasitoid flies. This is the
approach that was used by Zuk et al. (1993) and Simmons et al. (2001).
However, any correlation between male calling behavior and fly abundance may
also be explained by environmental or other differences between the two
populations (for example, populations without flies may be at higher latitudes


8
than those with flies). This limitation of working with geographically distinct
populations is overcome when studying a population that is exposed to seasonal
changes in fly abundance. This is the case for the field cricket G. rubens. The
calling song of this species attracts gravid females of the parasitoid fly Ormia
ochracea (Walker 1986, 1993). In northern Florida, 0. ochracea is abundant in
the fall and rare in the spring (Walker 1986), and fly parasitism rates are higher in
the fall than in the spring (10% versus 0%; Walker & Wineriter 1991). These
seasonal differences in fly abundance and rates of parasitism suggest that the
costs of calling are higher in the fall than in the spring. This seasonal variation in
natural selection serves as a natural experiment to examine the effect of fly
parasitism risk on nightly calling duration, number of males calling and timing of
calling in male crickets.
I hypothesize that seasonal changes in natural selection favor seasonal
changes in male calling behavior in G. rubens. Accordingly, I made the following
predictions. First, I predicted that fewer males would call in the fall than in the
spring because some males do not sing (personal observation) and
experimentally muted males have lower rates of fly parasitism than non-muted
males under high fly densities (Walker & Wineriter 1991). Second, I expected
that males should spend less time calling per night in the fall than in the spring
because the number of parasitoid flies attracted to broadcasts of cricket songs
increases with broadcast duration (Cade et al. 1996). Third, I expected that fewer
males would call in the early evening in the fall than in the spring because
parasitoid fly activity is highest in the early evening (Cade et al. 1996). I tested


9
these predictions by recording the calling duration of individual males under field
conditions and conducting nightly surveys of singing males.
Finding that variation in male calling behavior in G. rubens is correlated
with seasonal changes in natural selection from the risk of fly parasitism would
be important because it would help us understand how males deal with the
conflicting demands of attracting females while avoiding parasitism. Because
males of many animal species have mating signals that attract both females and
predators and/or parasites (reviewed by Burk 1982 and Magnhagen 1991), the
results of this study may have important implications for many systems in which
males or females attract mates with conspicuous displays.
Methods
Study Species
The field cricket Gryllus rubens is found in lawns, pastures, and along
roadsides in southeastern United States (Alexander 1957). Unlike most other
species of field crickets, male G. rubens produce a trilling song in which sound
pulses are repeated and grouped into long pulse trains (Doherty & Callos 1991).
This trilling song attracts females who approach and mount males. It also attracts
gravid females of the parasitoid fly Ormia ochracea (Walker 1986, 1993).


10
Fly Abundance and Activity
To confirm that 0. ochracea flies are more abundant in the fall than in the
spring and that flies are most active in the early evening, I monitored nocturnal
changes in fly activity in a pasture at the University of Florida Beef Teaching Unit,
Alachua Co., FL that was 1-km from the transects used in the night surveys. To
monitor fly activity, I used a slit trap (Walker 1989) with an electronic sound
synthesizer that broadcasts calls of G. rubens (4763 Hz carrier frequency, 50.0
pulses per second, 50% duty cycle; continuous trill at 106 dB, 15 cm above the
speaker). I activated the synthesizer an hour before sunset and counted the
number of flies captured at the trap 3,7,11, and 13 h after sunset. I carried out
this procedure in the fall from 20 September to 17 November 2000 (N=8) and
spring from 3 April to 23 May 2001 (N=6).
Field Recordings of Individual Males
To study seasonal changes in the number of calling males and time spent
calling by males per night, I recorded the calling duration of individual males
under field conditions. For these recordings, I collected adult males from a field at
the University of Florida Environmental Landscape Horticulture Education Lab,
Alachua Co., FL, using a sound trap with an electronic sound synthesizer
(Walker 1986). Males were weighed to the nearest 0.001 g and placed
individually in square plastic containers measuring 13 x 13 x 9 cm (I x w x h) with
wire screen lids for 10 days to ascertain that they were not parasitized by O.
ochracea. Each container had an inch of sand, an inverted egg carton shelter,


11
and Purina Cat Chow and water ad lib. All crickets were held under ambient light
and temperature in a greenhouse less than 1 km from the collection site. After 10
days, I transferred the containers to the field and laid them out in a straight line
keeping a distance of 15 m between containers. This distance was chosen so
that males could not hear the singing of other males being recorded. Each plastic
container was set in a square aluminum pan measuring 20 x 20 x 4 cm (I x w x h)
with an inch of water to prevent fire ants from invading the containers. Next to
each container, I placed an Optimus model CTR-115 voice-activated cassette
tape recorder. I switched on all tape recorders an hour before sunset and allowed
them to run for 16 h. All tape recorders were checked hourly to replace any tapes
that were full. Two to four males were recorded each night. I electronically
monitored 20 males over nine nights in the spring (April/May 2001) and 23 males
over nine nights in the fall (September/October 2001). Each male was monitored
only once. Fall and spring field recordings did not differ in the minimum
temperature recorded each night (Meanfaii SE = 15.441.35 C, Meanspnng SE
= 15.591.64 C, P=0.943). There was also no seasonal difference in the
maximum temperature recorded each night (Meanfaii SE = 30.060.86 C,
Meanspnng SE = 28.161.37 C, P=0.260).
I calculated the nightly calling duration for each male by listening to the
tapes with a stopwatch. If the male stopped calling for more than 2 sec, then the
stopwatch was stopped until the male resumed calling. The nightly calling
duration was the total time spent calling during the 16 h that males were
monitored.


12
Night Surveys
I studied seasonal changes in the number of calling males and the timing
of calling by conducting night surveys of singing males in a pasture used to graze
cows at the University of Florida Beef Teaching Unit. This pasture was divided
into 10 130 x 20 (I x w) m parallel transects. A night survey consisted of a 16 h
sampling period beginning an hour before sunset in which I walked in a straight
line through the center of each of five transects randomly selected at 4 h intervals
counting the number of calling males. At the start of each 4 h interval, I recorded
the temperature in each transect 2 cm above the soil. On a given night, the same
five transects were surveyed four times during the 16 h survey period. Each
transect was surveyed two to three nights each season. Fall surveys were
conducted from 23 September to 17 November 2000 and spring surveys from 3
April to 23 May 2001 because these are the dates when flies were shown to be
abundant and rare, respectively (Walker 1986).
To test the prediction that fewer males will call in the early evening in the
fall than in the spring, I performed a Repeated Measures ANOVA on the number
of calling males in the night surveys. This variable was transformed to achieve
normality by adding 0.375 to the variable and taking the square root of this sum
(Ott 1993). This transformation achieved normality and validated the sphericity
assumption (variance-covariance matrix of dependent variable is circular in form)
of the Repeated Measures ANOVA. The terms season (fall and spring) and time
(4 h interval) were used as factors. The average temperature across the four time
intervals in each transect was used as a covariate. Only one night survey per


13
transect was used in this analysis. For each transect, I chose the first night
survey where the average temperature recorded in the four time intervals fell
between 15 and 25 C. This restriction was used to limit the effect of extreme
temperatures on male calling behavior. For this analysis, I assumed that
transects were independent between seasons because the lifespan of males is
likely to be shorter than the time elapsed between the fall and spring night
surveys.
Data were analyzed using SYSTAT 6.0 and SPSS 11.5. Results are
presented as means standard error.
Results
Fly Abundance and Activity
Ormia ochracea flies were much more common in the fall than the spring.
In the fall, a mean (SE) of 17.6 6.2 flies/ night were captured at the slit trap
(N=8). In contrast, no flies were ever captured in the spring (N=6). In the fall, the
number of 0. ochracea flies captured was highest in the first few hours after
sunset (Fig. 2-1). Flight activity then steadily declined throughout the remainder
of the night but showed a small increase around dawn (i.e., 13 hours after
sunset).


14
Field Recordings
A higher proportion of males sang in the spring, 15/20 (75%), than in the
fall, 10/23 (43%) (Fishers exact test: P=0.030). Overall, fall males did not differ in
nightly calling duration (44.5 min 13.8) from spring males (31.8 min 9.1) (t-test,
Nfaii=23, Nspring=20, t=0.765, P=0.449). However, when males that did not sing at
any time during the 16 h test period were excluded from this analysis, then fall
males that sang spent more time calling (102.3 min 20.3) than spring males
that sang (42.4 min 10.9) (Fig. 2-2; t-test, Nfaii=10, Nspring=15, t=2.60, P=0.020).
Although fall males were heavier than spring males (Fig. 2-3; t-test, Nfan=23,
Nspring=20, t=3.693, P=0.001), male mass did not have a significant effect on
nightly calling duration for males that sang in either the fall (Fig. 2-4; N=10,
R2=0.090, P=0.399) or the spring (Fig. 2-4; N=15, R2=0.002, P=0.885).
Night Surveys
More males were observed singing in the spring than in the fall across all
sampling periods in all surveys (Fig. 2-5; Repeated Measures ANOVA, season,
F1i18=8.769, P=0.008). Time of day had a significant effect on the number of
calling males (Fig. 2-6; Repeated Measures ANOVA, 4 h interval, F3,54=7.319,
P<0.001). More males sang in the first few hours after sunset in the spring than
in the fall (Fig. 2-6; Repeated Measures ANOVA, time*season, F3i54=3.776,
P=0.016). These results came from a model that excluded temperature as a
covariate. Temperature was excluded because a preliminary model with season


15
and time as factors and temperature as a covariate showed that temperature did
not have a significant effect on the number of calling males (Repeated Measures
ANOVA, temperature, Fi,i7=0.058, P=0.812).
Discussion
This study has shown that seasonal changes in male G. rubens calling
behavior are correlated with seasonal changes in the risk of fly parasitism. The
probability of encountering parasitoid flies on a given night is higher in the fall
than in the spring. In the fall, this probability is highest in the early evening and
then declines steadily throughout the night. This suggests that the natural
selection costs of calling per night are higher in the fall than in the spring, and
that these costs decrease within a night in the fall. As predicted by these
seasonal changes in the risk of fly parasitism, a lower proportion of males sang in
the fall than in the spring and fewer males sang in the early evening in the fall
than in the spring. These findings are consistent with the hypothesis that natural
selection affects the evolution of sexually selected male traits. When the natural
selection costs of conspicuous traits increase relative to the benefits, then males
behave less conspicuously.
However, other factors besides the risk of fly parasitism change
seasonally and may explain the seasonal variation in the number of calling
males. These factors include differences in density, differences in the proportion
of parasitized males, and differences in male size between seasons. Differences
between seasons in female behavior will be discussed in chapter 3. Fewer


16
males may have sung in the fall than in the spring because fewer males were
present. Several lines of evidence argue against this possibility. First, Veazey et
al. (1976) found that 6. rubens densities in northern Florida are higher in the fall
than in the spring. Second, my field recordings of individual males showed that a
smaller proportion of males sang in the fall than in the spring. Alternatively, fewer
males may have sung in the fall than in the spring because a higher proportion of
males were parasitized. Parasites have been shown to decrease the ability of
males to sing (Cade 1984, Kolluru et al. 2002). However, my field recordings
show that this cannot entirely explain the pattern because healthy fall and spring
males differed in calling behavior. Finally, fewer males may have sung in the fall
than in the spring because spring males were larger than fall males. This is
possible because male size has been shown to be correlated with calling
duration in sagebrush and ground crickets (Sakaluk & Snedden 1990, Forrest et
al. 1991). However, this cannot be an explanation for the seasonal differences in
the number of calling males because fall males were heavier than spring males.
The combined evidence suggests that fewer males sang in the fall than in the
spring because the risk of fly parasitism is higher in the fall than in the spring.
More than half of the males that were recorded in the fall did not sing over
the 16 h period they were monitored. If females approach calling males, how do
these silent males get mated? It is likely that these males walk or fly around
searching for females. This mating strategy has been described in several
species of field crickets (French & Cade 1989, Hissmann 1990, Zuk et al. 1993,
Hack 1998). Cade (1979) proposed that searching may be an alternative mating


17
tactic. He hypothesized that males using this tactic may have lower mating
success on any given day than calling males. However, because searching
males may also have lower rates of fly parasitism, they may have higher survival
and longer lifespans than calling males. This type of alternative mating tactic has
been described for guppies. Males usually display their bright color patterns to
females by performing sigmoid displays in which males quiver their bodies in an
S-shape (Baerends et al. 1955). Some males, however, do not perform these
displays and instead attempt a sneak copulation by performing a gonopodial
thrust. This mating tactic may be favored by natural selection because it is less
conspicuous to predators than sigmoid displays (Endler 1987). This hypothesis
has been supported by studies showing that males switch from sigmoid displays
to gonopodial thrusts when the risk of predation is high (Endler 1987, Magurran &
Seghers 1990, Reynolds et al. 1993, Godin 1995). Taken together, studies on
crickets and guppies suggest that when the natural selection costs of
conspicuous male traits increase, males may use alternative mating tactics.
Males using these tactics are favored because even though they may have
reduced mating success, they may have higher survival when compared to
males using the more conspicuous tactic. Additionally, the success of searching
males may increase as density increases.
As predicted, I found that more male G. rubens sang in the early evening
in the spring than in the fall when flies were more active. French and Cade
(1987) studied the timing of male calling in G. texensis, G. veletis, and G.
pennsylvanicus. Their results showed that male calling increased at dawn in field


18
populations of all three species. Male G. rubens in this study did not increase
calling around dawn. One possible explanation for this finding is that the small
surge of parasitoid flies around dawn at my study site may have increased the
natural selection costs of calling at those times. Consistent with this interpretation
is the fact that the populations of G. texensis that French and Cade (1987)
studied did not experience a surge in fly activity around dawn, and the
populations of G. veletis and G. pennsylvanicus were not parasitized by O.
ochracea (Cade & Wyatt 1984). Collectively, these studies suggest that the
timing of male calling in crickets has responded to natural selection from
parasitoid flies.
I predicted that males should spend less time calling per night in the fall
than in the spring because the number of parasitoid flies attracted to broadcasts
of cricket songs increased with broadcast duration (this study, Cade et al. 1996).
The field recordings of individual males found the opposite result: fall males that
sang spent more time calling per night than spring males that sang. Bertram
(2002) also found the same seasonal pattern in male G. texensis. One factor that
may explain these results is density. As density increases, males are expected to
call less and search more because the gains from searching increase (Alexander
1961 & 1975, Otte 1977, Cade 1979). This effect has been demonstrated in
experimental studies by showing that male calling decreases with density (Cade
& Wyatt 1984, Cade & Cade 1992). However, seasonal differences in density
cannot explain the seasonal differences in nightly calling duration from this study


19
nor the Bertram (2002) study because cricket densities at both sites were higher
in the fall than in the spring (Veazey et al. 1976, Bertram 2002).
One possible explanation for the seasonal differences in nightly calling
duration in 6. rubens and 6. texensis is sexual selection. If sexual selection for
nightly calling duration is stronger in the fall than in the spring, then this seasonal
difference in mating benefit may explain why fall males that sang spent more
time calling than spring males that sang. I evaluate this hypothesis in chapter 4.


Hours after sunset
Fig. 2-1. Nocturnal changes in the mean number of Ormia ochracea flies captured at a slit trap
broadcasting Gryllus rubens male calls over eight nights in the fall.


Nightly calling duration (min)
fall (N=23)
spring (N=20)
Fig. 2-2. Seasonal changes in the nightly calling duration of male Gryllus rubens individually recorded in
the field during the fall and spring.


Season
Fig. 2-3. Seasonal changes in male Gryllus rubens mass. Males were collected in the fall (N=23)
and spring (N=20) using a sound trap. Each male was weighed to the nearest 0.001 g.


x Spring (N=15)
O Fall (N=10)
Male mass (g)
Fig. 2-4. Effect of mass on nightly calling duration of male Gryllus rubens individually recorded in
the field during the fall and spring.


Season
Fig. 2-5. Seasonal changes in the mean number of calling male Gryllus rubens over the night in
surveys of 10 transects sampled in the fall and spring.


Mean number of calling males/ transect/ night
to
Hours after sunset
Fig. 2-6. Seasonal changes in the timing of male Gryllus rubens calling over the night in surveys of
10 transects sampled in the fall and spring.


CHAPTER 3
SEASONAL VARIATION IN FEMALE PHONOTAXIS TO MALE CALLING SONG
OF THE FIELD CRICKET GRYLLUS RUBENS
Introduction
Males with sexually-selected traits are often at higher risk from predators
and parasites (reviewed by Burk 1982 and Magnhagen 1991) than those without.
Understanding how these risks affect female choice is important because
theoretical models on the evolution of female preferences show that choosiness
decreases when the risks of choice increase relative to the benefits (Parker
1983, Kirkpatrick 1987, Pomiankowski 1987, Real 1990, Iwasa etal. 1991). This
is because females that choose conspicuous males may also experience
increased risk of predation or parasitism because of their association with these
showy males. A decline in choosiness will reduce the strength of sexual selection
on male traits, which will cause males to become less extravagant. Therefore,
males with sexually selected traits evolve to be less conspicuous when predation
or parasitism is high not only because less conspicuous males survive better, but
also because the benefits of conspicuousness through female choice are
reduced. Despite the potential importance of these risks to females, few studies
have examined how predation risk affects female choice (although see Dill et al.
1999). Similarly, while many studies have investigated female choice for males
differing in parasitism levels (Borgia & Collis 1989, Hoglund et al. 1992, Houde &
Torio 1992, Fitzgerald et al. 1994, Buchholz 1995, Kraaijeveld et al. 1997, Lopez
26


27
1998), no study has evaluated how parasitism risk affects female choice. In this
study, I hypothesize that risk of parasitism decreases female attraction to males
with sexually-selected traits. I evaluated this hypothesis by examining female
phonotaxis to male calling song in the field cricket Gryllus rubens in two seasons
that differed in the risk of parasitism from acoustically orienting flies.
Female G. rubens approach calling males and mount them for mating.
The same male vocalizations that attract female crickets also attract gravid
females of the parasitoid fly Ormia ochracea. Female flies deposit larvae on and
around calling males, killing infected crickets within 7-10 days (Walker 1986,
1993). Even though females do not sing, Walker and Wineriter (1991) collected
females during the fall of 1990 and found 10% parasitism (3 of 10) and a similar
rate, 9%, among males (1 of 11). It is likely that females acquire parasites by
interacting with calling males that have been attacked by O. ochracea that same
night. Several lines of evidence support this idea. First, Walker and Wineriter
(1991) experimentally muted males and found that unmuted males under high fly
densities were not parasitized by O. ochracea (0 of 10) whereas non-muted
males were (7 of 13). This suggests that calling is associated with fly parasitism.
Because females do not sing and males only sing calling songs to attract females
for mating (Loher & Dambach 1989), females are likely to be parasitized as they
approach calling males. Second, O. ochracea flies deposit larvae around calling
males. Females may become parasitized by walking over these larvae which
adhere to them (Cade 1975, 1979, 1984). These scattered larvae may also
explain how male and female G. firmus are parasitized by O. ochracea (Walker &


28
Wineriter 1991); G. firmus co-occur with G. rubens but their calls do not attract
parasitoid flies (Walker 1986). This apparent link between female searching
behavior and fly parasitism make G. rubens an ideal species to test the
hypothesis that risk decreases female attraction to males with sexually-selected
traits.
Parasitoid flies may decrease female G. rubens attraction to male calling
song to minimize the risks of choice. In northern Florida, 0. ochracea is abundant
in the fall and rare in the spring (Walker 1986, Chapter 2), and fly parasitism
rates are higher in the fall than in the spring (10% versus 0%, Walker & Wineriter
1991; chapter 2). This seasonality in fly abundance and parasitism rates
suggests that fall females may incur higher risks of choice than spring females.
Such costs may be greatest in the early evening because O. ochracea flight
activity is highest just around dusk and then steadily declines (Cade et al. 1996,
Chapter 2). These nocturnal changes in parasitoid fly activity may favor females
that are less phonotactic to male calls in the early evening. This would be
consistent with laboratory experiments on gobiid fish, crickets, and guppies,
which have shown that female choosiness decreases when predation risk
increases (Forsgren 1992, Hedrick & Dill 1993, Godin & Briggs 1996). Because
O. ochracea flies are only active after dusk in the fall and not the spring (Chapter
2), I hypothesized that fall females would be less phonotactic to male calling
song in the early evening than spring females. I evaluated this hypothesis by
conducting a phonotactic experiment on fall and spring females shortly after
dusk. In this experiment, females were given a choice between a silent speaker


29
and a speaker broadcasting male calls. I predicted that fewer females would
choose the broadcasting speaker in the fall than in the spring, and fall females
would spend less time near the broadcasting speaker than spring females. The
potential finding that fall females are less attracted to male calling song in the
early evening than spring females would be important because it would reduce
the strength of sexual selection on males in the fall and help explain why fewer
males sing around dusk in the fall than in the spring (Chapter 2).
Methods
Phonotactic Experiment
In this experiment, I quantified and compared among seasons the
phonotactic behavior of wild-caught females for male calling song at the time
when parasitoid flies are most active. Gryllus rubens exhibits a wing dimorphism:
males and females are either long-winged and capable of flight or short-winged
and flightless. Both morph types were used in this phonotactic experiment in the
proportion that they were available. I used two methods of collecting crickets;
some were collected by turning over objects and debris and others were
captured using sound traps (Walker 1986). I used the two methods because I
collected very few long-winged individuals by turning over objects and debris and
sound traps captured long-winged individuals exclusively. All collections were
made at the University of Florida Organic Garden, Alachua Co., FL.
Females were weighed to the nearest 0.001 g and housed individually in
square plastic containers measuring 13 x 13 x 9 cm (I x w x h) with wire screen


30
lids. Each container had an inch of sand, an inverted egg carton shelter, and
Purina Cat Chow and water ad lib. All containers were held under ambient light
and temperature in a greenhouse less than 1 km from the collection site. After 10
days (the number of days necessary to ensure that the crickets were not
parasitized by O. ochracea), I brought females to the lab and tested their
preference for male calling song. Parasitized females were not used in these
experiments. The actual sample size for the fall was N|0ng-winged=24 and NShort-
winged=16. The Sample size for the spring was N|ong-winged23 and Nshort-winged=17.
Phonotactic tests were conducted in a rectangular arena (Fig. 3-1)
measuring 1.0 x 0.3 x 0.3 m (I x w x h). The floor of the arena was made of
compressed foam, and the walls consisted of metal 2 mm mesh wire screen To
prevent the crickets from climbing out of the arena, I taped a 5 cm wide paper
strip along the wire screen walls 15 cm from the floor. On the outside at each end
of the arena, I placed a Radio Shack model 277-1008 mini-audio amplifier
speaker. These speakers were connected to a Sony model D-EJ721 portable
CD-Walkman. Prior to a test, a female was placed in the center of the arena
under an inverted egg carton shelter. I covered this shelter with a plastic cup and
allowed the female to acclimate for 5 min. After that time, I lifted the plastic cup
with a string attached to it and gave the female 5 min to choose between a silent
speaker and a speaker broadcasting a male calling song. Output of the
broadcasting speaker was set at 72dB, 15 cm in front of the speaker, using a
Brel & Kjaer model 2219 sound level meter. This sound level falls within the


31
natural variation of sound intensities of calling males (M.J. Vlez, unpublished
data).
I observed females in the arena using a 40 watt red light. Each female
was tested only once two to three hours after sunset within the four-hour period
when O. ochracea flies are most active (Chapter 2). For each test, I recorded the
latency to leave the shelter, which end screen the female contacted first (criterion
for female choice), and the time spent within 10 cm of the broadcasting speaker.
The floor of the arena was lined with paper. Between tests, this paper was
replaced, the metal wire screen walls were wiped with paper towels, and the
position of the broadcasting speaker was randomized. No end biases were
detected. I examined female phonotactic behavior in the laboratory from 12 June
to 5 July (N=40) and 10 October to 6 December 2001 (N=40) because these are
the times of year when flies are rare and abundant, respectively (Walker 1986).
The temperature in these tests did not differ between the two times of the year
(Meanfan SE = 22.580.06 C, Meanspring SE = 22.660.05 C, P=0.463).
This phonotactic experiment was conducted in the absence of O.
ochracea flies because I assumed that females do not detect flies directly. I
based this assumption on the finding that male crickets in the presence of flies do
not sing differently than solitary males (S.M. Bertram, unpublished data).
I analyzed the time that females spent within 10 cm of the broadcasting
speaker using a general linear model with model: season (fall, spring), collection
method (turning over objects and debris, sound traps), morph (long-winged,
short-winged), mass, season*collection method, season*morph, season*mass.


32
The P-value reported for each variable is the value after all other variables were
taken into account. Data were analyzed using SPSS 11.5. Results are presented
as means standard error.
Source of Male Calls
Male G. rubens produce a trilling song in which sound pulses are repeated
and grouped into long pulse trains (Fig.3-2; Doherty & Callos 1991). Field-
collected males were individually recorded in the lab using an Optimus model
CTR-115 sound-activated cassette tape recorder. Ten males were recorded in
the fall and 10 males in the spring. Each male was recorded for at least five
minutes. The recordings of each male were digitized and divided to make four
one-minute calls. This generated 40 fall calls and 40 spring calls. The digitized 1-
min calls were burned onto CD discs, and one of these calls was played back
repeatedly to each female for 5 min during the preference test. Each female
received a different call. Fall females were tested with fall calls and spring
females with spring calls. The temperature at which males were recorded did not
differ between the fall and the spring (Meanfaii SE = 21,881.33 C, Meanspnng
SE = 22.750.80 C, P=0.584). This temperature also did not differ from the
temperature during the female phonotactic tests (Meanrec SE = 22.31 0.76 C,
Meanphon SE = 22.630.04C, P=0.684).


33
Fly Activity
To verify that O. ochracea flies are more active in the early evening in the
fall but not the spring, I monitored fly activity in a pasture at the University of
Florida Beef Teaching Unit, Alachua Co., FL. To monitor fly activity, I used a slit
trap (Walker 1989) with an electronic sound synthesizer that broadcasts calls of
G. rubens (4763 Hz carrier frequency, 50.0 pulses per second, 50% duty cycle;
continuous trill at 80 dB, 15 cm above the speaker). I activated the synthesizer
an hour before sunset and counted the number of flies captured at the trap 4 h
later. I carried out this procedure from 5 June to 12 July (N=20) and from 10
October to 26 November 2001 (N=20) because these were the times when
females were tested in the phonotactic experiment.
Results
Phonotactic Experiment
Fewer females chose the broadcasting speaker in the fall (20/40; 50%),
than in the spring (30/40; 75%) (Fishers exact test: P=0.013). Fall females also
spent less time within 10 cm of the broadcasting speaker (Fig. 3-3; ANOVA,
season, Fij2=9.736, P=0.003) than spring females. Furthermore, fall females
took longer to leave the shelter than spring females (Fig. 3-4; t-test: Nfaii=40,
NSpring=40, t=3.115, P=0.003). In general, upon leaving the shelter, fall females
tended to walk around it and after some time, many of them went to the
endscreen with the silent speaker and walked around that area. In contrast,
spring females tended to leave the shelter and walk directly to the endscreen


34
with the broadcasting speaker. Many of them climbed the metal screen wall
facing the speaker walking back and forth in front of the broadcasting speaker.
The time that females spent within 10 cm of the broadcasting speaker was
not significantly affected by collection method (ANOVA, collection method,
F1i72=0.227, P=0.635) or morph type (ANOVA, morph, Fii72=0.009, P=0.926).
Similarly, the time that females spent within 10 cm of the broadcasting speaker
was not significantly affected by the interaction between season and collection
method (ANOVA, season*collection method, Fii72=0.208, P=0.650) or the
interaction between season and morph (ANOVA, season*morph, Fit72=0.124,
P=0.726).
Female mass had a significant effect on the time that females spent within
10 cm of the broadcasting speaker (ANOVA, mass, Fit72=5.231, P=0.025). This
effect differed between the seasons. In the spring the time that females spent
within 10 cm of the broadcasting speaker decreased with female mass but not in
the fall (Fig. 3-5, ANOVA, season*mass, F1i72=8.219, P=0.005).
Fly Activity
More flies were captured in the early evening per night in the fall than in
the spring (Meanfan SE = 2.60.8, Meanspring SE = 0.20.1, Mann-Whitney U
test, U=317, nfaii=20, nspring=20, P<0.001).


35
Discussion
This study demonstrates seasonal changes in female G. rubens
phonotactic behavior to male calling song. Parasitoid flies are most abundant in
the fall and most active in the early evening (Chapter 2). In the spring, flies are
rare. Because females may be parasitized as they approach singing males, I
predicted that fall females should be less phonotactic to male calling song in the
early evening than spring females. The results of this study support this
prediction. Fewer females chose the speaker that was broadcasting male calls in
the fall than in the spring, and fall females spent less time near the broadcasting
speaker than spring females. These results are consistent with the hypothesis
that risk of parasitism decreases female attraction to males with sexually-
selected traits.
Few studies have examined the effect of risk on female choice for sexually
selected traits. French and Cade (1987) examined female mating behavior in G.
texensis housed in large outdoor arenas. This field cricket is also parasitized by
O. ochracea (Cade 1975). Their results showed that females mated more
frequently around dawn, which coincided with the time when parasitoid flies were
less active (Cade et al. 1996). Similarly, Hedrick and Hill (1993) examined female
preference for long-bout calls in G. integer in the presence and absence of cover.
In this species, females prefer long-bout calls over short bout calls (Hedrick
1986) all else being equal. However, when females were given a choice between
a long-bout call played across open space and a short-bout call played through
100% cover, females did not express a preference for long-bout calls, suggesting


36
that the increased predation risk associated with open space reduced the
attractiveness of long-bout calls. Similar results have also been found in sand
gobies and guppies (Forsgren 1992, Godin & Briggs 1996). Typically, females of
these two species prefer to mate with brightly colored males. However, this
preference disappeared when females had to choose in the presence of a
predator. Collectively, these results are consistent with the findings of this study
and support the hypothesis that when choice is risky, the intensity of sexual
selection declines.
In this study, females were collected using two methods; turning over
objects and debris, and using sound traps. The use of sound traps may have
introduced a bias since individuals that were captured using this method were
initially attracted to male calling songs. However, this possibility was not
supported because collection method and the interaction between season and
collection method did not have significant effects on the time that females spent
within 10 cm of the broadcasting speaker. Another bias that may have affected
the results of this study is that different numbers of long- and short-winged
females were collected in the fall and spring. However, again, this was not
supported because morph type and the interaction between season and morph
type did not significantly affect the time that females spent within 10 cm of the
broadcasting speaker. Collectively, these results suggest that seasonal changes
in female phonotactic behavior were not due to seasonal changes in the effects
of collection method or morph type.


37
The time that females spent near the broadcasting speaker decreased
with female mass in the spring but not the fall (Fig. 3-5). This seasonal difference
in the interaction between female mass and phonotactic behavior may be due to
seasonal differences in the mating experience of females. Female crickets mate
multiply (reviewed by Loher & Dambach 1989). Their mating experience has
been shown to affect their phonotactic behavior. Lickman et al. (1998) presented
females differing in mating experience with multi-stimulus songs. Their results
show that virgin females exhibited more directional movement towards male
songs than mated females. Virgin females may be more phonotactic than mated
females since this will increase the chances that they will mate at least once and
secure some sperm. In this study, if heavy females in the spring were old and
mated, and light females were young and virgin, then this relationship may
explain why female phonotaxis decreased with female mass in the spring. This is
possible because 6. rubens has two pulses of adults in the fall and spring, and
two smaller cohorts of adults in the summer and winter (T.J. Walker, unpublished
data). At the time that spring females were collected from the field (15 May to 25
July 2001), the summer cohort was starting to emerge as adults. Therefore, I
could have collected both mated spring females, and virgin summer females. In
contrast, because at the time that fall females were collected (1 October to 25
November 2001), no females from the winter cohort had yet emerged. This
would mean that my fall sample may have primarily consisted of mated fall
females. This could explain why phonotactic behavior did not decrease with
female mass in the fall. It is important to note, however, that even when this


38
seasonal interaction between female phonotactic behavior and female mass is
taken into account, season still has a significant effect on female phonotactic
behavior.
Fall females took longer to leave the shelter during the phonotactic
experiment than spring females. This seasonal difference in female behavior may
be interpreted as fall females behaving more cautiously around male calls than
spring females in order to reduce fly parasitism or other sources of increased
risk. This explanation is consistent with a study on G. integer in which males
differing in song conspicuousness were presented with a novel, potentially
dangerous predator stimulus (Hedrick 2000). Males with more conspicuous
songs took longer to emerge from a shelter after the predator stimulus than
males with less conspicuous songs. Hedrick (2000) interpreted these differences
in latency to leave the shelter as males compensating behaviorally for their
conspicuous mating displays. This interpretation is consistent with the findings of
this study for females and suggests that future work should examine how
parasitism risk affects female searching behavior.
Fall females took longer to leave the shelter than spring females. Could
this difference in behavior account for the shorter time that females spent within
10 cm of the broadcasting speaker in the fall than in the spring? This is possible
because time spent in the shelter is time that could not have been spent near the
broadcasting speaker. However, some evidence suggests that this is unlikely.
The mean time that fall females spent in the shelter was 98 sec (Fig. 3-4). This
means that fall females spent on average 202 sec outside of the shelter. This


39
time is much greater than the mean time that spring females spent within 10 cm
of the broadcasting speaker, 121 sec (Fig. 3-3). These results suggest that
seasonal differences in female latency to leave the shelter do not fully explain the
seasonal variation in the time that females spent within 10 cm of the
broadcasting speaker.
This study has important implications for understanding the evolution of
sexually-selected traits. Male G. rubens sing to attract females for mating. This
behavior, however, also attracts parasitoid flies in some populations, in some
seasons, and at some times of the night. Differences in male calling behavior
between populations, seasons, or time of the night have been interpreted as due
to the natural selection pressure, parasitism. However, if parasitism risk also
decreases female attraction to male calls, then males may derive fewer benefits
from calling. In this study, I have found that fall females are less attracted to male
calling song in the early evening than spring females. This reduction in the
strength of sexual selection, as well as the increased natural selection costs for
males, will weaken selection for calling in the early evening for fall but not spring
males. Therefore, fall and spring males should differ in the timing of calling
behavior. Fall males should not sing in the early evening, but spring males
should sing at that time. This prediction has been confirmed in G. rubens
(Chapter 2): surveys of calling males in the field showed that fewer males sing in
the early evening in the fall than in the spring. This correlation between female
behavior and male calling behavior suggests that future studies investigating
variation in sexually selected traits should examine not only how risk affects


40
natural selection on males with sexually selected traits, but also how risk affects
female choice for males with sexually selected traits, and hence the benefits of
conspicuous traits.


CD player
broadcasting
speaker
O
shelter
w=0.3 m
silent
speaker
<
1=1.0 m
>-
Fig. 3-1. Experimental arena used in phonotactic experiment.


Frequency (kHz)
10
s
Time (sec)
4^
NJ
Fig. 3-2. Sound spectrograph of Gryllus rubens calling song at 25.4C (SINA)


300
o
0
(/>
¡5 250 U
0
ro
0
CL
O)
c
'M
8
O
0
o
ro
0
c
200 U
150 U
r 100 U
c
0
CL
o
E
£ 50 U
c
(0
0
0
fall
spring
U)
Season
Fig. 3-3. Seasonal differences in the mean time spent by female Gryllus rubens within 10 cm of a
broadcasting speaker in a phonotactic experiment where fall (N=40) and spring (N=40) females
chose between a silent speaker and a speaker broadcasting a male calling song.


300
250
o'
0)
| 200
JZ
tn
0)
1 150
o
4-
£
S ioo
5
c
ro
S 50
0
Fig. 3-4. Seasonal differences in the mean latency to leave the shelter by female Gryllus rubens
in a phonotactic experiment where fall (N=40) and spring (N=40) females chose between a silent
speaker and a speaker broadcasting a male calling song.
fall spring
Season
£


x spring
O fall
Female mass (g)
Fig. 3-5. Seasonal differences in the association between female Gryllus rubens mass and time spent
by females within 10 cm of a broadcasting speaker in a phonotactic experiment where fall (N=40)
and spring (N=40) females chose between a silent speaker and a speaker broadcasting a male calling
song.


CHAPTER 4
VARIATION IN SEXUAL SELECTION FOR MALE SONG
IN THE FIELD CRICKET GRYLLUS RUBENS
Introduction
A primary goal of sexual selection theory is to understand how natural and
sexual selection interact to favor variation in male sexually-selected traits
(Andersson 1994). Crickets represent an ideal system in which to study this
question for several reasons. First, calling song is highly variable among males
(for review see Bertram 2002). Such variation can be easily quantified by
individually recording males and digitally analyzing their songs. Second, male
song attracts conspecific females for mating. Typically, females approach singing
males and mount them (Loher & Dambach 1989). This suggests that male calling
is affected by sexual selection. Finally, male vocalizations also attract parasitoid
flies (Cade 1975, Walker 1986, 1993). Female flies deposit larvae on and around
calling males, killing infected crickets within 7-10 days (Walker 1986, 1993). This
means that male calling may also be affected by natural selection. Collectively,
these conditions make crickets an ideal system to study how natural and sexual
selection interact to favor variation in male sexually-selected traits.
One of the most variable song characters in male crickets is nightly calling
duration or the total time spent calling per night. Using an artificial selection
experiment, Cade (1981) demonstrated that much of the variation in nightly
calling duration in the cricket, Gryllus texensis, was due to genetic differences
46


47
among males. Subsequently, Cade and Wyatt (1984) found that nightly calling
duration was highly variable among male G. texensis, G. pennsylvanicus, G.
veletis, and Teleogryllus africanus. Similarly, Cade (1991) determined that nightly
calling duration was also highly variable among male G. integer and G. rubens.
Despite the potential role of sexual selection in explaining variation in male
cricket song, most studies examining variation in nightly calling duration have
ignored the effect of sexual selection (Cade 1984, Cade & Wyatt 1984, French &
Cade 1989, Sakaluk & Snedden 1990, Forrest et al. 1991, Cade & Cade 1992,
Gray 1997, Wagner & Hoback 1999, Kolluru et al. 2002, but see Bertram 2002).
The goal of this study is to examine whether seasonal variation in male G.
rubens nightly calling duration is correlated with seasonal variation in sexual
selection.
In northern Florida, male G. rubens that sing spend more time calling in
the fall than in the spring (Chapter 2). Three factors have been used to explain
variation in male cricket calling behavior: male condition, parasites, and density.
Nightly calling duration should increase with male condition because males in
good condition are better able to bear the energetic costs of calling than males in
poor condition (Sakaluk & Snedden 1990, Forrest et al. 1991, Gray 1997,
Wagner & Hoback 1999). However, this possibility is not supported by the
available data for G. rubens. Although fall males are heavier than spring males,
nightly calling duration does not increase with male mass (Chapter 2). Similarly,
parasitized males should call less than unparasitized males because parasites
decrease the ability of males to sing (Cade 1984, Kolluru et al. 2002). However,


48
this hypothesis does not explain seasonal variation in G. rubens calling since
unparasitized fall and spring males differed in nightly calling duration (Chapter 2).
Finally, males at high densities should sing less than males at low densities
because as density increases, the costs of defending territories increase as well
as the chances of encountering a female by searching without calling (Alexander
1961 & 1975, Otte 1977, Cade 1979, Cade & Wyatt 1984, French & Cade 1989,
Cade & Cade 1992). But, differences in density cannot explain the longer calling
duration of fall G. rubens because male densities are higher in the fall than in the
spring (Veazey et al. 1976). Taken together, it is unlikely that the three most
common explanations can account for the seasonal changes in male G. rubens
nightly calling duration.
The same male G. rubens vocalizations that attract females also attract
gravid females of the parasitoid fly Ormia ochracea (Walker 1986,1993). These
females deposit larvae on and around calling males, killing infected crickets
within 7-10 days. If parasitoid flies attack males more in the spring than in the fall,
then this seasonal difference in parasitism risk may explain the seasonal
differences in nightly calling duration. Flowever, for G. rubens this is not an
explanation because O. ochracea in northern Florida is abundant in the fall and
rare in the spring (Walker 1986, Chapter 2) and fly parasitism rates are higher in
the fall than in the spring (10% versus 0%, Walker & Wineriter 1991). Therefore,
seasonal changes in parasitism risk cannot explain the seasonal changes in
male G. rubens calling duration.


49
Males that sing may have longer calling durations in the fall than in the
spring because seasonal changes in photoperiod and temperature cause fall and
spring males to differ in nightly calling duration. This possibility is supported by
rearing studies showing that photoperiod and temperature may affect calling
song parameters. Walker (2000) reared male G. rubens under different
photoperiods and temperatures to examine whether developmental conditions
affect pulse rate. He found that temperature influenced pulse rate, but
photoperiod did not. Similarly, Olvido and Mousseau (1995) reared striped
ground crickets (Allonemobius fasciatus) under two environments differing in
photoperiod and temperature. Their results showed that chirp rate, chirp duration,
interchip interval, pulse number, and carrier frequency are affected by rearing
environments. Despite this work, no study has ever found a significant effect of
photoperiod on male nightly calling duration in crickets. Bertram and Bellani
(2002) reared male G. texensis under fall-like and spring-like photoperiods and
found that rearing photoperiod does not significantly affect nightly calling duration
in males. This suggests that seasonal differences in male G. rubens nightly
calling duration are not due to seasonal changes in photoperiod. However, the
effect of temperature during development on nightly calling duration has not been
tested in any cricket species.
Male G. rubens may exhibit seasonal changes in nightly calling duration if
the benefits of singing change seasonally. These benefits may vary seasonally if
sexual selection operates differently in the two seasons. Accordingly, I
hypothesized that males sing longer in the fall than in the spring because the


50
benefit of singing increases with calling duration in the fall but not in the spring. I
evaluated this hypothesis by using pitfall traps fitted with speakers broadcasting
male calling songs of different duration. I counted the number of females
captured at the traps in the fall and spring, and predicted that the number of
captured females should increase with calling duration in the fall but not in the
spring. This prediction is important because it would be the first evidence to
suggest seasonal variation in sexual selection. Such variation could favor
variation in the sexually-selected trait, male calling duration.
Methods
Pitfall Trap Experiment
I examined seasonal variation in sexual selection for nightly calling
duration from 14 April to 12 June and 23 September to 5 November, 2002
because these are times when males sing for short and long durations,
respectively (Chapter 2). During both fall and spring, female and male G. rubens
occur in two forms, long- and short- winged. The two morphs differ in their ability
to fly as well as in other life-history characteristics such as the onset of
reproduction (Roff 1984, Zera & Rankin 1989, Holtmeier & Zera 1993). Females
of both morphs were used in this study in the proportions in which they occurred.
All experiments were conducted in a field at the University of Florida
Environmental Landscape Horticulture Education Lab, Alachua Co., FL. I chose
this field because male G. rubens sing all year long at this site.


51
Tests for seasonal variation in sexual selection for nightly calling duration
were conducted using five pitfall traps (Fig. 4-1) laid out in a straight line 10 m
apart. Each trap was made by excavating a hole in the ground and placing a 2.4
L plastic bucket measuring 17 x 15 cm (height x diameter) in the hole so that the
rim was level with the ground. Over each bucket I placed a plastic funnel 10 cm
tall with an upper diameter of 18 cm and a spout diameter of 6 cm. The funnel
rested on the rim of the bucket and the spout was 7 cm above the bottom of the
bucket. I placed a 33-cm square PVC tube frame (1.9 cm diameter) with four 7-
cm legs over the bucket-funnel trap. The frame rested on the ground surrounding
the bucket-funnel trap and supported a metal grill top with 1.5 cm spaces
between the bars. In the center of this grill top, I positioned a Sony model D-
EJ721 portable CD-Walkman with a Radio Shack model 277-1008 mini-audio
amplifier speaker lying flat over the CD player. The speaker pointed upward and
rested 9 cm over the center of the trap. Crickets that were attracted to calls
played on the CD player, flew (long-winged morph) or walked (short-winged
morph) into the bucket-funnel trap where they were collected hourly. Five
different treatments of male calling duration were used, one for each pitfall trap.
The treatments consisted of broadcasting male calling songs for 0,1,2, 3, or 4
hours. These calling durations fall within the natural variation of time spent calling
by males (Chapter 2). Output of the broadcasting speakers was set at 80dB,
measured 15 cm above each speaker, using a Briiel & Kjaer model 2219 sound
level meter. This sound level falls within the natural range of sound intensities of
calling males (M.J. Vlez, unpublished data).


52
Each experiment began an hour before sunset when I activated the CD
player over the pitfall trap set to broadcast for 4 h. An hour later, I turned on the
CD player over the pitfall trap set to broadcast for 3 h. This continued until all CD
players broadcasting songs had been activated. The outcome of this staggering
the onset of the calling was that all CD players finished broadcasting male calling
songs three hours after sunset. This pattern was used because individual
recordings of males under field conditions show that calling start times are
inversely related to calling duration (M.J. Vlez, unpublished data). The pitfall
trap with the 0-h treatment was handled like the pitfall trap set to broadcast for 4
h: a CD player was placed over the trap an hour before sunset. However, this
player did not broadcast any songs. This treatment controlled for the number of
non-phonotactic females that fell into a trap. All pitfall traps were checked hourly
to count the number of captured females from one hour after the experiment
started to the end of the experiment three hours later. Trapped females were
removed from the field to ensure that they were not sampled in more than one
treatment or night. Any trapped males or juveniles were released back into the
field. Each night constituted one replicate of the experiment. All treatments within
a replicate used the same male calling song, but different replicates used
different songs. Between replicates, I randomized the position of the treatments.
Randomization was achieved by generating all possible combinations of
treatments and picking one at random each night. This experiment was
conducted on 35 nights in the spring and 25 nights in the fall.


53
Source of Male Calls
Male 6. rubens were collected from the field using sound traps (Walker
1986). They were housed individually in square plastic containers measuring 13
x 13 x 9 cm (I x w x h) with wire screen lids. Each container had an inch of sand,
a few pieces of Purina Cat Chow (changed daily) and water ad lib. All containers
were held under ambient light and temperature in a greenhouse less than 1 km
from the collection site. After 10 days (the number of days necessary to ensure
that the crickets were not parasitized by O. ochracea), I brought the males into
the lab and individually recorded each for at least 5 min using an Optimus model
CTR-115 sound-activated cassette tape recorder. All of the males used in these
recordings were unparasitized and long-winged because short-winged males
could not fly into the sound traps used to collect crickets. Ten males were
recorded in the fall and 10 males in the spring. The recordings of each male were
digitized and used to make four 1 min calling songs. This generated 40 fall and
40 spring calling songs. These songs were burned onto CD discs and played
back repeatedly in the pitfall trap experiment for the duration of the treatment. In
the fall, I used fall calling songs over pitfall traps, and in the spring, I used spring
calling songs. The mean temperature at which males were recorded was 23.1C
and did not differ between fall and spring calls (t-test, P=0.702).
Data Analysis
A generalized linear model with a Poisson distribution and a logit link was
used to regress the calling duration of a treatment on the number of females


54
captured in that treatments pitfall trap. This model was used because the data
could not be transformed to achieve normality. Because treatments within a
replicate were not statistically independent, I used a mixed model to analyze the
data. In this model, calling duration was a fixed effect, and replicate number was
a random effect. Fall and spring data were analyzed separately. The data were
tested for overdispersion to validate the use of the Poisson model.
If females search for mates at different times of the night in the fall and
spring, then such seasonal differences in female behavior could affect the
relationship between calling duration and number of trapped females. To
evaluate this possibility, I compared the capture time to the nearest hour of each
female in the pitfall traps during the fall and spring using a Mann-Whitney U test.
Data were analyzed using R 1.7.1 (Dalgaard 2002) and Systat 6.0
(Wilkinson 1997). Descriptive results are presented as means standard error.
Results
The number of captured females increased with treatment calling duration
in the fall but not in the spring (Table 4-1; Fig. 4-2; fall: t=4.532, df=99, P<0.001;
spring: t=0.419, df=139, P=0.676). Females were captured in all calling duration
treatments in the spring and fall with the exception of the 0-h treatment in which
no female was captured in either the fall or the spring (Fig. 4-2). Sixteen females
arrived at the pitfall traps in the fall (niong=12, nShort=4), and 10 in the spring
(niong=6, nShort=4). Fall and spring females did not differ in the time at which they
were captured in the pitfall traps (Fig. 4-3; nfaii=16, nspring=10, Mann-Whitney U


55
test=56.5, P=0.189). This non-significant result remains even when the number
of females captured in the first two hours are compared with the number of
females captured in the last two hours in the fall and spring (X2=0.005, df=1,
P=0.940)
Discussion
Male G. rubens that sing spend more time calling in the fall than in the
spring (Chapter 2). This is surprising because there are more phonotactic,
parasitoid flies present in the fall (Walker 1986, Chapter 2) so one would expect
males to call less to avoid parasitism. Furthermore, as I argued in Chapter 3,
females should be less attracted to calling males in the fall because they too are
parasitized by these flies (Walker & Wineriter 1991). The results of the pitfall trap
study provide an explanation: males that sing longer are more likely to attract
females during the fall but not during the spring (Table 4-1). What might explain
this result?
If fall females search for mates in the early evening and spring females
search for mates in the late evening, then these temporal differences in female
activity may explain why the number of captured females in my pitfall traps
increased with calling duration in the fall but not the spring. This is possible
because of the way I set up the experiment: the long calling duration treatments
began earlier in the evening than the short calling duration treatments.
Consequently, fall females, preferring to search for mates in the early evening,
would have a higher probability of encountering a pitfall trap with a long calling


56
duration whereas spring females, preferring to search for mates in the late
evening, would have an equal probability of encountering a long or short calling
duration treatment. However, the results of this study do not support this
explanation. Fall and spring females did not differ in the time at which they were
captured in the pitfall traps. This suggests that male calling duration increases in
the fall but not the spring because long duration calls are more effective at
attracting females in the fall but not in the spring. This pattern is consistent with
seasonal variation in sexual selection.
Female G. rubens densities in northern Florida are higher in the fall than in
the spring (Veazey al. 1976). If females choose males randomly with respect to
calling duration, then males with long duration calls should have a higher
probability of attracting females than males with short duration calls. Because
this probability should increase proportionally with density, seasonal changes in
female density cannot explain the seasonal changes in the success of long
duration calls. Instead, the seasonal differences in the success of long duration
calls suggest that fall females do not have a preference for calling duration and
spring females prefer short duration calls. Theoretical models on the evolution of
female preferences show that choosiness decreases when the risks of choice
increase relative to the benefits (Parker 1983, Kirkpatrick 1987, Pomiankowski
1987, Real 1990, Iwasa et al. 1991). Parasitoid flies are abundant in the fall and
rare in the spring (Walker 1986, Chapter 2). Fall females may not have a
preference for calling duration because the costs of choice due to fly parasitism
may be high in that season. Even though females do not sing, they are


57
nonetheless parasitized to the same levels as males are (Walker & Wineriter
1991). In contrast, females are more phonotactic in the spring than in the fall
(Chapter 3). In an experiment giving females a choice between a silent speaker
and a speaker broadcasting a male calling song, fewer females chose the
speaker that was broadcasting male calls in the fall than in the spring, and fall
females spent less time near the broadcasting speaker than spring females. This
may mean that less singing is required to attract females in the spring than in the
fall. This seasonal variation in sexual selection is consistent with the seasonal
changes in the success of long duration calls.
The results of this study are consistent with recent work showing within-
and between-population variation in the female preference functions of field
crickets. Wagner et al. (1995) sequentially presented female G. integer from a
single population with songs differing in the number of pulses per trill, inter-trill
interval, or the proportion of missing pulses within a trill. These presentations
revealed that individual females differed significantly in their preference functions
based on the number of pulses per trill and inter-trill interval. Furthermore, these
preference functions were highly repeatable within females. Similarly, Simmons
et al. (2001) examined geographic variation in the female preference function for
the long chirp portion of male Teleogryllus oceanicus calling song. Their results
show that the female preference function for the amount of long chirps differed
between populations. The findings from these studies suggest that selection can
act on female preference functions. The outcome of such process may favor
variation in male sexually-selected traits.


58
To evaluate the general importance of temporal variation in sexual
selection as a mechanism favoring variation in male sexually-selected traits,
future studies should examine how variation in female choice is affected by
factors that change the costs and benefits of choice, such as predation,
parasitism, sex ratios, and population densities. If any of these factors fluctuates
in time, then the costs and benefits of choice as well as the resulting female
choice patterns may also vary temporally. In turn, this temporal variation in
female choice could act to favor variation in sexually-selected traits


Top view
Side view
PVC frame with metal
Ground
level
Fig. 4-1. Pitfall trap used to test Gryllus rubens female choice for male calling duration. Each trap
was made by excavating a hole in the ground. This hole was deep enough to accommodate a 2.4 L
bucket measuring 17 x 15 cm (height x diameter). The rim of the bucket was level with the ground.
The bucket was covered with a large plastic funnel 10 cm tall that rested on the rim of the bucket,
spout down. The funnel had an upper diameter of 18 cm and a spout diameter of 6 cm. A square
PVC frame (length = 33 cm) with four 7-cm legs was positioned over the funnel and bucket. This
frame rested on the ground surrounding the bucket-funnel trap. The frame supported a metal grill top.
A CD-Walkman with an external speaker facing upward rested on the metal grill top. The speaker was
positioned 9 cm over the center of the trap.


Table 4-1. The effect of season on male Gryllus rubens nightly calling duration. The table shows the
results of a generalized linear model with a Poisson distribution used to regress treatment calling
duration on the number of captured females in that treatments pitfall trap. Because treatments
within a replicate were not statistically independent, a mixed model was used to analyze the data. In
this model, calling duration was a fixed effect, and replicate number was a random effect. Each
season was analyzed separately.
Season
Parameter
Value
Std. Error
DF*
t-Value
P-value
Fall
Intercept
-4.238
0.577
99
-7.342
< 0.001
Slope
0.716
0.158
99
4.532
< 0.001
Spring
Intercept
-3.073
0.625
139
-4.916
< 0.001
Slope
0.100
0.239
139
0.419
0.676
* Degrees of freedom were calculated with the formula: #nights (# treatments 1) 1.


fall (N=25 nights)
Calling duration treatment (h)
Fig. 4-2. Seasonal differences in the mean total number of female Gryllus rubens captured in the
pitfall trap experiment. Five treatments were presented in each replicate; each broadcasted male
calling song for a different duration (0, 1, 2, 3, and 4 h). Each pitfall trap was checked hourly to count
the number of captured females (which were then removed). Each night constituted one replicate of
the experiment.


4 -
6 -
8 -
10 1 1 1
2 3 4
Female capture time (hours after experiment started)
fall (N=16)
spring (N=10)
Fig. 4-3. Seasonal differences in female Gryllus rubens capture time in the pitfall trap experiment. In
this experiment, five pitfall traps broadcasting male calling songs for 0,1,2, 3, and 4 h were checked
hourly to estimate to the nearest hour the capture time of each female that fell into the trap.


CHAPTER 5
GENERAL MODEL: A TEST OF THE FACTORS AFFECTING MALE CALLING
DURATION IN THE FIELD CRICKET GRYLLUS RUBENS
Factors Affecting Male Calling Duration
Fall and spring generations of the field cricket Gryllus rubens in northern
Florida experience different selective pressures and social environments. Males
of this species sing which attracts females for mating. Their calling song also
attracts gravid females of the parasitoid fly Ormia ochracea (Walker 1986, 1993).
These female flies deposit larvae on and around calling males, killing infected
crickets within 7-10 days. In the fall when parasitoid flies are abundant (Walker
1986, Chapter 2), and cricket densities are high (Veazey al. 1976), calling songs
of long duration attract more females than short duration calling songs (Table 4-
1, Fig. 4-2). This is quite different from the spring when parasitoid flies are rare
(Walker 1986, Chapter 2), cricket densities are low (Veazey al. 1976) and calling
song duration does not affect female attraction rates (Table 4-1, Fig. 4-2). In this
chapter, I examine how seasonal changes in fly parasitism, female visitation
rates, and cricket density should alter the costs and benefits of calling for fall and
spring males. I evaluate these effects by building a model that calculates the
fitness of different calling durations in each season. I then use this model to
predict observed seasonal patterns in calling duration. Additionally, I
independently vary the effects of fly parasitism and female choice on male calling
duration within each season to evaluate the relative importance of natural versus
63


64
sexual selection in favoring seasonal patterns in male calling duration. This
model is valuable because it is the first to evaluate simultaneously the effects of
fly parasitism, female attraction rates, and cricket density on the male character,
calling duration. This model also provides insights into how different selective
pressures and social environments can interact to favor variation in a male trait.
Model: Predicting Seasonal Patterns in Male Calling Duration
Male Calling Strategies
In this model, males can adopt one of four calling strategies: (1) call for 1
h, (2) call for 2 h, (3) call for 3 h, and (4) call for 4 h. These calling durations fall
within the natural range of time spent calling by male G. rubens under field
conditions (Fig. 2-4; Cade 1991). I assume that males adopt one calling strategy
and use it throughout their lifetime. This assumption is supported by the finding
that the mean time spent calling per 24 h under laboratory conditions does not
change with male age in G. texensis, G. pennsylvanicus, G. veletis, and
Teleogryllus africanus (Cade & Wyatt 1984). Furthermore, Cade (1981) found
that some of differences between male G. texensis in calling duration are due to
genetic differences among males.
Individual field recordings of male G. mbens show that some males do not
call (Fig. 2-4). These males may obtain mates by walking around searching for
females. This alternative male tactic has been documented in several species of
field crickets (French & Cade 1989, Hissmann 1990, Zuk et al. 1993, Hack 1998).
I did not include this behavior in my model because little is known about the


65
mating success and parasitism risk of walking males. Cade (1979) proposed that
walking males may have decreased mating success on any given day, but they
may have higher survival rates and longer lifespans than calling males because
they do not attract as many parasitoid flies. This hypothesis, however, remains
untested and hence I exclude non-calling as a strategy in my model.
Calculating Fitness of Male Calling Strategies
Male calling attracts conspecific females and parasitoid flies. Each calling
strategy carries a mating benefit, M, and a probability of escaping fly parasitism,
E. Each day, males mate before they face fly parasitism. The daily fitness of
each calling strategy F¡ is defined as:
F¡ = EjM¡ + (1-E¡)M¡,
where i is the calling strategy.
If a male sings and is not parasitized on a given night, he accrues the
mating benefit of that day and sings again in the next day. If a male sings and is
parasitized on a given night, he accrues the mating benefit of that day. After fly
parasitism, the infected male has one more day to carry out his calling strategy
and receive the corresponding mating benefit. After that day, the infected male
does not get any further reproduction. This restriction is meant to mimic the
temporal dynamics of the effects of fly parasitism on male calling duration.
Kolluru et al. (2002) experimentally infected male T. oceanicus with O. ochracea
larvae and monitored the proportion of time spent calling by males each night
after fly parasitism. Their results show that the proportion of time spent calling by


66
males remained unchanged the day after fly parasitism but declined sharply to
almost zero after the second day.
I calculated the lifetime fitness of each calling strategy using the formula:
d=60
XMi(d+1)(1-Ei)E>1
d=1
where d equals day and the length of the breeding season is 60 days. This
formula individually adds the daily fitness of each strategy over the 60 days of the
breeding season for all possible outcomes (i.e., male is parasitized on the first
day, male avoids fly parasitism on the first day and is parasitized on the second
day, male avoids fly parasitism on the first and second day and is parasitized on
the third day, etc.). Fitness was calculated using R 1.7.1 (Dalgaard 2002).
Seasonal Changes in Mating Benefit (M)
Long duration calling songs are more likely to attract females than short
duration calling songs in the fall but not the spring (Table 4-1; Fig. 4-2). This
seasonal pattern in female attraction suggests that the mating benefit of singing
should increase with calling duration in the fall but not the spring. I estimated the
mating benefit of the different calling strategies in each season using data from a
field experiment carried out in the fall and spring in which I counted the number
of females captured at pitfall traps fitted with speakers broadcasting male calling
songs differing in duration (1 h, 2 h, 3 h, and 4 h; Fig. 4-2). Because the number
of captured females increased with calling duration treatment in the fall, I used
the mean number of captured females per night for each treatment to estimate


67
the mating benefit of each calling strategy in the fall (Table 5-1). Because there
was no significant relationship between calling duration treatment and number of
captured females in the spring, I assumed that all calling strategies have a similar
mating benefit in the spring. I estimated this mating benefit by calculating the
mean number of captured females per night for all calling treatments combined
(Table 5-1). These seasonal changes in mating benefit with calling duration
indicate that males that call more attract more females in the fall but not in the
spring.
Female density may also affect the mating benefit of the different calling
strategies. Males should call less at high densities because females may be
easier to locate without calling and calling territories may be more expensive to
defend at high than at low densities (Alexander 1961 & 1975, Otte 1977, Cade
1979). Because cricket densities are higher in the fall than in the spring, males
should call less in the fall than in the spring. This potential seasonal effect of
density on the mating benefit of the different calling strategies is already taken
into account in the empirical estimates of seasonal changes in mating benefit due
to differences in female visitation rates. This is because the pitfall traps used to
collect these data captured females at their natural densities in each season.
Seasonal Changes in Probability of Escaping Flv Parasitism (E)
Calling males attract parasitoid flies (Cade 1975). Walker and Wineriter
(1991) experimentally muted G. rubens males by removing the right tegmen.
They found that muted males have lower fly parasitism rates (0/10) than unmuted


68
males (7/13) under field conditions at high fly densities. This finding suggests that
the probability of escaping fly parasitism should decrease with calling duration.
To estimate this relationship more precisely, I used data collected in the fall on
the arrival times of flies captured at a slit trap broadcasting male G. rubens
calling song (Chapter 2). The number of flies arriving at the trap decayed rapidly
with time (Fig. 2-1). These data can be used to estimate the probability of
escaping fly parasitism for different calling strategies because male calling start
times are inversely related to calling duration. To achieve this, I fitted a negative
exponential equation to the data on fly arrival times using a generalized mixed
linear model with a Poisson distribution. I used this equation to estimate the
probability of at least one fly infecting a male of each calling strategy. To
accomplish this, I used data collected in the fall on the probability of at least one
fly arriving at a slit trap broadcasting male G. rubens calling song for 4 h (Chapter
2). I set this probability as the y-intercept in the negative exponential equation I
derived earlier and used this equation to calculate the probability of at least one
fly infecting a male that sings for 1 h, 2 h, and 3 h. The outcome of these
manipulations is that the probability of escaping fly parasitism decreased with
calling duration in the fall (Table 5-1). This relationship between the probability of
escaping fly parasitism and calling duration indicates that males that call more
have lower survival rates and shorter lifespans than males that call less. This is
because the cumulative probability of escaping fly parasitism in this model
decreases more rapidly for long calling durations than for short calling durations


69
over the 60 days of the breeding season. The end result is that this model
incorporates the tradeoff between calling duration and life span for fall males.
Walker (1986) estimated that fly abundance in the spring is 1 % of the fly
abundance in the fall. This means that the probability of escaping fly parasitism
should be higher for all calling strategies in this season. I estimated this
probability for each strategy by increasing their counterpart fall probability by
99%. This assumes that there are 99% fewer flies in the fall than in the spring, fly
parasitism risk increases linearly with parasitoid fly abundance, and the
probability of escaping fly parasitism decreases with calling duration in the same
way as it does in the fall (Table 5-1).
Model Results: Seasonal Changes in Fitness of Male Calling Strategies
The fitness of calling strategies increased with calling duration in the fall
but not the spring (Fig. 5-1). In the fall, the strategy call 4 h had the highest
fitness, and call 1 h had the lowest fitness (Fig. 5-1). In the spring, the strategies
call 1 h and call 2 h had the highest fitness, and call 3 h and call 4 h had the
lowest fitness (Fig. 5-1).
Comparison of Optimal Seasonal Strategies with Observed Calling Durations
Even though parasitoid flies are more common in the fall than in the spring
(Walker 1986, Chapter 2), male G. rubens spent more time calling in the fall than
in the spring (Fig. 2-5). This finding agrees with the predictions of my model. The
fitness of calling strategies increased with calling duration in the fall but not the


70
spring. The probability of escaping fly parasitism decreased with calling duration
in both the fall and spring. In the fall, however, this increased cost of singing with
calling duration was offset by a disproportionate increase in the mating benefit
with calling duration. This resulted in calling strategies with long calling durations
obtaining higher fitness than calling strategies with short calling durations. In
contrast, the increased cost of singing with calling duration in the spring was not
offset by the mating benefit because this benefit did not change with calling
duration. This resulted in calling strategies with short calling durations achieving
higher fitness than calling strategies with long calling durations.
A comparison of the mean calling duration of male G. rubens in the fall
and spring with the optimal seasonal calling durations predicted by my model
reveals that males sang about as much as expected in the spring (0.71 h vs. 1 or
2 h) and less than expected (1.71 h vs. 4 h) in the fall. One possible explanation
for this discrepancy is that my model did not include the energetic costs of
calling. Such costs are known to increase linearly with calling duration in trilling
species of crickets like G. rubens (Prestwich & Walker 1981). Fall males may
have not sung as much as predicted because it may be more costly to sing in the
fall than in the spring. This could occur if temperatures are higher in the fall than
in the spring. It is clear that if these seasonal changes in the energetic costs of
calling were incorporated into my model, then the predicted calling durations
would be lower than the current ones in the fall.


71
Variations of the Model
To evaluate the relative importance of natural versus sexual selection in
favoring seasonal patterns in male calling duration, I independently varied the
effects of fly parasitism and female visitation rates on male calling duration within
each season. In the first variation of the model, I drastically reduced fly
parasitism by setting the probability of escaping fly parasitism to 0.999 for all
calling strategies in the fall and spring (Table 5-2). I then examined the fitness of
the different calling strategies given only seasonal variation in the mating benefit
(Table 5-2). This would be the situation, for example, in a population with few
flies, which may be found in the northern extent of the G. rubens range. In the
second variation of the model, I held the mating benefit constant for all calling
strategies in the fall and spring by assigning all strategies the average mating
benefit for both seasons combined (Table 5-3). I then evaluated the fitness of the
different calling strategies given only seasonal variation in the probability of
escaping fly parasitism (Table 5-3). This would simulate the effect of a population
in which there was no relationship between calling duration and female visitation
rates.
Results of Model Variations
In the model with reduced and constant fly parasitism, and variable mating
benefit, the results are the same as when fly parasitism was included: the fitness
of longer duration calling strategies increased in the fall but not the spring (Fig. 5-
2). In the fall, the calling strategy call 4 h had the highest fitness, and the


72
strategy call 1 h had the lowest fitness (Fig. 5-2). In the spring, all calling
strategies had the same fitness (Fig. 5-2). In the model with variable fly
parasitism and constant mating benefit (Fig. 5-3), however, the results are not
the same as when mating benefit varied: the fitness of calling strategies call 1 h
and call 2 h were higher than the fitness of strategies call 3 h and call 4 h in
both the fall and the spring
Relative Importance of Natural versus Sexual Selection
Reducing fly parasitism and holding it constant did not affect the general
pattern of the original model: the fitness of longer calling duration strategies
increased with calling duration in the fall but not the spring in both models. This
finding suggests that sexual selection may be more important than natural
selection in favoring seasonal patterns in male calling duration. Male G. rubens
calling durations may have been higher in the fall than in the spring because long
duration calls were more effective at attracting females in the fall than in the
spring. This implication is significant because it suggests that variation in sexual
selection may be more important than variation in natural selection in shaping the
evolution of male traits.
Even though sexual selection may be more important than natural
selection, nonetheless sexual selection and natural selection clearly interact to
shape seasonal patterns in male G. rubens calling duration. In the model with
variable fly parasitism and constant mating benefit, short calling strategies had


73
higher fitness than long calling strategies. This suggests that natural selection
can modulate the effects of sexual selection on male calling duration.
Implications of the Model
The conclusions of this model enhance our understanding of how natural
and sexual selection may interact to favor variation in sexually selected male
traits. In G. rubens, male song attracts both females and parasitoid flies. Signals
that attract both mates and predators or parasites are common in species where
males have sexually selected traits (for review see Burk 1982). The outcomes of
my model show that in systems like this one, sexual selection may be more
important than natural selection in favoring variation in male traits, but that
natural selection can modify the effects of sexual selection.
Seasonal variation in calling duration has been observed in other Gryllus
species. Male G. texensis sing to attract females for mating and their calling song
also attracts O. ochracea (Cade 1975). The risk of fly parasitism in male G.
texensis may increase with calling duration because the number of O. ochracea
flies attracted to broadcasts of G. texensis song increases with calling duration
(Cade et al. 1996). Because O. ochracea is more abundant in the fall than in the
spring, the risk of fly parasitism should be higher in the fall than in the spring.
Cade (1989) studied female G. texensis attracted to male calling song in late
summer and early fall. He found that the number of females attracted to male
song increased with calling duration. Unfortunately, Cade (1989) did not examine
female G. texensis attraction to male song in the spring. However, if female G.


74
texensis attraction to male song does not increase with calling duration in the
spring, as in the closely related species G. rubens, then G. texensis would be
similar to G. rubens in the seasonal changes in mating benefit and probability of
escaping fly parasitism. Accordingly, my model would predict that male G.
texensis should spend more time calling in the fall than in the spring. Bertram
(2002) found evidence for this prediction, although she had a different
explanation for the cause. This finding supports the general conclusions of this
model in explaining seasonal variation in male G. rubens calling duration.
This model is valuable because its formulation is general enough to
explain variation in any sexually selected male trait that attracts both females and
predators or parasites. Such tradeoffs in sexually selected traits are well known
(Burk 1982; Godin & Briggs 1996). For example, this model could be used to
evaluate geographical variation in male color patterns of guppies (Poecilia
reticulata). Male guppies show extensive geographical variation in their
conspicuous color patterns. Their bright colors are preferred by females (Kodric-
Brown 1985, Houde 1987, Long & Houde 1989) but at the same time make the
males more vulnerable to predation (Endler 1987). This variation has been
explained by a balance between female preference for conspicuous males and
natural selection against conspicuous males (Endler 1983). However, studies
over the last two decades have shown that female preferences also vary with
predation levels (Breden & Stoner 1987, Stoner & Breden 1988, Godin & Briggs
1996; Brooks & Endler 2001). Female preference for conspicuous males
disappears when predation is high. This means that the geographical variation in


75
male guppy color patterns may be due to differences among populations in
sexual selection as well as natural selection (Dill et al. 1999).
My model could help examine the relative importance of natural versus
sexual selection in favoring geographical patterns of male color patterns in
species like guppies. The strategies of the model could be different areas of
conspicuous body colors or different levels of bright coloration. The mating
benefit of the different color areas could be estimated from studies examining
female preference for male color patterns in low and high predation areas.
Similarly, the probability of escaping predation in geographic areas with different
color strategies could be estimated from studies examining the survival of males
under low and high predation levels. Once these parameters are calculated, they
could be manipulated using my model to assess the relative importance of
natural versus sexual selection in favoring geographical variation in male guppy
color patterns. If the gain in fitness through sexual selection (female preference)
shows an accelerating effect whereas natural selection (risk of predation) is
linear with color brightness, then the results will be similar to my model with
Gryllus: the effect of sexual selection has a stronger effect on driving variation in
male color variation than natural selection. This application shows how my
model could be used to examine variation in any sexually selected male trait that
attracts both females and predators.


Table 5-1. Model Variables: Mating benefit and probability of escaping fly parasitism of different
male Gryllus rubens calling strategies in the fall and spring.
Male Calling Strategy
Mating Benefit (mean #
females attracted/ night)
Probability of Escaping Fly
Parasitism/ night
Fall
Spring
Fall
Spring
Call 1 h
0.040
0.070
0.500
0.995
Call 2h
0.120
0.070
0.500
0.995
Call 3h
0.160
0.070
0.350
0.697
Call 4h
0.320
0.070
0.350
0.697


Lifetime fitness
call 1 h call 2 h call 3 h call 4 h
fall
spring
Male calling strategy
Fig. 5-1. Model Results: Seasonal changes in the lifetime fitness of different male Gryllus rubens
calling strategies.


Table 5-2. Model variables with reduced and constant fly parasitism and variable mating benefit.
The table shows the mating benefit and probability of escaping fly parasitism of different male
Gryllus rubens calling strategies in the fall and spring.
Male Calling Strategy
Mating Benefit (mean #
females attracted/ night)
Probability of Escaping Fly
Parasitism/ night
Fall
Spring
Fall
Spring
Call 1 h
0.040
0.070
0.999
0.999
Call 2h
0.120
0.070
0.999
0.999
Call 3h
0.160
0.070
0.999
0.999
Call 4h
0.320
0.070
0.999
0.999


Table 5-3. Mating benefit and probability of escaping fly parasitism of different male Gryllus rubens
calling strategies in the fall and spring for model with variable fly parasitism and constant mating
benefit.
Male Calling Strategy
Mating Benefit (mean #
females attracted/ night)
Probability of Escaping Fly
Parasitism/ night
Fall
Spring
Fall
Spring
Call 1 h
0.120
0.120
0.500
0.995
Call 2h
0.120
0.120
0.500
0.995
Call 3h
0.120
0.120
0.350
0.697
Call 4h
0.120
0.120
0.350
0.697


Lifetime fitness
0.8
0.6
call 1 h call 2 h call 3 h call 4 h
Male calling strategy
fall
spring
Fig. 5-2. Seasonal changes in the lifetime fitness of different male Gryllus rubens calling strategies
in model with reduced and constant fly parasitism, and variable mating benefit.


Lifetime fitness
0.8
0.6
0.4
call 1 h call 2 h call 3 h call 4 h
fall
spring
Male calling strategy
Fig. 5-3. Model Results with variable fly parasitism risk and constant mating benefit. The figure shows
seasonal changes in the lifetime fitness of different male Gryllus rubens calling strategies. "


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BIOGRAPHICAL SKETCH
Manuel J. Vlez was born in San Juan, Puerto Rico, on February 4,1974.
He is the son of Manuel J. Vlez Jr., a college professor of invertebrate zoology
at the University of Puerto Rico, and Alsacia Bosch de Vlez, a social worker and
housewife. He grew up in San Juan, where he attended elementary school at
Colegio Nuestra Seora de la Providencia, and middle and high school at
Colegio San Ignacio de Loyola. After graduating from high school, he left San
Juan and attended Cornell University, where he earned a B.A. in biology with a
concentration in neurobiology and behavior. While at Cornell University, he
conducted behavioral research both in the field and lab in a wide range of taxa
including fish, birds, and mammals. After graduating from Cornell University, he
attended the University of Florida, where he earned an M.S. degree in zoology.
His masters thesis explored parental care strategies in the cichlid fish Aequidens
coeruleopunctatus. After he obtains his Ph.D. in zoology, he will attend Boston
University School of Law, where he will study intellectual property.
89


I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
H. Jne Brockmann, Chair
Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
Colette M. St. Mary t"
Associate Professor of Zoology '
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
Benja
Assist
Bolker
t Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Rhitosopftyx
Marta L. Wayne
Assistant Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of philosophy.
Thomas J. W^iker
Professor Emeritus of Entomology and
Nematology
This dissertation was submitted to the Graduate Faculty of the Department
of Zoology in the College of Liberal Arts and Sciences and to the Graduate
School and was accepted as partial fulfillment of the requirements for the degree
of Doctor of Philosophy.


This thesis was submitted to the Graduate Faculty of the Department of
Zoology in the College of Liberal Arts and Sciences and to the Graduate School
and was accepted as partial fulfillment of the requirements for the degree of
Doctor of Philosophy.
August 2004
Dean, Graduate School


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TEMPORAL VARIATION IN NATURAL AND SEXUAL SELECTION OF MALE
CALLING BEHAVIOR IN THE FIELD CRICKET GRYLLUS RUBENS
By
MANUEL J. VELEZ
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2004

ACKNOWLEDGMENTS
I am indebted to my advisor, H. Jane Brockmann, for her invaluable support
in the design and analysis of this research project. I am also thankful to Colette
M. St. Mary, Benjamin Bolker, Marta L. Wayne, and Thomas J. Walker for their
helpful feedback on my research. My deepest gratitude goes to Laura Sirot,
Suhel Quader, Kavita Isravan, Rebecca Hale, and other colleagues at the
Department of Zoology for their support in the development and analysis of this
project. I am also thankful to George Casella for giving me statistical advice.
My deepest gratitude goes to the University of Florida Beef Teaching Unit
and Environmental Landscape Horticulture Education Lab for allowing me to
conduct field research in their pastures.
This work was funded by an NSF Graduate Predoctoral fellowship. My PhD
committee consisted of Drs. H. Jane Brockmann, Colette M. St. Mary, Benjamin
Bolker, Marta L. Wayne, and Thomas J. Walker.
n

TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ü
ABSTRACT v
CHAPTERS
1 GENERAL INTRODUCTION 1
2 VARIABLE SELECTION FOR MALE SONG IN THE
FIELD CRICKET GRYLLUS RUBENS 5
Introduction 5
Methods 9
Study Species 9
Fly Abundance and Activity 10
Field Recordings of Individual Males 10
Night Surveys 12
Results 13
Fly Abundance and Activity 13
Field Recordings 14
Night Surveys 14
Discussion 15
3 SEASONAL VARIATION IN FEMALE PHONOTAXIS
TO MALE CALLING SONG OF THE FIELD CRICKET
GRYLLUS RUBENS 26
Introduction 26
Methods 29
Phonotactic Experiment 29
Source of Male Calls 32
Fly Activity 33
Results 33
Phonotactic Experiment 33
Fly Activity 34
Discussion 35
in

4 VARIATION IN SEXUAL SELECTION FOR MALE
SONG IN THE FIELD CRICKET
GRYLLUS RUBENS 46
Introduction 46
Methods 50
Pitfall Trap Experiment 50
Source of Male Calls 53
Data Analysis 53
Results 54
Discussion 55
5 GENERAL MODEL: A TEST OF THE FACTORS
AFFECTING MALE CALLING DURATION IN THE FIELD
CRICKET GRYLLUS RUBENS 63
Factors Affecting Male Calling Duration 63
Model: Predicting Seasonal Patterns in Male Calling Duration 64
Male Calling Strategies 64
Calculating Fitness of Male Calling Strategies 65
Seasonal Changes in Mating Benefit (M) 66
Seasonal Changes in Probability of Escaping Fly
Parasitism (E) 67
Model Results: Seasonal Changes in Fitness of
Male Calling Strategies 69
Comparison of Optimal Seasonal Strategies with
Observed Calling Durations 69
Variations of the Model 71
Results of Model Variations 71
Relative Importance of Natural versus Sexual Selection 72
Implications of the Model 73
LIST OF REFERENCES 82
BIOGRAPHICAL SKETCH 89
IV

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
TEMPORAL VARIATION IN NATURAL AND SEXUAL SELECTION OF MALE
CALLING BEHAVIOR IN THE FIELD CRICKET GRYLLUS RUBENS
By
Manuel J. Vélez
August 2004
Chair: H. Jane Brockmann
Major Department: Zoology
A primary goal of sexual selection theory is to understand how natural and
sexual selection interact to favor variation in sexually-selected traits. The goal of
this dissertation is to examine variation in male calling behavior of the field cricket
Gryllus rubens and how temporal changes in natural and sexual selection may
favor this variation. The results of this dissertation are important because they
show that temporal variation in natural and sexual selection may favor variation
in male sexually-selected traits.
Male calling song in G. rubens attracts females. It also attracts gravid
females of the parasitoid fly Ormia ochracea. These female flies deposit larvae
on and around calling males, killing infected crickets within 7-10 days. In northern
Florida, O. ochracea is abundant in the fall and rare in the spring, and fly
parasitism rates are higher in the fall than in the spring. This temporal variation in
natural selection could favor different male songs in the fall than in the spring. I
v

evaluated this possibility by examining seasonal variation in male calling
behavior. I found that fewer males sing each night in the fall than in the spring
and that fewer males sing at times when parasitoid flies are most active.
Because seasonal changes in parasitoid fly abundance may change the
costs and benefit of choice, fall and spring females may behave differently
towards calling males. I evaluated this hypothesis by observing the phonotactic
behavior of fall and spring females. I found that fall females are less attracted to
male songs than spring females at times when flies are most active.
Seasonal changes in sexual selection may change the seasonal benefits
males derive from singing. I explored this hypothesis by examining whether the
benefits of singing change seasonally and whether such changes are correlated
with seasonal differences in male calling duration. The results of a playback
experiment show that long duration calls attract more females in the fall, where
call durations are long, than in the spring, where call durations are short.
Temporal changes in natural and sexual selection may favor seasonal
variation in male G. rubens calling behavior. To evaluate how such fluctuating
selection may affect seasonal patterns in calling duration, I developed a model
evaluating the fitness of different calling strategies in the fall and spring given
seasonal changes in natural and sexual selection. By varying these selective
forces independently and comparing the fitness of calling strategies to observed
calling durations, I found that temporal changes in sexual selection may have a
greater effect on calling duration than changes in natural selection.
vi

CHAPTER 1
GENERAL INTRODUCTION
A primary goal of sexual selection theory is to understand how natural and
sexual selection interact to favor variation in male sexually-selected traits
(Andersson 1994). Crickets are ideal subjects for the study of sexually selected
traits for several reasons. First, males produce calling songs to attract females
for mating, and females select mates based on the quality of their songs (for
review see Loher & Dambach 1989). This means that male song is likely to be
influenced by sexual selection. Second, many song characters are highly
variable among males (Cade & Wyatt 1984, Cade 1991, Walker 1998, 2000).
Third, song characters can be manipulated independently from male phenotype
by tape recording male calling songs and editing the recordings. This provides a
means of isolating different song characters and testing for their separate effects
on female choice (Hedrick 1986). Fourth, females choose males overtly: they
approach males and mount them. This female behavior offers the opportunity to
separate the effects of female choice from male-male competition. Additionally,
because females approach speakers broadcasting male songs in the same way
they would approach calling males, female choice can be studied without the
problem of males adjusting their song in response to females, and females
modifying their preference in response to changes in male song. Collectively,
these characteristics make crickets model subjects for studies on sexual
1

2
selection (Cade 1979, Crankshaw 1979, Boake 1984, Hedrick 1986 & 1988,
Simmons 1988).
The goal of this dissertation is to examine variation in male calling
behavior of the field cricket Gryllus rubens and how temporal changes in natural
and sexual selection may favor this variation. The results of this dissertation are
important because they show that temporal variation in natural and sexual
selection may favor variation in male sexually-selected traits.
The field cricket G. rubens is well suited for studying the effects of natural
selection on calling song traits. In addition to attracting females, the calling song
of male G. rubens also attracts gravid females of the parasitoid fly Ormia
ochracea (Walker 1986,1993). These female flies deposit larvae on and around
calling males, killing infected crickets within 7-10 days. In northern Florida, O.
ochracea is abundant in the fall and rare in the spring (Walker 1986), and fly
parasitism rates are higher in the fall than in the spring (Walker & Wineriter
1991). This seasonal variation in the costs of calling suggests that natural
selection on male calling song varies temporally. This temporal variation in
natural selection can result in different calling songs in different seasons. I
examine this hypothesis in the second chapter of this dissertation by exploring
whether male G. rubens exhibit seasonal variation in calling behavior and
whether this variation is associated with seasonal changes in parasitoid fly
abundance.
Gryllus rubens is also well suited for studying the effects of temporal
variation in sexual selection. Even though females do not sing, Walker and

3
Wineriter (1991) have shown that females can also be parasitized. If females are
infected as they approach calling males, then the costs of mate choice may be
higher in the fall when flies are abundant than in the spring when flies are rare.
These seasonal changes in the costs of mate choice may cause females to be
less attracted to male calling song in the fall than in the spring to reduce the risk
of fly parasitism. This temporal variation in female behavior may alter the
strength of sexual selection seasonally. I evaluate this hypothesis in the third
chapter of this dissertation by examining female attraction to male calling song at
different times of the year.
Seasonal variation in sexual selection may alter the seasonal benefits that
males derive from singing. As a result, such variation may favor seasonal
changes in male calling behavior. I evaluate this hypothesis in the fourth chapter
of this dissertation by examining whether the benefits of singing change
seasonally and whether such changes are correlated with seasonal differences in
male calling duration.
I conclude this dissertation with a discussion of how temporal changes in
natural and sexual selection should affect seasonal variation in male G. rubens
calling duration. I test these effects by calculating the fitness of different calling
strategies in the fall and spring given seasonal changes in natural and sexual
selection. By varying natural and sexual selection independently, I estimate the
relative importance of natural versus sexual selection in favoring variation in male
calling duration.

4
By studying male calling behavior and the benefits that males derive from
singing when flies are rare and abundant, this dissertation examines how
temporal variation in natural and sexual selection may interact to favor variation
in male sexually-selected traits.

CHAPTER 2
VARIABLE SELECTION FOR MALE SONG IN THE
FIELD CRICKET GRYLLUS RUBENS
Introduction
Conspicuous male traits such as bright colors, large size, or elaborate
behavior evolve through the joint action of sexual and natural selection
(Andersson 1994). Sexual selection favors males with conspicuous traits since
these traits are advantageous in mate acquisition through either female choice or
male-male competition (Darwin 1871). Natural selection, on the other hand, acts
against males with conspicuous traits if these traits lower male survival.
Conspicuous traits may lower survival by being energetically costly or by making
males more vulnerable to predators. Therefore, conspicuous male traits evolve
depending on the relative strength of sexual and natural selection. This
hypothesis predicts that when the costs of natural selection increase relative to
the benefits of sexual selection, then males should become less conspicuous. In
this study, I evaluate this prediction by examining the male calling behavior of the
field cricket Gryllus rubens in two seasons that differ in the risk of parasitism from
acoustically orienting flies.
Male crickets produce calling songs by rubbing their raised forewings
together, which attracts conspecific females for mating (Loher & Dambach 1989).
In some species, the same calling song also attracts parasitoid flies which
deposit larvae on and around calling males, killing infected crickets within 7-10
5

6
days (Cade 1975, Walker 1986,1993). This fly parasitism risk has been used to
explain variation between species in the nightly calling duration of male field
crickets (Orthoptera; Gryllidae). Cade and Wyatt (1984) found that male G.
texensis, which is parasitized, spend less time calling per night than male G.
veletis, G. pennsylvanicus, and Teleogryllus africanus, which are not parasitized
by phonotactic flies. They argue that fly parasitism has selected against long
nightly calling in species that are parasitized. They also found that the distribution
of male G. texensis nightly calling duration is highly skewed because some
males do not call. It is thought that males that do not call or males that sing less
search for females so there is some expectation of success for these males,
particularly when female densities are high. Cade and Wyatt (1984) attribute the
skew distribution of nightly calling duration in G. texensis to the higher rates of
parasitism in this species.
However, species differences in time spent calling per night may be due to
species differences in the energetic costs of singing. The song of G. texensis is
made up of sound pulses organized into trills. In contrast, the songs of G. veletis
and G. pennsylvanicus are composed of sound pulses grouped into chirps with
quiet periods between them. Because trilling songs may be more energetically
costly than chirping songs (Prestwich & Walker 1981, Prestwich 1994),
differences in time spent calling per night between the trilling G. texensis and the
chirping G. veletis and G. pennsylvanicus may be explained by energetic
differences in song production. Collectively, these findings cannot separate the

7
effects of fly parasitism risk and energetic costs of calling on nightly calling
duration in male crickets.
Fly parasitism risk has also been used to explain variation in the timing of
male calling in crickets. French and Cade (1987) found that male G. texensis call
more around dawn than in the early evening. They hypothesized that dawn
calling is common in G. texensis because parasitoid flies are less active at dawn.
However, G. veletis and G. pennsylvanicus also call more around dawn (French
& Cade 1987), and they are not infected by parasitoid flies. Furthermore, in all
three species, male calling around dawn coincided with increased female
movement and mating activities (French & Cade 1987). Therefore, it is not clear
whether male G. texensis call more around dawn because flies are less active at
that time or because females are more sexually receptive around dawn.
Testing the effect of fly parasitism risk on nightly calling duration and
timing of calling in male crickets under laboratory conditions is difficult because it
is possible that crickets do not respond directly to flies but rather to
environmental cues associated with fly activity. If so, then examining male calling
behavior in an experimental arena with and without flies will not reveal the effect
of parasitoid flies on behavior. One solution to this problem is to study male
calling behavior in populations with and without parasitoid flies. This is the
approach that was used by Zuk et al. (1993) and Simmons et al. (2001).
However, any correlation between male calling behavior and fly abundance may
also be explained by environmental or other differences between the two
populations (for example, populations without flies may be at higher latitudes

8
than those with flies). This limitation of working with geographically distinct
populations is overcome when studying a population that is exposed to seasonal
changes in fly abundance. This is the case for the field cricket G. rubens. The
calling song of this species attracts gravid females of the parasitoid fly Ormia
ochracea (Walker 1986, 1993). In northern Florida, 0. ochracea is abundant in
the fall and rare in the spring (Walker 1986), and fly parasitism rates are higher in
the fall than in the spring (10% versus 0%; Walker & Wineriter 1991). These
seasonal differences in fly abundance and rates of parasitism suggest that the
costs of calling are higher in the fall than in the spring. This seasonal variation in
natural selection serves as a natural experiment to examine the effect of fly
parasitism risk on nightly calling duration, number of males calling and timing of
calling in male crickets.
I hypothesize that seasonal changes in natural selection favor seasonal
changes in male calling behavior in G. rubens. Accordingly, I made the following
predictions. First, I predicted that fewer males would call in the fall than in the
spring because some males do not sing (personal observation) and
experimentally muted males have lower rates of fly parasitism than non-muted
males under high fly densities (Walker & Wineriter 1991). Second, I expected
that males should spend less time calling per night in the fall than in the spring
because the number of parasitoid flies attracted to broadcasts of cricket songs
increases with broadcast duration (Cade et al. 1996). Third, I expected that fewer
males would call in the early evening in the fall than in the spring because
parasitoid fly activity is highest in the early evening (Cade et al. 1996). I tested

9
these predictions by recording the calling duration of individual males under field
conditions and conducting nightly surveys of singing males.
Finding that variation in male calling behavior in G. rubens is correlated
with seasonal changes in natural selection from the risk of fly parasitism would
be important because it would help us understand how males deal with the
conflicting demands of attracting females while avoiding parasitism. Because
males of many animal species have mating signals that attract both females and
predators and/or parasites (reviewed by Burk 1982 and Magnhagen 1991), the
results of this study may have important implications for many systems in which
males or females attract mates with conspicuous displays.
Methods
Study Species
The field cricket Gryllus rubens is found in lawns, pastures, and along
roadsides in southeastern United States (Alexander 1957). Unlike most other
species of field crickets, male G. rubens produce a trilling song in which sound
pulses are repeated and grouped into long pulse trains (Doherty & Callos 1991).
This trilling song attracts females who approach and mount males. It also attracts
gravid females of the parasitoid fly Ormia ochracea (Walker 1986, 1993).

10
Fly Abundance and Activity
To confirm that 0. ochracea flies are more abundant in the fall than in the
spring and that flies are most active in the early evening, I monitored nocturnal
changes in fly activity in a pasture at the University of Florida Beef Teaching Unit,
Alachua Co., FL that was 1-km from the transects used in the night surveys. To
monitor fly activity, I used a slit trap (Walker 1989) with an electronic sound
synthesizer that broadcasts calls of G. rubens (4763 Hz carrier frequency, 50.0
pulses per second, 50% duty cycle; continuous trill at 106 dB, 15 cm above the
speaker). I activated the synthesizer an hour before sunset and counted the
number of flies captured at the trap 3,7,11, and 13 h after sunset. I carried out
this procedure in the fall from 20 September to 17 November 2000 (N=8) and
spring from 3 April to 23 May 2001 (N=6).
Field Recordings of Individual Males
To study seasonal changes in the number of calling males and time spent
calling by males per night, I recorded the calling duration of individual males
under field conditions. For these recordings, I collected adult males from a field at
the University of Florida Environmental Landscape Horticulture Education Lab,
Alachua Co., FL, using a sound trap with an electronic sound synthesizer
(Walker 1986). Males were weighed to the nearest 0.001 g and placed
individually in square plastic containers measuring 13 x 13 x 9 cm (I x w x h) with
wire screen lids for 10 days to ascertain that they were not parasitized by O.
ochracea. Each container had an inch of sand, an inverted egg carton shelter,

11
and Purina Cat Chow and water ad lib. All crickets were held under ambient light
and temperature in a greenhouse less than 1 km from the collection site. After 10
days, I transferred the containers to the field and laid them out in a straight line
keeping a distance of 15 m between containers. This distance was chosen so
that males could not hear the singing of other males being recorded. Each plastic
container was set in a square aluminum pan measuring 20 x 20 x 4 cm (I x w x h)
with an inch of water to prevent fire ants from invading the containers. Next to
each container, I placed an Optimus model CTR-115 voice-activated cassette
tape recorder. I switched on all tape recorders an hour before sunset and allowed
them to run for 16 h. All tape recorders were checked hourly to replace any tapes
that were full. Two to four males were recorded each night. I electronically
monitored 20 males over nine nights in the spring (April/May 2001) and 23 males
over nine nights in the fall (September/October 2001). Each male was monitored
only once. Fall and spring field recordings did not differ in the minimum
temperature recorded each night (Meanfaii ± SE = 15.44±1.35 °C, Meanspnng ± SE
= 15.59±1.64 °C, P=0.943). There was also no seasonal difference in the
maximum temperature recorded each night (Meanfaii ± SE = 30.06±0.86 °C,
Meanspnng ± SE = 28.16±1.37 °C, P=0.260).
I calculated the nightly calling duration for each male by listening to the
tapes with a stopwatch. If the male stopped calling for more than 2 sec, then the
stopwatch was stopped until the male resumed calling. The nightly calling
duration was the total time spent calling during the 16 h that males were
monitored.

12
Night Surveys
I studied seasonal changes in the number of calling males and the timing
of calling by conducting night surveys of singing males in a pasture used to graze
cows at the University of Florida Beef Teaching Unit. This pasture was divided
into 10 130 x 20 (I x w) m parallel transects. A night survey consisted of a 16 h
sampling period beginning an hour before sunset in which I walked in a straight
line through the center of each of five transects randomly selected at 4 h intervals
counting the number of calling males. At the start of each 4 h interval, I recorded
the temperature in each transect 2 cm above the soil. On a given night, the same
five transects were surveyed four times during the 16 h survey period. Each
transect was surveyed two to three nights each season. Fall surveys were
conducted from 23 September to 17 November 2000 and spring surveys from 3
April to 23 May 2001 because these are the dates when flies were shown to be
abundant and rare, respectively (Walker 1986).
To test the prediction that fewer males will call in the early evening in the
fall than in the spring, I performed a Repeated Measures ANOVA on the number
of calling males in the night surveys. This variable was transformed to achieve
normality by adding 0.375 to the variable and taking the square root of this sum
(Ott 1993). This transformation achieved normality and validated the sphericity
assumption (variance-covariance matrix of dependent variable is circular in form)
of the Repeated Measures ANOVA. The terms season (fall and spring) and time
(4 h interval) were used as factors. The average temperature across the four time
intervals in each transect was used as a covariate. Only one night survey per

13
transect was used in this analysis. For each transect, I chose the first night
survey where the average temperature recorded in the four time intervals fell
between 15 and 25 °C. This restriction was used to limit the effect of extreme
temperatures on male calling behavior. For this analysis, I assumed that
transects were independent between seasons because the lifespan of males is
likely to be shorter than the time elapsed between the fall and spring night
surveys.
Data were analyzed using SYSTAT 6.0 and SPSS 11.5. Results are
presented as means ± standard error.
Results
Fly Abundance and Activity
Ormia ochracea flies were much more common in the fall than the spring.
In the fall, a mean (±SE) of 17.6 ± 6.2 flies/ night were captured at the slit trap
(N=8). In contrast, no flies were ever captured in the spring (N=6). In the fall, the
number of 0. ochracea flies captured was highest in the first few hours after
sunset (Fig. 2-1). Flight activity then steadily declined throughout the remainder
of the night but showed a small increase around dawn (i.e., 13 hours after
sunset).

14
Field Recordings
A higher proportion of males sang in the spring, 15/20 (75%), than in the
fall, 10/23 (43%) (Fisher’s exact test: P=0.030). Overall, fall males did not differ in
nightly calling duration (44.5 min ±13.8) from spring males (31.8 min ±9.1) (t-test,
Nfaii=23, Nspring=20, t=0.765, P=0.449). However, when males that did not sing at
any time during the 16 h test period were excluded from this analysis, then fall
males that sang spent more time calling (102.3 min ± 20.3) than spring males
that sang (42.4 min ± 10.9) (Fig. 2-2; t-test, Nfaii=10, Nspring=15, t=2.60, P=0.020).
Although fall males were heavier than spring males (Fig. 2-3; t-test, Nfan=23,
Nspring=20, t=3.693, P=0.001), male mass did not have a significant effect on
nightly calling duration for males that sang in either the fall (Fig. 2-4; N=10,
R2=0.090, P=0.399) or the spring (Fig. 2-4; N=15, R2=0.002, P=0.885).
Night Surveys
More males were observed singing in the spring than in the fall across all
sampling periods in all surveys (Fig. 2-5; Repeated Measures ANOVA, season,
F1i18=8.769, P=0.008). Time of day had a significant effect on the number of
calling males (Fig. 2-6; Repeated Measures ANOVA, 4 h interval, F3,54=7.319,
P<0.001). More males sang in the first few hours after sunset in the spring than
in the fall (Fig. 2-6; Repeated Measures ANOVA, time*season, F3i54=3.776,
P=0.016). These results came from a model that excluded temperature as a
covariate. Temperature was excluded because a preliminary model with season

15
and time as factors and temperature as a covariate showed that temperature did
not have a significant effect on the number of calling males (Repeated Measures
ANOVA, temperature, Fi,i7=0.058, P=0.812).
Discussion
This study has shown that seasonal changes in male G. rubens calling
behavior are correlated with seasonal changes in the risk of fly parasitism. The
probability of encountering parasitoid flies on a given night is higher in the fall
than in the spring. In the fall, this probability is highest in the early evening and
then declines steadily throughout the night. This suggests that the natural
selection costs of calling per night are higher in the fall than in the spring, and
that these costs decrease within a night in the fall. As predicted by these
seasonal changes in the risk of fly parasitism, a lower proportion of males sang in
the fall than in the spring and fewer males sang in the early evening in the fall
than in the spring. These findings are consistent with the hypothesis that natural
selection affects the evolution of sexually selected male traits. When the natural
selection costs of conspicuous traits increase relative to the benefits, then males
behave less conspicuously.
However, other factors besides the risk of fly parasitism change
seasonally and may explain the seasonal variation in the number of calling
males. These factors include differences in density, differences in the proportion
of parasitized males, and differences in male size between seasons. Differences
between seasons in female behavior will be discussed in chapter 3. Fewer

16
males may have sung in the fall than in the spring because fewer males were
present. Several lines of evidence argue against this possibility. First, Veazey et
al. (1976) found that 6. rubens densities in northern Florida are higher in the fall
than in the spring. Second, my field recordings of individual males showed that a
smaller proportion of males sang in the fall than in the spring. Alternatively, fewer
males may have sung in the fall than in the spring because a higher proportion of
males were parasitized. Parasites have been shown to decrease the ability of
males to sing (Cade 1984, Kolluru et al. 2002). However, my field recordings
show that this cannot entirely explain the pattern because healthy fall and spring
males differed in calling behavior. Finally, fewer males may have sung in the fall
than in the spring because spring males were larger than fall males. This is
possible because male size has been shown to be correlated with calling
duration in sagebrush and ground crickets (Sakaluk & Snedden 1990, Forrest et
al. 1991). However, this cannot be an explanation for the seasonal differences in
the number of calling males because fall males were heavier than spring males.
The combined evidence suggests that fewer males sang in the fall than in the
spring because the risk of fly parasitism is higher in the fall than in the spring.
More than half of the males that were recorded in the fall did not sing over
the 16 h period they were monitored. If females approach calling males, how do
these silent males get mated? It is likely that these males walk or fly around
searching for females. This mating strategy has been described in several
species of field crickets (French & Cade 1989, Hissmann 1990, Zuk et al. 1993,
Hack 1998). Cade (1979) proposed that searching may be an alternative mating

17
tactic. He hypothesized that males using this tactic may have lower mating
success on any given day than calling males. However, because searching
males may also have lower rates of fly parasitism, they may have higher survival
and longer lifespans than calling males. This type of alternative mating tactic has
been described for guppies. Males usually display their bright color patterns to
females by performing sigmoid displays in which males quiver their bodies in an
S-shape (Baerends et al. 1955). Some males, however, do not perform these
displays and instead attempt a sneak copulation by performing a gonopodial
thrust. This mating tactic may be favored by natural selection because it is less
conspicuous to predators than sigmoid displays (Endler 1987). This hypothesis
has been supported by studies showing that males switch from sigmoid displays
to gonopodial thrusts when the risk of predation is high (Endler 1987, Magurran &
Seghers 1990, Reynolds et al. 1993, Godin 1995). Taken together, studies on
crickets and guppies suggest that when the natural selection costs of
conspicuous male traits increase, males may use alternative mating tactics.
Males using these tactics are favored because even though they may have
reduced mating success, they may have higher survival when compared to
males using the more conspicuous tactic. Additionally, the success of searching
males may increase as density increases.
As predicted, I found that more male G. rubens sang in the early evening
in the spring than in the fall when flies were more active. French and Cade
(1987) studied the timing of male calling in G. texensis, G. veletis, and G.
pennsylvanicus. Their results showed that male calling increased at dawn in field

18
populations of all three species. Male G. rubens in this study did not increase
calling around dawn. One possible explanation for this finding is that the small
surge of parasitoid flies around dawn at my study site may have increased the
natural selection costs of calling at those times. Consistent with this interpretation
is the fact that the populations of G. texensis that French and Cade (1987)
studied did not experience a surge in fly activity around dawn, and the
populations of G. veletis and G. pennsylvanicus were not parasitized by O.
ochracea (Cade & Wyatt 1984). Collectively, these studies suggest that the
timing of male calling in crickets has responded to natural selection from
parasitoid flies.
I predicted that males should spend less time calling per night in the fall
than in the spring because the number of parasitoid flies attracted to broadcasts
of cricket songs increased with broadcast duration (this study, Cade et al. 1996).
The field recordings of individual males found the opposite result: fall males that
sang spent more time calling per night than spring males that sang. Bertram
(2002) also found the same seasonal pattern in male G. texensis. One factor that
may explain these results is density. As density increases, males are expected to
call less and search more because the gains from searching increase (Alexander
1961 & 1975, Otte 1977, Cade 1979). This effect has been demonstrated in
experimental studies by showing that male calling decreases with density (Cade
& Wyatt 1984, Cade & Cade 1992). However, seasonal differences in density
cannot explain the seasonal differences in nightly calling duration from this study

19
nor the Bertram (2002) study because cricket densities at both sites were higher
in the fall than in the spring (Veazey et al. 1976, Bertram 2002).
One possible explanation for the seasonal differences in nightly calling
duration in 6. rubens and 6. texensis is sexual selection. If sexual selection for
nightly calling duration is stronger in the fall than in the spring, then this seasonal
difference in mating benefit may explain why fall males that sang spent more
time calling than spring males that sang. I evaluate this hypothesis in chapter 4.

Hours after sunset
Fig. 2-1. Nocturnal changes in the mean number of Ormia ochracea flies captured at a slit trap
broadcasting Gryllus rubens male calls over eight nights in the fall.

Nightly calling duration (min)
â–  fall (N=23)
â–¡ spring (N=20)
Fig. 2-2. Seasonal changes in the nightly calling duration of male Gryllus rubens individually recorded in
the field during the fall and spring.

Season
Fig. 2-3. Seasonal changes in male Gryllus rubens mass. Males were collected in the fall (N=23)
and spring (N=20) using a sound trap. Each male was weighed to the nearest 0.001 g.

x Spring (N=15)
O Fall (N=10)
Male mass (g)
Fig. 2-4. Effect of mass on nightly calling duration of male Gryllus rubens individually recorded in
the field during the fall and spring.

Season
Fig. 2-5. Seasonal changes in the mean number of calling male Gryllus rubens over the night in
surveys of 10 transects sampled in the fall and spring.

Mean number of calling males/ transect/ night
to
Hours after sunset
Fig. 2-6. Seasonal changes in the timing of male Gryllus rubens calling over the night in surveys of
10 transects sampled in the fall and spring.

CHAPTER 3
SEASONAL VARIATION IN FEMALE PHONOTAXIS TO MALE CALLING SONG
OF THE FIELD CRICKET GRYLLUS RUBENS
Introduction
Males with sexually-selected traits are often at higher risk from predators
and parasites (reviewed by Burk 1982 and Magnhagen 1991) than those without.
Understanding how these risks affect female choice is important because
theoretical models on the evolution of female preferences show that choosiness
decreases when the risks of choice increase relative to the benefits (Parker
1983, Kirkpatrick 1987, Pomiankowski 1987, Real 1990, Iwasa etal. 1991). This
is because females that choose conspicuous males may also experience
increased risk of predation or parasitism because of their association with these
showy males. A decline in choosiness will reduce the strength of sexual selection
on male traits, which will cause males to become less extravagant. Therefore,
males with sexually selected traits evolve to be less conspicuous when predation
or parasitism is high not only because less conspicuous males survive better, but
also because the benefits of conspicuousness through female choice are
reduced. Despite the potential importance of these risks to females, few studies
have examined how predation risk affects female choice (although see Dill et al.
1999). Similarly, while many studies have investigated female choice for males
differing in parasitism levels (Borgia & Collis 1989, Hoglund et al. 1992, Houde &
Torio 1992, Fitzgerald et al. 1994, Buchholz 1995, Kraaijeveld et al. 1997, Lopez
26

27
1998), no study has evaluated how parasitism risk affects female choice. In this
study, I hypothesize that risk of parasitism decreases female attraction to males
with sexually-selected traits. I evaluated this hypothesis by examining female
phonotaxis to male calling song in the field cricket Gryllus rubens in two seasons
that differed in the risk of parasitism from acoustically orienting flies.
Female G. rubens approach calling males and mount them for mating.
The same male vocalizations that attract female crickets also attract gravid
females of the parasitoid fly Ormia ochracea. Female flies deposit larvae on and
around calling males, killing infected crickets within 7-10 days (Walker 1986,
1993). Even though females do not sing, Walker and Wineriter (1991) collected
females during the fall of 1990 and found 10% parasitism (3 of 10) and a similar
rate, 9%, among males (1 of 11). It is likely that females acquire parasites by
interacting with calling males that have been attacked by O. ochracea that same
night. Several lines of evidence support this idea. First, Walker and Wineriter
(1991) experimentally muted males and found that unmuted males under high fly
densities were not parasitized by O. ochracea (0 of 10) whereas non-muted
males were (7 of 13). This suggests that calling is associated with fly parasitism.
Because females do not sing and males only sing calling songs to attract females
for mating (Loher & Dambach 1989), females are likely to be parasitized as they
approach calling males. Second, O. ochracea flies deposit larvae around calling
males. Females may become parasitized by walking over these larvae which
adhere to them (Cade 1975, 1979, 1984). These scattered larvae may also
explain how male and female G. firmus are parasitized by O. ochracea (Walker &

28
Wineriter 1991); G. firmus co-occur with G. rubens but their calls do not attract
parasitoid flies (Walker 1986). This apparent link between female searching
behavior and fly parasitism make G. rubens an ideal species to test the
hypothesis that risk decreases female attraction to males with sexually-selected
traits.
Parasitoid flies may decrease female G. rubens attraction to male calling
song to minimize the risks of choice. In northern Florida, 0. ochracea is abundant
in the fall and rare in the spring (Walker 1986, Chapter 2), and fly parasitism
rates are higher in the fall than in the spring (10% versus 0%, Walker & Wineriter
1991; chapter 2). This seasonality in fly abundance and parasitism rates
suggests that fall females may incur higher risks of choice than spring females.
Such costs may be greatest in the early evening because O. ochracea flight
activity is highest just around dusk and then steadily declines (Cade et al. 1996,
Chapter 2). These nocturnal changes in parasitoid fly activity may favor females
that are less phonotactic to male calls in the early evening. This would be
consistent with laboratory experiments on gobiid fish, crickets, and guppies,
which have shown that female choosiness decreases when predation risk
increases (Forsgren 1992, Hedrick & Dill 1993, Godin & Briggs 1996). Because
O. ochracea flies are only active after dusk in the fall and not the spring (Chapter
2), I hypothesized that fall females would be less phonotactic to male calling
song in the early evening than spring females. I evaluated this hypothesis by
conducting a phonotactic experiment on fall and spring females shortly after
dusk. In this experiment, females were given a choice between a silent speaker

29
and a speaker broadcasting male calls. I predicted that fewer females would
choose the broadcasting speaker in the fall than in the spring, and fall females
would spend less time near the broadcasting speaker than spring females. The
potential finding that fall females are less attracted to male calling song in the
early evening than spring females would be important because it would reduce
the strength of sexual selection on males in the fall and help explain why fewer
males sing around dusk in the fall than in the spring (Chapter 2).
Methods
Phonotactic Experiment
In this experiment, I quantified and compared among seasons the
phonotactic behavior of wild-caught females for male calling song at the time
when parasitoid flies are most active. Gryllus rubens exhibits a wing dimorphism:
males and females are either long-winged and capable of flight or short-winged
and flightless. Both morph types were used in this phonotactic experiment in the
proportion that they were available. I used two methods of collecting crickets;
some were collected by turning over objects and debris and others were
captured using sound traps (Walker 1986). I used the two methods because I
collected very few long-winged individuals by turning over objects and debris and
sound traps captured long-winged individuals exclusively. All collections were
made at the University of Florida Organic Garden, Alachua Co., FL.
Females were weighed to the nearest 0.001 g and housed individually in
square plastic containers measuring 13 x 13 x 9 cm (I x w x h) with wire screen

30
lids. Each container had an inch of sand, an inverted egg carton shelter, and
Purina Cat Chow and water ad lib. All containers were held under ambient light
and temperature in a greenhouse less than 1 km from the collection site. After 10
days (the number of days necessary to ensure that the crickets were not
parasitized by O. ochracea), I brought females to the lab and tested their
preference for male calling song. Parasitized females were not used in these
experiments. The actual sample size for the fall was N|0ng-winged=24 and NShort-
winged=16. The Sample size for the spring was N|ong-winged—23 and Nshort-winged=17.
Phonotactic tests were conducted in a rectangular arena (Fig. 3-1)
measuring 1.0 x 0.3 x 0.3 m (I x w x h). The floor of the arena was made of
compressed foam, and the walls consisted of metal 2 mm mesh wire screen . To
prevent the crickets from climbing out of the arena, I taped a 5 cm wide paper
strip along the wire screen walls 15 cm from the floor. On the outside at each end
of the arena, I placed a Radio Shack model 277-1008 mini-audio amplifier
speaker. These speakers were connected to a Sony model D-EJ721 portable
CD-Walkman. Prior to a test, a female was placed in the center of the arena
under an inverted egg carton shelter. I covered this shelter with a plastic cup and
allowed the female to acclimate for 5 min. After that time, I lifted the plastic cup
with a string attached to it and gave the female 5 min to choose between a silent
speaker and a speaker broadcasting a male calling song. Output of the
broadcasting speaker was set at 72dB, 15 cm in front of the speaker, using a
Brüel & Kjaer model 2219 sound level meter. This sound level falls within the

31
natural variation of sound intensities of calling males (M.J. Vélez, unpublished
data).
I observed females in the arena using a 40 watt red light. Each female
was tested only once two to three hours after sunset within the four-hour period
when O. ochracea flies are most active (Chapter 2). For each test, I recorded the
latency to leave the shelter, which end screen the female contacted first (criterion
for female choice), and the time spent within 10 cm of the broadcasting speaker.
The floor of the arena was lined with paper. Between tests, this paper was
replaced, the metal wire screen walls were wiped with paper towels, and the
position of the broadcasting speaker was randomized. No end biases were
detected. I examined female phonotactic behavior in the laboratory from 12 June
to 5 July (N=40) and 10 October to 6 December 2001 (N=40) because these are
the times of year when flies are rare and abundant, respectively (Walker 1986).
The temperature in these tests did not differ between the two times of the year
(Meanfan ± SE = 22.58±0.06 °C, Meanspring ± SE = 22.66±0.05 °C, P=0.463).
This phonotactic experiment was conducted in the absence of O.
ochracea flies because I assumed that females do not detect flies directly. I
based this assumption on the finding that male crickets in the presence of flies do
not sing differently than solitary males (S.M. Bertram, unpublished data).
I analyzed the time that females spent within 10 cm of the broadcasting
speaker using a general linear model with model: season (fall, spring), collection
method (turning over objects and debris, sound traps), morph (long-winged,
short-winged), mass, season*collection method, season*morph, season*mass.

32
The P-value reported for each variable is the value after all other variables were
taken into account. Data were analyzed using SPSS 11.5. Results are presented
as means ± standard error.
Source of Male Calls
Male G. rubens produce a trilling song in which sound pulses are repeated
and grouped into long pulse trains (Fig.3-2; Doherty & Callos 1991). Field-
collected males were individually recorded in the lab using an Optimus model
CTR-115 sound-activated cassette tape recorder. Ten males were recorded in
the fall and 10 males in the spring. Each male was recorded for at least five
minutes. The recordings of each male were digitized and divided to make four
one-minute calls. This generated 40 fall calls and 40 spring calls. The digitized 1-
min calls were burned onto CD discs, and one of these calls was played back
repeatedly to each female for 5 min during the preference test. Each female
received a different call. Fall females were tested with fall calls and spring
females with spring calls. The temperature at which males were recorded did not
differ between the fall and the spring (Meanfaii ± SE = 21,88±1.33 °C, Meanspnng ±
SE = 22.75±0.80 °C, P=0.584). This temperature also did not differ from the
temperature during the female phonotactic tests (Meanrec± SE = 22.31 ±0.76 °C,
Meanphon ± SE = 22.63±0.04°C, P=0.684).

33
Fly Activity
To verify that O. ochracea flies are more active in the early evening in the
fall but not the spring, I monitored fly activity in a pasture at the University of
Florida Beef Teaching Unit, Alachua Co., FL. To monitor fly activity, I used a slit
trap (Walker 1989) with an electronic sound synthesizer that broadcasts calls of
G. rubens (4763 Hz carrier frequency, 50.0 pulses per second, 50% duty cycle;
continuous trill at 80 dB, 15 cm above the speaker). I activated the synthesizer
an hour before sunset and counted the number of flies captured at the trap 4 h
later. I carried out this procedure from 5 June to 12 July (N=20) and from 10
October to 26 November 2001 (N=20) because these were the times when
females were tested in the phonotactic experiment.
Results
Phonotactic Experiment
Fewer females chose the broadcasting speaker in the fall (20/40; 50%),
than in the spring (30/40; 75%) (Fisher’s exact test: P=0.013). Fall females also
spent less time within 10 cm of the broadcasting speaker (Fig. 3-3; ANOVA,
season, Fij2=9.736, P=0.003) than spring females. Furthermore, fall females
took longer to leave the shelter than spring females (Fig. 3-4; t-test: Nfaii=40,
NSpring=40, t=3.115, P=0.003). In general, upon leaving the shelter, fall females
tended to walk around it and after some time, many of them went to the
endscreen with the silent speaker and walked around that area. In contrast,
spring females tended to leave the shelter and walk directly to the endscreen

34
with the broadcasting speaker. Many of them climbed the metal screen wall
facing the speaker walking back and forth in front of the broadcasting speaker.
The time that females spent within 10 cm of the broadcasting speaker was
not significantly affected by collection method (ANOVA, collection method,
F1i72=0.227, P=0.635) or morph type (ANOVA, morph, F1i72=0.009, P=0.926).
Similarly, the time that females spent within 10 cm of the broadcasting speaker
was not significantly affected by the interaction between season and collection
method (ANOVA, season*collection method, Fii72=0.208, P=0.650) or the
interaction between season and morph (ANOVA, season*morph, Fit72=0.124,
P=0.726).
Female mass had a significant effect on the time that females spent within
10 cm of the broadcasting speaker (ANOVA, mass, F1i72=5.231, P=0.025). This
effect differed between the seasons. In the spring the time that females spent
within 10 cm of the broadcasting speaker decreased with female mass but not in
the fall (Fig. 3-5, ANOVA, season*mass, F1i72=8.219, P=0.005).
Fly Activity
More flies were captured in the early evening per night in the fall than in
the spring (Meanfan ± SE = 2.6±0.8, Meanspring ± SE = 0.2±0.1, Mann-Whitney U
test, U=317, nfaii=20, nspring=20, P<0.001).

35
Discussion
This study demonstrates seasonal changes in female G. rubens
phonotactic behavior to male calling song. Parasitoid flies are most abundant in
the fall and most active in the early evening (Chapter 2). In the spring, flies are
rare. Because females may be parasitized as they approach singing males, I
predicted that fall females should be less phonotactic to male calling song in the
early evening than spring females. The results of this study support this
prediction. Fewer females chose the speaker that was broadcasting male calls in
the fall than in the spring, and fall females spent less time near the broadcasting
speaker than spring females. These results are consistent with the hypothesis
that risk of parasitism decreases female attraction to males with sexually-
selected traits.
Few studies have examined the effect of risk on female choice for sexually
selected traits. French and Cade (1987) examined female mating behavior in G.
texensis housed in large outdoor arenas. This field cricket is also parasitized by
O. ochracea (Cade 1975). Their results showed that females mated more
frequently around dawn, which coincided with the time when parasitoid flies were
less active (Cade et al. 1996). Similarly, Hedrick and Hill (1993) examined female
preference for long-bout calls in G. integer in the presence and absence of cover.
In this species, females prefer long-bout calls over short bout calls (Hedrick
1986) all else being equal. However, when females were given a choice between
a long-bout call played across open space and a short-bout call played through
100% cover, females did not express a preference for long-bout calls, suggesting

36
that the increased predation risk associated with open space reduced the
attractiveness of long-bout calls. Similar results have also been found in sand
gobies and guppies (Forsgren 1992, Godin & Briggs 1996). Typically, females of
these two species prefer to mate with brightly colored males. However, this
preference disappeared when females had to choose in the presence of a
predator. Collectively, these results are consistent with the findings of this study
and support the hypothesis that when choice is risky, the intensity of sexual
selection declines.
In this study, females were collected using two methods; turning over
objects and debris, and using sound traps. The use of sound traps may have
introduced a bias since individuals that were captured using this method were
initially attracted to male calling songs. However, this possibility was not
supported because collection method and the interaction between season and
collection method did not have significant effects on the time that females spent
within 10 cm of the broadcasting speaker. Another bias that may have affected
the results of this study is that different numbers of long- and short-winged
females were collected in the fall and spring. However, again, this was not
supported because morph type and the interaction between season and morph
type did not significantly affect the time that females spent within 10 cm of the
broadcasting speaker. Collectively, these results suggest that seasonal changes
in female phonotactic behavior were not due to seasonal changes in the effects
of collection method or morph type.

37
The time that females spent near the broadcasting speaker decreased
with female mass in the spring but not the fall (Fig. 3-5). This seasonal difference
in the interaction between female mass and phonotactic behavior may be due to
seasonal differences in the mating experience of females. Female crickets mate
multiply (reviewed by Loher & Dambach 1989). Their mating experience has
been shown to affect their phonotactic behavior. Lickman et al. (1998) presented
females differing in mating experience with multi-stimulus songs. Their results
show that virgin females exhibited more directional movement towards male
songs than mated females. Virgin females may be more phonotactic than mated
females since this will increase the chances that they will mate at least once and
secure some sperm. In this study, if heavy females in the spring were old and
mated, and light females were young and virgin, then this relationship may
explain why female phonotaxis decreased with female mass in the spring. This is
possible because 6. rubens has two pulses of adults in the fall and spring, and
two smaller cohorts of adults in the summer and winter (T.J. Walker, unpublished
data). At the time that spring females were collected from the field (15 May to 25
July 2001), the summer cohort was starting to emerge as adults. Therefore, I
could have collected both mated spring females, and virgin summer females. In
contrast, because at the time that fall females were collected (1 October to 25
November 2001), no females from the winter cohort had yet emerged. This
would mean that my fall sample may have primarily consisted of mated fall
females. This could explain why phonotactic behavior did not decrease with
female mass in the fall. It is important to note, however, that even when this

38
seasonal interaction between female phonotactic behavior and female mass is
taken into account, season still has a significant effect on female phonotactic
behavior.
Fall females took longer to leave the shelter during the phonotactic
experiment than spring females. This seasonal difference in female behavior may
be interpreted as fall females behaving more cautiously around male calls than
spring females in order to reduce fly parasitism or other sources of increased
risk. This explanation is consistent with a study on G. integer in which males
differing in song conspicuousness were presented with a novel, potentially
dangerous predator stimulus (Hedrick 2000). Males with more conspicuous
songs took longer to emerge from a shelter after the predator stimulus than
males with less conspicuous songs. Hedrick (2000) interpreted these differences
in latency to leave the shelter as males compensating behaviorally for their
conspicuous mating displays. This interpretation is consistent with the findings of
this study for females and suggests that future work should examine how
parasitism risk affects female searching behavior.
Fall females took longer to leave the shelter than spring females. Could
this difference in behavior account for the shorter time that females spent within
10 cm of the broadcasting speaker in the fall than in the spring? This is possible
because time spent in the shelter is time that could not have been spent near the
broadcasting speaker. However, some evidence suggests that this is unlikely.
The mean time that fall females spent in the shelter was 98 sec (Fig. 3-4). This
means that fall females spent on average 202 sec outside of the shelter. This

39
time is much greater than the mean time that spring females spent within 10 cm
of the broadcasting speaker, 121 sec (Fig. 3-3). These results suggest that
seasonal differences in female latency to leave the shelter do not fully explain the
seasonal variation in the time that females spent within 10 cm of the
broadcasting speaker.
This study has important implications for understanding the evolution of
sexually-selected traits. Male G. rubens sing to attract females for mating. This
behavior, however, also attracts parasitoid flies in some populations, in some
seasons, and at some times of the night. Differences in male calling behavior
between populations, seasons, or time of the night have been interpreted as due
to the natural selection pressure, parasitism. However, if parasitism risk also
decreases female attraction to male calls, then males may derive fewer benefits
from calling. In this study, I have found that fall females are less attracted to male
calling song in the early evening than spring females. This reduction in the
strength of sexual selection, as well as the increased natural selection costs for
males, will weaken selection for calling in the early evening for fall but not spring
males. Therefore, fall and spring males should differ in the timing of calling
behavior. Fall males should not sing in the early evening, but spring males
should sing at that time. This prediction has been confirmed in G. rubens
(Chapter 2): surveys of calling males in the field showed that fewer males sing in
the early evening in the fall than in the spring. This correlation between female
behavior and male calling behavior suggests that future studies investigating
variation in sexually selected traits should examine not only how risk affects

40
natural selection on males with sexually selected traits, but also how risk affects
female choice for males with sexually selected traits, and hence the benefits of
conspicuous traits.

CD player
broadcasting
speaker
O
shelter
w=0.3 m
silent
speaker
â—„
1=1.0 m
>-
Fig. 3-1. Experimental arena used in phonotactic experiment.

Frequency (kHz)
10
s
' m mn i ii i n 41 mu 11111 ii i in 11 in 11 >i mui i m m 11 i u >iu hi I) m M 11111 ill 11 11 M 111 tu I IIMttiliilllllll
r-""1 i i 1 i i i r i
C 0 2 C4 Oft 08 t 12 14 1 6 18 2
Time (sec)
4^
NJ
Fig. 3-2. Sound spectrograph of Gryllus rubens calling song at 25.4°C (SINA)

300
o
0
(/>
¡5 250 L
0
ro
CL
O)
c
8
â– O
0
o
ro
0
c
200 U
150 L
r 100 U
c
0
Q.
o
E
•£ 50 U
c
(0
0
0
fall
spring
4^
OJ
Season
Fig. 3-3. Seasonal differences in the mean time spent by female Gryllus rubens within 10 cm of a
broadcasting speaker in a phonotactic experiment where fall (N=40) and spring (N=40) females
chose between a silent speaker and a speaker broadcasting a male calling song.

300
250
o'
0)
| 200
JZ
tn
0)
1 150
o
■4-»
&
S ioo
«5
c
ro
S 50
0
Fig. 3-4. Seasonal differences in the mean latency to leave the shelter by female Gryllus rubens
in a phonotactic experiment where fall (N=40) and spring (N=40) females chose between a silent
speaker and a speaker broadcasting a male calling song.
fall spring
Season
£

x spring
O fall
Female mass (g)
Fig. 3-5. Seasonal differences in the association between female Gryllus rubens mass and time spent
by females within 10 cm of a broadcasting speaker in a phonotactic experiment where fall (N=40)
and spring (N=40) females chose between a silent speaker and a speaker broadcasting a male calling
song.

CHAPTER 4
VARIATION IN SEXUAL SELECTION FOR MALE SONG
IN THE FIELD CRICKET GRYLLUS RUBENS
Introduction
A primary goal of sexual selection theory is to understand how natural and
sexual selection interact to favor variation in male sexually-selected traits
(Andersson 1994). Crickets represent an ideal system in which to study this
question for several reasons. First, calling song is highly variable among males
(for review see Bertram 2002). Such variation can be easily quantified by
individually recording males and digitally analyzing their songs. Second, male
song attracts conspecific females for mating. Typically, females approach singing
males and mount them (Loher & Dambach 1989). This suggests that male calling
is affected by sexual selection. Finally, male vocalizations also attract parasitoid
flies (Cade 1975, Walker 1986, 1993). Female flies deposit larvae on and around
calling males, killing infected crickets within 7-10 days (Walker 1986, 1993). This
means that male calling may also be affected by natural selection. Collectively,
these conditions make crickets an ideal system to study how natural and sexual
selection interact to favor variation in male sexually-selected traits.
One of the most variable song characters in male crickets is nightly calling
duration or the total time spent calling per night. Using an artificial selection
experiment, Cade (1981) demonstrated that much of the variation in nightly
calling duration in the cricket, Gryllus texensis, was due to genetic differences
46

47
among males. Subsequently, Cade and Wyatt (1984) found that nightly calling
duration was highly variable among male G. texensis, G. pennsylvanicus, G.
veletis, and Teleogryllus africanus. Similarly, Cade (1991) determined that nightly
calling duration was also highly variable among male G. integer and G. rubens.
Despite the potential role of sexual selection in explaining variation in male
cricket song, most studies examining variation in nightly calling duration have
ignored the effect of sexual selection (Cade 1984, Cade & Wyatt 1984, French &
Cade 1989, Sakaluk & Snedden 1990, Forrest et al. 1991, Cade & Cade 1992,
Gray 1997, Wagner & Hoback 1999, Kolluru et al. 2002, but see Bertram 2002).
The goal of this study is to examine whether seasonal variation in male G.
rubens nightly calling duration is correlated with seasonal variation in sexual
selection.
In northern Florida, male G. rubens that sing spend more time calling in
the fall than in the spring (Chapter 2). Three factors have been used to explain
variation in male cricket calling behavior: male condition, parasites, and density.
Nightly calling duration should increase with male condition because males in
good condition are better able to bear the energetic costs of calling than males in
poor condition (Sakaluk & Snedden 1990, Forrest et al. 1991, Gray 1997,
Wagner & Hoback 1999). However, this possibility is not supported by the
available data for G. rubens. Although fall males are heavier than spring males,
nightly calling duration does not increase with male mass (Chapter 2). Similarly,
parasitized males should call less than unparasitized males because parasites
decrease the ability of males to sing (Cade 1984, Kolluru et al. 2002). However,

48
this hypothesis does not explain seasonal variation in G. rubens calling since
unparasitized fall and spring males differed in nightly calling duration (Chapter 2).
Finally, males at high densities should sing less than males at low densities
because as density increases, the costs of defending territories increase as well
as the chances of encountering a female by searching without calling (Alexander
1961 & 1975, Otte 1977, Cade 1979, Cade & Wyatt 1984, French & Cade 1989,
Cade & Cade 1992). But, differences in density cannot explain the longer calling
duration of fall G. rubens because male densities are higher in the fall than in the
spring (Veazey et al. 1976). Taken together, it is unlikely that the three most
common explanations can account for the seasonal changes in male G. rubens
nightly calling duration.
The same male G. rubens vocalizations that attract females also attract
gravid females of the parasitoid fly Ormia ochracea (Walker 1986,1993). These
females deposit larvae on and around calling males, killing infected crickets
within 7-10 days. If parasitoid flies attack males more in the spring than in the fall,
then this seasonal difference in parasitism risk may explain the seasonal
differences in nightly calling duration. Flowever, for G. rubens this is not an
explanation because O. ochracea in northern Florida is abundant in the fall and
rare in the spring (Walker 1986, Chapter 2) and fly parasitism rates are higher in
the fall than in the spring (10% versus 0%, Walker & Wineriter 1991). Therefore,
seasonal changes in parasitism risk cannot explain the seasonal changes in
male G. rubens calling duration.

49
Males that sing may have longer calling durations in the fall than in the
spring because seasonal changes in photoperiod and temperature cause fall and
spring males to differ in nightly calling duration. This possibility is supported by
rearing studies showing that photoperiod and temperature may affect calling
song parameters. Walker (2000) reared male G. rubens under different
photoperiods and temperatures to examine whether developmental conditions
affect pulse rate. He found that temperature influenced pulse rate, but
photoperiod did not. Similarly, Olvido and Mousseau (1995) reared striped
ground crickets (Allonemobius fasciatus) under two environments differing in
photoperiod and temperature. Their results showed that chirp rate, chirp duration,
interchip interval, pulse number, and carrier frequency are affected by rearing
environments. Despite this work, no study has ever found a significant effect of
photoperiod on male nightly calling duration in crickets. Bertram and Bellani
(2002) reared male G. texensis under fall-like and spring-like photoperiods and
found that rearing photoperiod does not significantly affect nightly calling duration
in males. This suggests that seasonal differences in male G. rubens nightly
calling duration are not due to seasonal changes in photoperiod. However, the
effect of temperature during development on nightly calling duration has not been
tested in any cricket species.
Male G. rubens may exhibit seasonal changes in nightly calling duration if
the benefits of singing change seasonally. These benefits may vary seasonally if
sexual selection operates differently in the two seasons. Accordingly, I
hypothesized that males sing longer in the fall than in the spring because the

50
benefit of singing increases with calling duration in the fall but not in the spring. I
evaluated this hypothesis by using pitfall traps fitted with speakers broadcasting
male calling songs of different duration. I counted the number of females
captured at the traps in the fall and spring, and predicted that the number of
captured females should increase with calling duration in the fall but not in the
spring. This prediction is important because it would be the first evidence to
suggest seasonal variation in sexual selection. Such variation could favor
variation in the sexually-selected trait, male calling duration.
Methods
Pitfall Trap Experiment
I examined seasonal variation in sexual selection for nightly calling
duration from 14 April to 12 June and 23 September to 5 November, 2002
because these are times when males sing for short and long durations,
respectively (Chapter 2). During both fall and spring, female and male G. rubens
occur in two forms, long- and short- winged. The two morphs differ in their ability
to fly as well as in other life-history characteristics such as the onset of
reproduction (Roff 1984, Zera & Rankin 1989, Holtmeier & Zera 1993). Females
of both morphs were used in this study in the proportions in which they occurred.
All experiments were conducted in a field at the University of Florida
Environmental Landscape Horticulture Education Lab, Alachua Co., FL. I chose
this field because male G. rubens sing all year long at this site.

51
Tests for seasonal variation in sexual selection for nightly calling duration
were conducted using five pitfall traps (Fig. 4-1) laid out in a straight line 10 m
apart. Each trap was made by excavating a hole in the ground and placing a 2.4
L plastic bucket measuring 17 x 15 cm (height x diameter) in the hole so that the
rim was level with the ground. Over each bucket I placed a plastic funnel 10 cm
tall with an upper diameter of 18 cm and a spout diameter of 6 cm. The funnel
rested on the rim of the bucket and the spout was 7 cm above the bottom of the
bucket. I placed a 33-cm square PVC tube frame (1.9 cm diameter) with four 7-
cm legs over the bucket-funnel trap. The frame rested on the ground surrounding
the bucket-funnel trap and supported a metal grill top with 1.5 cm spaces
between the bars. In the center of this grill top, I positioned a Sony model D-
EJ721 portable CD-Walkman with a Radio Shack model 277-1008 mini-audio
amplifier speaker lying flat over the CD player. The speaker pointed upward and
rested 9 cm over the center of the trap. Crickets that were attracted to calls
played on the CD player, flew (long-winged morph) or walked (short-winged
morph) into the bucket-funnel trap where they were collected hourly. Five
different treatments of male calling duration were used, one for each pitfall trap.
The treatments consisted of broadcasting male calling songs for 0,1,2, 3, or 4
hours. These calling durations fall within the natural variation of time spent calling
by males (Chapter 2). Output of the broadcasting speakers was set at 80dB,
measured 15 cm above each speaker, using a Briiel & Kjaer model 2219 sound
level meter. This sound level falls within the natural range of sound intensities of
calling males (M.J. Vélez, unpublished data).

52
Each experiment began an hour before sunset when I activated the CD
player over the pitfall trap set to broadcast for 4 h. An hour later, I turned on the
CD player over the pitfall trap set to broadcast for 3 h. This continued until all CD
players broadcasting songs had been activated. The outcome of this staggering
the onset of the calling was that all CD players finished broadcasting male calling
songs three hours after sunset. This pattern was used because individual
recordings of males under field conditions show that calling start times are
inversely related to calling duration (M.J. Vélez, unpublished data). The pitfall
trap with the 0-h treatment was handled like the pitfall trap set to broadcast for 4
h: a CD player was placed over the trap an hour before sunset. However, this
player did not broadcast any songs. This treatment controlled for the number of
non-phonotactic females that fell into a trap. All pitfall traps were checked hourly
to count the number of captured females from one hour after the experiment
started to the end of the experiment three hours later. Trapped females were
removed from the field to ensure that they were not sampled in more than one
treatment or night. Any trapped males or juveniles were released back into the
field. Each night constituted one replicate of the experiment. All treatments within
a replicate used the same male calling song, but different replicates used
different songs. Between replicates, I randomized the position of the treatments.
Randomization was achieved by generating all possible combinations of
treatments and picking one at random each night. This experiment was
conducted on 35 nights in the spring and 25 nights in the fall.

53
Source of Male Calls
Male 6. rubens were collected from the field using sound traps (Walker
1986). They were housed individually in square plastic containers measuring 13
x 13 x 9 cm (I x w x h) with wire screen lids. Each container had an inch of sand,
a few pieces of Purina Cat Chow (changed daily) and water ad lib. All containers
were held under ambient light and temperature in a greenhouse less than 1 km
from the collection site. After 10 days (the number of days necessary to ensure
that the crickets were not parasitized by O. ochracea), I brought the males into
the lab and individually recorded each for at least 5 min using an Optimus model
CTR-115 sound-activated cassette tape recorder. All of the males used in these
recordings were unparasitized and long-winged because short-winged males
could not fly into the sound traps used to collect crickets. Ten males were
recorded in the fall and 10 males in the spring. The recordings of each male were
digitized and used to make four 1 min calling songs. This generated 40 fall and
40 spring calling songs. These songs were burned onto CD discs and played
back repeatedly in the pitfall trap experiment for the duration of the treatment. In
the fall, I used fall calling songs over pitfall traps, and in the spring, I used spring
calling songs. The mean temperature at which males were recorded was 23.1°C
and did not differ between fall and spring calls (t-test, P=0.702).
Data Analysis
A generalized linear model with a Poisson distribution and a logit link was
used to regress the calling duration of a treatment on the number of females

54
captured in that treatment’s pitfall trap. This model was used because the data
could not be transformed to achieve normality. Because treatments within a
replicate were not statistically independent, I used a mixed model to analyze the
data. In this model, calling duration was a fixed effect, and replicate number was
a random effect. Fall and spring data were analyzed separately. The data were
tested for overdispersion to validate the use of the Poisson model.
If females search for mates at different times of the night in the fall and
spring, then such seasonal differences in female behavior could affect the
relationship between calling duration and number of trapped females. To
evaluate this possibility, I compared the capture time to the nearest hour of each
female in the pitfall traps during the fall and spring using a Mann-Whitney U test.
Data were analyzed using R 1.7.1 (Dalgaard 2002) and Systat 6.0
(Wilkinson 1997). Descriptive results are presented as means ± standard error.
Results
The number of captured females increased with treatment calling duration
in the fall but not in the spring (Table 4-1; Fig. 4-2; fall: t=4.532, df=99, P<0.001;
spring: t=0.419, df=139, P=0.676). Females were captured in all calling duration
treatments in the spring and fall with the exception of the 0-h treatment in which
no female was captured in either the fall or the spring (Fig. 4-2). Sixteen females
arrived at the pitfall traps in the fall (niong=12, nShort=4), and 10 in the spring
(niong=6, nShort=4). Fall and spring females did not differ in the time at which they
were captured in the pitfall traps (Fig. 4-3; nfaii=16, nspring=10, Mann-Whitney U

55
test=56.5, P=0.189). This non-significant result remains even when the number
of females captured in the first two hours are compared with the number of
females captured in the last two hours in the fall and spring (X2=0.005, df=1,
P=0.940)
Discussion
Male G. rubens that sing spend more time calling in the fall than in the
spring (Chapter 2). This is surprising because there are more phonotactic,
parasitoid flies present in the fall (Walker 1986, Chapter 2) so one would expect
males to call less to avoid parasitism. Furthermore, as I argued in Chapter 3,
females should be less attracted to calling males in the fall because they too are
parasitized by these flies (Walker & Wineriter 1991). The results of the pitfall trap
study provide an explanation: males that sing longer are more likely to attract
females during the fall but not during the spring (Table 4-1). What might explain
this result?
If fall females search for mates in the early evening and spring females
search for mates in the late evening, then these temporal differences in female
activity may explain why the number of captured females in my pitfall traps
increased with calling duration in the fall but not the spring. This is possible
because of the way I set up the experiment: the long calling duration treatments
began earlier in the evening than the short calling duration treatments.
Consequently, fall females, preferring to search for mates in the early evening,
would have a higher probability of encountering a pitfall trap with a long calling

56
duration whereas spring females, preferring to search for mates in the late
evening, would have an equal probability of encountering a long or short calling
duration treatment. However, the results of this study do not support this
explanation. Fall and spring females did not differ in the time at which they were
captured in the pitfall traps. This suggests that male calling duration increases in
the fall but not the spring because long duration calls are more effective at
attracting females in the fall but not in the spring. This pattern is consistent with
seasonal variation in sexual selection.
Female G. rubens densities in northern Florida are higher in the fall than in
the spring (Veazey al. 1976). If females choose males randomly with respect to
calling duration, then males with long duration calls should have a higher
probability of attracting females than males with short duration calls. Because
this probability should increase proportionally with density, seasonal changes in
female density cannot explain the seasonal changes in the success of long
duration calls. Instead, the seasonal differences in the success of long duration
calls suggest that fall females do not have a preference for calling duration and
spring females prefer short duration calls. Theoretical models on the evolution of
female preferences show that choosiness decreases when the risks of choice
increase relative to the benefits (Parker 1983, Kirkpatrick 1987, Pomiankowski
1987, Real 1990, Iwasa et al. 1991). Parasitoid flies are abundant in the fall and
rare in the spring (Walker 1986, Chapter 2). Fall females may not have a
preference for calling duration because the costs of choice due to fly parasitism
may be high in that season. Even though females do not sing, they are

57
nonetheless parasitized to the same levels as males are (Walker & Wineriter
1991). In contrast, females are more phonotactic in the spring than in the fall
(Chapter 3). In an experiment giving females a choice between a silent speaker
and a speaker broadcasting a male calling song, fewer females chose the
speaker that was broadcasting male calls in the fall than in the spring, and fall
females spent less time near the broadcasting speaker than spring females. This
may mean that less singing is required to attract females in the spring than in the
fall. This seasonal variation in sexual selection is consistent with the seasonal
changes in the success of long duration calls.
The results of this study are consistent with recent work showing within-
and between-population variation in the female preference functions of field
crickets. Wagner et al. (1995) sequentially presented female G. integer from a
single population with songs differing in the number of pulses per trill, inter-trill
interval, or the proportion of missing pulses within a trill. These presentations
revealed that individual females differed significantly in their preference functions
based on the number of pulses per trill and inter-trill interval. Furthermore, these
preference functions were highly repeatable within females. Similarly, Simmons
et al. (2001) examined geographic variation in the female preference function for
the long chirp portion of male Teleogryllus oceanicus calling song. Their results
show that the female preference function for the amount of long chirps differed
between populations. The findings from these studies suggest that selection can
act on female preference functions. The outcome of such process may favor
variation in male sexually-selected traits.

58
To evaluate the general importance of temporal variation in sexual
selection as a mechanism favoring variation in male sexually-selected traits,
future studies should examine how variation in female choice is affected by
factors that change the costs and benefits of choice, such as predation,
parasitism, sex ratios, and population densities. If any of these factors fluctuates
in time, then the costs and benefits of choice as well as the resulting female
choice patterns may also vary temporally. In turn, this temporal variation in
female choice could act to favor variation in sexually-selected traits

Top view
Side view
PVC frame with metal
Ground
level
Fig. 4-1. Pitfall trap used to test Gryllus rubens female choice for male calling duration. Each trap
was made by excavating a hole in the ground. This hole was deep enough to accommodate a 2.4 L
bucket measuring 17 x 15 cm (height x diameter). The rim of the bucket was level with the ground.
The bucket was covered with a large plastic funnel 10 cm tall that rested on the rim of the bucket,
spout down. The funnel had an upper diameter of 18 cm and a spout diameter of 6 cm. A square
PVC frame (length = 33 cm) with four 7-cm legs was positioned over the funnel and bucket. This
frame rested on the ground surrounding the bucket-funnel trap. The frame supported a metal grill top.
A CD-Walkman with an external speaker facing upward rested on the metal grill top. The speaker was
positioned 9 cm over the center of the trap.

Table 4-1. The effect of season on male Gryllus rubens nightly calling duration. The table shows the
results of a generalized linear model with a Poisson distribution used to regress treatment calling
duration on the number of captured females in that treatment’s pitfall trap. Because treatments
within a replicate were not statistically independent, a mixed model was used to analyze the data. In
this model, calling duration was a fixed effect, and replicate number was a random effect. Each
season was analyzed separately.
Season
Parameter
Value
Std. Error
DF*
t-Value
P-value
Fall
Intercept
-4.238
0.577
99
-7.342
< 0.001
Slope
0.716
0.158
99
4.532
< 0.001
Spring
Intercept
-3.073
0.625
139
-4.916
< 0.001
Slope
0.100
0.239
139
0.419
0.676
* Degrees of freedom were calculated with the formula: #nights (# treatments - 1) - 1.

fall (N=25 nights)
Calling duration treatment (h)
Fig. 4-2. Seasonal differences in the mean total number of female Gryllus rubens captured in the
pitfall trap experiment. Five treatments were presented in each replicate; each broadcasted male
calling song for a different duration (0, 1, 2, 3, and 4 h). Each pitfall trap was checked hourly to count
the number of captured females (which were then removed). Each night constituted one replicate of
the experiment.

4 -
6 - -
8 -
10 1 1 1
2 3 4
Female capture time (hours after experiment started)
â–  fall (N=16)
â–¡ spring (N=10)
Fig. 4-3. Seasonal differences in female Gryllus rubens capture time in the pitfall trap experiment. In
this experiment, five pitfall traps broadcasting male calling songs for 0,1,2, 3, and 4 h were checked
hourly to estimate to the nearest hour the capture time of each female that fell into the trap.

CHAPTER 5
GENERAL MODEL: A TEST OF THE FACTORS AFFECTING MALE CALLING
DURATION IN THE FIELD CRICKET GRYLLUS RUBENS
Factors Affecting Male Calling Duration
Fall and spring generations of the field cricket Gryllus rubens in northern
Florida experience different selective pressures and social environments. Males
of this species sing which attracts females for mating. Their calling song also
attracts gravid females of the parasitoid fly Ormia ochracea (Walker 1986, 1993).
These female flies deposit larvae on and around calling males, killing infected
crickets within 7-10 days. In the fall when parasitoid flies are abundant (Walker
1986, Chapter 2), and cricket densities are high (Veazey al. 1976), calling songs
of long duration attract more females than short duration calling songs (Table 4-
1, Fig. 4-2). This is quite different from the spring when parasitoid flies are rare
(Walker 1986, Chapter 2), cricket densities are low (Veazey al. 1976) and calling
song duration does not affect female attraction rates (Table 4-1, Fig. 4-2). In this
chapter, I examine how seasonal changes in fly parasitism, female visitation
rates, and cricket density should alter the costs and benefits of calling for fall and
spring males. I evaluate these effects by building a model that calculates the
fitness of different calling durations in each season. I then use this model to
predict observed seasonal patterns in calling duration. Additionally, I
independently vary the effects of fly parasitism and female choice on male calling
duration within each season to evaluate the relative importance of natural versus
63

64
sexual selection in favoring seasonal patterns in male calling duration. This
model is valuable because it is the first to evaluate simultaneously the effects of
fly parasitism, female attraction rates, and cricket density on the male character,
calling duration. This model also provides insights into how different selective
pressures and social environments can interact to favor variation in a male trait.
Model: Predicting Seasonal Patterns in Male Calling Duration
Male Calling Strategies
In this model, males can adopt one of four calling strategies: (1) call for 1
h, (2) call for 2 h, (3) call for 3 h, and (4) call for 4 h. These calling durations fall
within the natural range of time spent calling by male G. rubens under field
conditions (Fig. 2-4; Cade 1991). I assume that males adopt one calling strategy
and use it throughout their lifetime. This assumption is supported by the finding
that the mean time spent calling per 24 h under laboratory conditions does not
change with male age in G. texensis, G. pennsylvanicus, G. veletis, and
Teleogryllus africanus (Cade & Wyatt 1984). Furthermore, Cade (1981) found
that some of differences between male G. texensis in calling duration are due to
genetic differences among males.
Individual field recordings of male G. mbens show that some males do not
call (Fig. 2-4). These males may obtain mates by walking around searching for
females. This alternative male tactic has been documented in several species of
field crickets (French & Cade 1989, Hissmann 1990, Zuk et al. 1993, Hack 1998).
I did not include this behavior in my model because little is known about the

65
mating success and parasitism risk of walking males. Cade (1979) proposed that
walking males may have decreased mating success on any given day, but they
may have higher survival rates and longer lifespans than calling males because
they do not attract as many parasitoid flies. This hypothesis, however, remains
untested and hence I exclude non-calling as a strategy in my model.
Calculating Fitness of Male Calling Strategies
Male calling attracts conspecific females and parasitoid flies. Each calling
strategy carries a mating benefit, M, and a probability of escaping fly parasitism,
E. Each day, males mate before they face fly parasitism. The daily fitness of
each calling strategy F¡ is defined as:
F¡ = EjM¡ + (1-E¡)M¡,
where i is the calling strategy.
If a male sings and is not parasitized on a given night, he accrues the
mating benefit of that day and sings again in the next day. If a male sings and is
parasitized on a given night, he accrues the mating benefit of that day. After fly
parasitism, the infected male has one more day to carry out his calling strategy
and receive the corresponding mating benefit. After that day, the infected male
does not get any further reproduction. This restriction is meant to mimic the
temporal dynamics of the effects of fly parasitism on male calling duration.
Kolluru et al. (2002) experimentally infected male T. oceanicus with O. ochracea
larvae and monitored the proportion of time spent calling by males each night
after fly parasitism. Their results show that the proportion of time spent calling by

66
males remained unchanged the day after fly parasitism but declined sharply to
almost zero after the second day.
I calculated the lifetime fitness of each calling strategy using the formula:
d=60
XMi(d+1)(1-Ei)E>1
d=1
where d equals day and the length of the breeding season is 60 days. This
formula individually adds the daily fitness of each strategy over the 60 days of the
breeding season for all possible outcomes (i.e., male is parasitized on the first
day, male avoids fly parasitism on the first day and is parasitized on the second
day, male avoids fly parasitism on the first and second day and is parasitized on
the third day, etc.). Fitness was calculated using R 1.7.1 (Dalgaard 2002).
Seasonal Changes in Mating Benefit (M)
Long duration calling songs are more likely to attract females than short
duration calling songs in the fall but not the spring (Table 4-1; Fig. 4-2). This
seasonal pattern in female attraction suggests that the mating benefit of singing
should increase with calling duration in the fall but not the spring. I estimated the
mating benefit of the different calling strategies in each season using data from a
field experiment carried out in the fall and spring in which I counted the number
of females captured at pitfall traps fitted with speakers broadcasting male calling
songs differing in duration (1 h, 2 h, 3 h, and 4 h; Fig. 4-2). Because the number
of captured females increased with calling duration treatment in the fall, I used
the mean number of captured females per night for each treatment to estimate

67
the mating benefit of each calling strategy in the fall (Table 5-1). Because there
was no significant relationship between calling duration treatment and number of
captured females in the spring, I assumed that all calling strategies have a similar
mating benefit in the spring. I estimated this mating benefit by calculating the
mean number of captured females per night for all calling treatments combined
(Table 5-1). These seasonal changes in mating benefit with calling duration
indicate that males that call more attract more females in the fall but not in the
spring.
Female density may also affect the mating benefit of the different calling
strategies. Males should call less at high densities because females may be
easier to locate without calling and calling territories may be more expensive to
defend at high than at low densities (Alexander 1961 & 1975, Otte 1977, Cade
1979). Because cricket densities are higher in the fall than in the spring, males
should call less in the fall than in the spring. This potential seasonal effect of
density on the mating benefit of the different calling strategies is already taken
into account in the empirical estimates of seasonal changes in mating benefit due
to differences in female visitation rates. This is because the pitfall traps used to
collect these data captured females at their natural densities in each season.
Seasonal Changes in Probability of Escaping Flv Parasitism (E)
Calling males attract parasitoid flies (Cade 1975). Walker and Wineriter
(1991) experimentally muted G. rubens males by removing the right tegmen.
They found that muted males have lower fly parasitism rates (0/10) than unmuted

68
males (7/13) under field conditions at high fly densities. This finding suggests that
the probability of escaping fly parasitism should decrease with calling duration.
To estimate this relationship more precisely, I used data collected in the fall on
the arrival times of flies captured at a slit trap broadcasting male G. rubens
calling song (Chapter 2). The number of flies arriving at the trap decayed rapidly
with time (Fig. 2-1). These data can be used to estimate the probability of
escaping fly parasitism for different calling strategies because male calling start
times are inversely related to calling duration. To achieve this, I fitted a negative
exponential equation to the data on fly arrival times using a generalized mixed
linear model with a Poisson distribution. I used this equation to estimate the
probability of at least one fly infecting a male of each calling strategy. To
accomplish this, I used data collected in the fall on the probability of at least one
fly arriving at a slit trap broadcasting male G. rubens calling song for 4 h (Chapter
2). I set this probability as the y-intercept in the negative exponential equation I
derived earlier and used this equation to calculate the probability of at least one
fly infecting a male that sings for 1 h, 2 h, and 3 h. The outcome of these
manipulations is that the probability of escaping fly parasitism decreased with
calling duration in the fall (Table 5-1). This relationship between the probability of
escaping fly parasitism and calling duration indicates that males that call more
have lower survival rates and shorter lifespans than males that call less. This is
because the cumulative probability of escaping fly parasitism in this model
decreases more rapidly for long calling durations than for short calling durations

69
over the 60 days of the breeding season. The end result is that this model
incorporates the tradeoff between calling duration and life span for fall males.
Walker (1986) estimated that fly abundance in the spring is 1 % of the fly
abundance in the fall. This means that the probability of escaping fly parasitism
should be higher for all calling strategies in this season. I estimated this
probability for each strategy by increasing their counterpart fall probability by
99%. This assumes that there are 99% fewer flies in the fall than in the spring, fly
parasitism risk increases linearly with parasitoid fly abundance, and the
probability of escaping fly parasitism decreases with calling duration in the same
way as it does in the fall (Table 5-1).
Model Results: Seasonal Changes in Fitness of Male Calling Strategies
The fitness of calling strategies increased with calling duration in the fall
but not the spring (Fig. 5-1). In the fall, the strategy ‘call 4 h’ had the highest
fitness, and ‘call 1 h’ had the lowest fitness (Fig. 5-1). In the spring, the strategies
‘call 1 h’ and ‘call 2 h’ had the highest fitness, and ‘call 3 h’ and ‘call 4 h’ had the
lowest fitness (Fig. 5-1).
Comparison of Optimal Seasonal Strategies with Observed Calling Durations
Even though parasitoid flies are more common in the fall than in the spring
(Walker 1986, Chapter 2), male G. rubens spent more time calling in the fall than
in the spring (Fig. 2-5). This finding agrees with the predictions of my model. The
fitness of calling strategies increased with calling duration in the fall but not the

70
spring. The probability of escaping fly parasitism decreased with calling duration
in both the fall and spring. In the fall, however, this increased cost of singing with
calling duration was offset by a disproportionate increase in the mating benefit
with calling duration. This resulted in calling strategies with long calling durations
obtaining higher fitness than calling strategies with short calling durations. In
contrast, the increased cost of singing with calling duration in the spring was not
offset by the mating benefit because this benefit did not change with calling
duration. This resulted in calling strategies with short calling durations achieving
higher fitness than calling strategies with long calling durations.
A comparison of the mean calling duration of male G. rubens in the fall
and spring with the optimal seasonal calling durations predicted by my model
reveals that males sang about as much as expected in the spring (0.71 h vs. 1 or
2 h) and less than expected (1.71 h vs. 4 h) in the fall. One possible explanation
for this discrepancy is that my model did not include the energetic costs of
calling. Such costs are known to increase linearly with calling duration in trilling
species of crickets like G. rubens (Prestwich & Walker 1981). Fall males may
have not sung as much as predicted because it may be more costly to sing in the
fall than in the spring. This could occur if temperatures are higher in the fall than
in the spring. It is clear that if these seasonal changes in the energetic costs of
calling were incorporated into my model, then the predicted calling durations
would be lower than the current ones in the fall.

71
Variations of the Model
To evaluate the relative importance of natural versus sexual selection in
favoring seasonal patterns in male calling duration, I independently varied the
effects of fly parasitism and female visitation rates on male calling duration within
each season. In the first variation of the model, I drastically reduced fly
parasitism by setting the probability of escaping fly parasitism to 0.999 for all
calling strategies in the fall and spring (Table 5-2). I then examined the fitness of
the different calling strategies given only seasonal variation in the mating benefit
(Table 5-2). This would be the situation, for example, in a population with few
flies, which may be found in the northern extent of the G. rubens range. In the
second variation of the model, I held the mating benefit constant for all calling
strategies in the fall and spring by assigning all strategies the average mating
benefit for both seasons combined (Table 5-3). I then evaluated the fitness of the
different calling strategies given only seasonal variation in the probability of
escaping fly parasitism (Table 5-3). This would simulate the effect of a population
in which there was no relationship between calling duration and female visitation
rates.
Results of Model Variations
In the model with reduced and constant fly parasitism, and variable mating
benefit, the results are the same as when fly parasitism was included: the fitness
of longer duration calling strategies increased in the fall but not the spring (Fig. 5-
2). In the fall, the calling strategy ‘call 4 h’ had the highest fitness, and the

72
strategy ‘call 1 h’ had the lowest fitness (Fig. 5-2). In the spring, all calling
strategies had the same fitness (Fig. 5-2). In the model with variable fly
parasitism and constant mating benefit (Fig. 5-3), however, the results are not
the same as when mating benefit varied: the fitness of calling strategies ‘call 1 h’
and ‘call 2 h’ were higher than the fitness of strategies ‘call 3 h’ and ‘call 4 h’ in
both the fall and the spring
Relative Importance of Natural versus Sexual Selection
Reducing fly parasitism and holding it constant did not affect the general
pattern of the original model: the fitness of longer calling duration strategies
increased with calling duration in the fall but not the spring in both models. This
finding suggests that sexual selection may be more important than natural
selection in favoring seasonal patterns in male calling duration. Male G. rubens
calling durations may have been higher in the fall than in the spring because long
duration calls were more effective at attracting females in the fall than in the
spring. This implication is significant because it suggests that variation in sexual
selection may be more important than variation in natural selection in shaping the
evolution of male traits.
Even though sexual selection may be more important than natural
selection, nonetheless sexual selection and natural selection clearly interact to
shape seasonal patterns in male G. rubens calling duration. In the model with
variable fly parasitism and constant mating benefit, short calling strategies had

73
higher fitness than long calling strategies. This suggests that natural selection
can modulate the effects of sexual selection on male calling duration.
Implications of the Model
The conclusions of this model enhance our understanding of how natural
and sexual selection may interact to favor variation in sexually selected male
traits. In G. rubens, male song attracts both females and parasitoid flies. Signals
that attract both mates and predators or parasites are common in species where
males have sexually selected traits (for review see Burk 1982). The outcomes of
my model show that in systems like this one, sexual selection may be more
important than natural selection in favoring variation in male traits, but that
natural selection can modify the effects of sexual selection.
Seasonal variation in calling duration has been observed in other Gryllus
species. Male G. texensis sing to attract females for mating and their calling song
also attracts O. ochracea (Cade 1975). The risk of fly parasitism in male G.
texensis may increase with calling duration because the number of O. ochracea
flies attracted to broadcasts of G. texensis song increases with calling duration
(Cade et al. 1996). Because O. ochracea is more abundant in the fall than in the
spring, the risk of fly parasitism should be higher in the fall than in the spring.
Cade (1989) studied female G. texensis attracted to male calling song in late
summer and early fall. He found that the number of females attracted to male
song increased with calling duration. Unfortunately, Cade (1989) did not examine
female G. texensis attraction to male song in the spring. However, if female G.

74
texensis attraction to male song does not increase with calling duration in the
spring, as in the closely related species G. rubens, then G. texensis would be
similar to G. rubens in the seasonal changes in mating benefit and probability of
escaping fly parasitism. Accordingly, my model would predict that male G.
texensis should spend more time calling in the fall than in the spring. Bertram
(2002) found evidence for this prediction, although she had a different
explanation for the cause. This finding supports the general conclusions of this
model in explaining seasonal variation in male G. rubens calling duration.
This model is valuable because its formulation is general enough to
explain variation in any sexually selected male trait that attracts both females and
predators or parasites. Such tradeoffs in sexually selected traits are well known
(Burk 1982; Godin & Briggs 1996). For example, this model could be used to
evaluate geographical variation in male color patterns of guppies (Poecilia
reticulata). Male guppies show extensive geographical variation in their
conspicuous color patterns. Their bright colors are preferred by females (Kodric-
Brown 1985, Houde 1987, Long & Houde 1989) but at the same time make the
males more vulnerable to predation (Endler 1987). This variation has been
explained by a balance between female preference for conspicuous males and
natural selection against conspicuous males (Endler 1983). However, studies
over the last two decades have shown that female preferences also vary with
predation levels (Breden & Stoner 1987, Stoner & Breden 1988, Godin & Briggs
1996; Brooks & Endler 2001). Female preference for conspicuous males
disappears when predation is high. This means that the geographical variation in

75
male guppy color patterns may be due to differences among populations in
sexual selection as well as natural selection (Dill et al. 1999).
My model could help examine the relative importance of natural versus
sexual selection in favoring geographical patterns of male color patterns in
species like guppies. The strategies of the model could be different areas of
conspicuous body colors or different levels of bright coloration. The mating
benefit of the different color areas could be estimated from studies examining
female preference for male color patterns in low and high predation areas.
Similarly, the probability of escaping predation in geographic areas with different
color strategies could be estimated from studies examining the survival of males
under low and high predation levels. Once these parameters are calculated, they
could be manipulated using my model to assess the relative importance of
natural versus sexual selection in favoring geographical variation in male guppy
color patterns. If the gain in fitness through sexual selection (female preference)
shows an accelerating effect whereas natural selection (risk of predation) is
linear with color brightness, then the results will be similar to my model with
Gryllus: the effect of sexual selection has a stronger effect on driving variation in
male color variation than natural selection. This application shows how my
model could be used to examine variation in any sexually selected male trait that
attracts both females and predators.

Table 5-1. Model Variables: Mating benefit and probability of escaping fly parasitism of different
male Gryllus rubens calling strategies in the fall and spring.
Male Calling Strategy
Mating Benefit (mean #
females attracted/ night)
Probability of Escaping Fly
Parasitism/ night
Fall
Spring
Fall
Spring
Call 1 h
0.040
0.070
0.500
0.995
Call 2h
0.120
0.070
0.500
0.995
Call 3h
0.160
0.070
0.350
0.697
Call 4h
0.320
0.070
0.350
0.697

Lifetime fitness
call 1 h call 2 h call 3 h call 4 h
â–  fall
â–¡ spring
Male calling strategy
Fig. 5-1. Model Results: Seasonal changes in the lifetime fitness of different male Gryllus rubens
calling strategies.

Table 5-2. Model variables with reduced and constant fly parasitism and variable mating benefit.
The table shows the mating benefit and probability of escaping fly parasitism of different male
Gryllus rubens calling strategies in the fall and spring.
Male Calling Strategy
Mating Benefit (mean #
females attracted/ night)
Probability of Escaping Fly
Parasitism/ night
Fall
Spring
Fall
Spring
Call 1 h
0.040
0.070
0.999
0.999
Call 2h
0.120
0.070
0.999
0.999
Call 3h
0.160
0.070
0.999
0.999
Call 4h
0.320
0.070
0.999
0.999

Table 5-3. Mating benefit and probability of escaping fly parasitism of different male Gryllus rubens
calling strategies in the fall and spring for model with variable fly parasitism and constant mating
benefit.
Male Calling Strategy
Mating Benefit (mean #
females attracted/ night)
Probability of Escaping Fly
Parasitism/ night
Fall
Spring
Fall
Spring
Call 1 h
0.120
0.120
0.500
0.995
Call 2h
0.120
0.120
0.500
0.995
Call 3h
0.120
0.120
0.350
0.697
Call 4h
0.120
0.120
0.350
0.697

Lifetime fitness
0.8
0.6
call 1 h call 2 h call 3 h call 4 h
Male calling strategy
â–  fall
â–¡ spring
Fig. 5-2. Seasonal changes in the lifetime fitness of different male Gryllus rubens calling strategies
in model with reduced and constant fly parasitism, and variable mating benefit.

Lifetime fitness
0.8
0.6
0.4
call 1 h call 2 h call 3 h call 4 h
â–  fall
â–¡ spring
Male calling strategy
Fig. 5-3. Model Results with variable fly parasitism risk and constant mating benefit. The figure shows
seasonal changes in the lifetime fitness of different male Gryllus rubens calling strategies. "

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BIOGRAPHICAL SKETCH
Manuel J. Vélez was born in San Juan, Puerto Rico, on February 4,1974.
He is the son of Manuel J. Vélez Jr., a college professor of invertebrate zoology
at the University of Puerto Rico, and Alsacia Bosch de Vélez, a social worker and
housewife. He grew up in San Juan, where he attended elementary school at
Colegio Nuestra Señora de la Providencia, and middle and high school at
Colegio San Ignacio de Loyola. After graduating from high school, he left San
Juan and attended Cornell University, where he earned a B.A. in biology with a
concentration in neurobiology and behavior. While at Cornell University, he
conducted behavioral research both in the field and lab in a wide range of taxa
including fish, birds, and mammals. After graduating from Cornell University, he
attended the University of Florida, where he earned an M.S. degree in zoology.
His master’s thesis explored parental care strategies in the cichlid fish Aequidens
coeruleopunctatus. After he obtains his Ph.D. in zoology, he will attend Boston
University School of Law, where he will study intellectual property.
89

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
H. Jáne Brockmann, Chair
Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
(
/ I
4-
Colette M. St. Mary
Associate Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
Benja
Assist
Bolker
t Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree
SáL
Marta L. Wayne
Assistant Professor of Zoology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of philosophy.
Thomas J. W^íker
Professor Emeritus of Entomology and
Nematology
This dissertation was submitted to the Graduate Faculty of the Department
of Zoology in the College of Liberal Arts and Sciences and to the Graduate
School and was accepted as partial fulfillment of the requirements for the degree
of Doctor of Philosophy.

This thesis was submitted to the Graduate Faculty of the Department of
Zoology in the College of Liberal Arts and Sciences and to the Graduate School
and was accepted as partial fulfillment of the requirements for the degree of
Doctor of Philosophy.
August 2004
Dean, Graduate School

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UNIVERSITY OF FLORIDA



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