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Effects of Environmental Heterogeneity on Male-Male Competitive Success, Size, and Scaling in a Cactus Bug Narnia femorata (Hemiptera: Coreidae)

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Effects of Environmental Heterogeneity on Male-Male Competitive Success, Size, and Scaling in a Cactus Bug Narnia femorata (Hemiptera: Coreidae)
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Journal of Undergraduate Research
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Nageon de Lestang, Fae
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Gainesville, Fla.
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University of Florida
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The effects of environment on sexual selection, specifically male competition, have yet to be fully explored. We examined the effects of natural environmental heterogeneity on male scaling, size, and competitive success in a cactus bug, Narnia femorata. Male N. femorata were raised in two alternate environments (cactus with fruits and cactus without fruits) from the fifth instar and checked daily until they reached adulthood. Males raised in a treatment with both cactus pads and fruit developed faster and weighed more overall. Upon reaching sexual maturity, a subset of males was randomly chosen, paired by treatment, and placed with a virgin female. Then their behaviors were recorded for time intervals over a period of 110 minutes. We observed matings in 8 of 29 contests, all by males raised in the treatment with both cactus pads and fruit. Noneof the males raised on a treatment of just cactus pads mated. Successful males were larger and expressed relatively larger sexually-selected traits for their body size. These results suggest that natural environmental variation has a significant effect on male competitive success for the species.

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Effects of Environmental Heterogeneity on Male-Male
Competitive Success, Size, and Scaling in a Cactus Bug
Narnia femorata (Hemiptera: Coreidae)

Fae Nageon de Lestang


The effects of environment on sexual selection, specifically male competition, have yet to be fully explored. We examined
the effects of natural environmental heterogeneity on male scaling, size, and competitive success in a cactus bug, Narnia
femorata. Male N femorata were raised in two alternate environments (cactus with fruits and cactus without fruits) from
the fifth instar and checked daily until they reached adulthood. Males raised in a treatment with both cactus pads and fruit
developed faster and weighed more overall. Upon reaching sexual maturity, a subset of males was randomly chosen,
paired by treatment, and placed with a virgin female. Then their behaviors were recorded for time intervals over a period
of 110 minutes. We observed matings in 8 of 29 contests, all by males raised in the treatment with both cactus pads and
fruit. Noneof the males raised on a treatment of just cactus pads mated. Successful males were larger and expressed
relatively larger sexually-selected traits for their body size. These results suggest that natural environmental variation has
a significant effect on male competitive success for the species.


INTRODUCTION

Sexual selection has been a greatly debated concept
since it was first defined by Darwin in 1871. Darwin
originally described sexual selection as an advantage in
reproductive success among individuals (Darwin, 1871;
Andersson, 1994). Traits from successful individuals
would presumably be passed on to their offspring, and thus
the process of sexual selection would ensue (Pagel, 2002).
An important facet of sexual selection, intrasexual
selection, involves competition between members of the
same sex for access to the opposite sex and is generally
most intense in males. Intersexual selection (mate choice)
occurs when individuals select mates based upon preferred
traits. Females are often the choosier sex; hence, this
process is commonly termed female choice (or female
preference). The products of sexual selection are usually a
combination of both intrasexual and intersexual selection
(Moore & Moore, 1999; Hunt et al., 2009).
The factors that determine the success of an individual
may be more complex than initially imagined. While
heredity undeniably affects sexual selection, we have only
recently begun to address effects of natural environmental
variation on the processes and outcomes of sexual
selection. Recent empirical research has shown both social
and physical environments to impact male competitive
success in contests across many taxa, yet our understanding
of male combat and environmental heterogeneity is still
incomplete (Hubbell & Johnson, 1987; Young & Stanton,


1990; Clark et al., 1997). In this study, we explored
influences of natural environmental variation on male
development, size, shape, and competitive success. The
following question was addressed in our research: what
effect does environmental variation have on individual
morphology, and consequently, male contest outcome?
Based on results from other species (Miller & Emlen,
2010), we predicted that natural environmental variation
would have a significant effect on all aspects of male
growth and development and ultimately affect competitive
success.
We tested our prediction using Namia femorata, a
species of cactus bug in the family Coreidae. Coreids, or
leaf-footed bugs, are diverse and commonly have enlarged
and ornate hind legs (Rodriguez, 2005). Males often use
their hind legs to fight for access to females (Miyatake,
1997; Eberhard, 1998). Although Coreids are a very
diverse and interesting group, they have been largely
overlooked as a study group (an exception, however, is
Miller & Emlen, 2010).
The study species, N femorata, is found primarily in
Central America, the southwestern United States, and, most
recently, Florida (Baranowski & Slater, 1986). N femorata
feeds primarily on prickly pear and cholla cactus in the
genus Opuntia,both on the cactus cladodes (or pads) and its
reproductive structures (flowers and fruit). Females lay
eggs directly along the spines of cactus, and juveniles
aggregate on the cactus cladodes until they mature.
Juveniles reach adulthood in five nymphal instars. There


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





FAE NAGEON DE LESTANG


are approximately two generations of N. femorata per year
(Baranowski & Slater, 1986). Females spend the majority
of their time feeding on cactus fruits, and males establish
and defend territories on the cactus fruit against other
males. Males have enlarged femurs complete with spurs
and use their large hind legs in combat, kicking other
males, or even gripping and squeezing the opposing male's
abdomen with their femurs during conflict. Prior to mating,
males typically touch females with their antennae and
proceed to mount them. Whether or not mating occurs from
this point is largely under female control.
Opuntia fruits are a favorite food of many animals,
such as deer, rodents, gophers, and rabbits, and are
removed quickly from cactus patches (Hellgren, 1994; CW
Miller, unpublished data). Also, because most Opuntia
species produce fruit only once per year, some generations
of N. femorata develop without Opuntia fruit. N. femorata
juveniles do not have wings until they reach adulthood, and
thus are largely immobile, leaving many juveniles without
access to Opuntia fruit for large parts or all of their
development. Previous work examining effects of food
treatment on N. femorata as of the first instar found that
95% of nymphs reared with fruit and cactus cladodes had
reached adulthood at 10 weeks of development, while only
7% of nymphs raised without cactus fruit reached
adulthood (Nageon de Lestang & Miller, 2009).
To mimic two different natural environments N.
femorata experience during maturation, we raised two
groups ofN. femorata from the fifth instar to adulthood on
either Opuntia humifusa cladodes and fruit or 0. humifusa
cladodes only. We hypothesized that males raised on a diet
of 0. humifusa cladodes and fruit would be more
competitively successful, be larger in size and weight, and
have faster development (Nageon de Lestang & Miller,
2009; CW Miller, unpublished data). We recorded weights
and development timing of N femorata from the fifth
instar to adulthood. After males were sexually mature, we
randomly paired them by treatment and allowed them to
compete for access to a female. Morphology and body size
were recorded for all competitions in which a male mated
with the female.

METHODS

We collected over 40 adult N femorata adults and
ample 0. humifusa cactus pads and fruits over the span of a
40-hectare area to ensure genetic diversity of the lab
population at Ordway-Swisher Biological Station,
University of Florida (820W, 29041'N) in September, 2008.
Males and females were paired, placed into separate
containers with cactus and fruits, and allowed to mate.
Each day we checked containers for egg clutches. Clutches
were placed into discrete containers where groups of
nymphs hatched and developed with ample cactus and


cactus fruit available. All insects were raised in a
greenhouse with natural lighting supplemented with
artificial lighting for a 13-hour photoperiod and a
temperature range of 21 to 380 C. All containers included
both 0. humifusa cladodes and fruits, planted in a mixture
of potting soil and topsoil. Within 24 hours of reaching the
fifth instar, we weighed each nymph within 0.001 g and
randomly transferred each juvenile to a plastic container
with a treatment of 0. humifusa with or without cactus
fruit. Fifth instars were checked daily until reaching
adulthood and were then weighed for the second time as
new adults within 24 hours. A small subset was weighed
after 72 hours. The weights and weight gain of these
individuals did not differ significantly from those weighed
after 24 hours; thus, they were included in all analyses.
From these 296 new adults, we randomly selected 58
sexually mature male N femorata for competitions. When
these 58 males were from two- to five-week-old adults, we
randomly paired and placed them into an arena. We
allowed males 24 hours to acclimate to the arena and
establish dominancy roles before any observations were
taken. None of the males used in the experiment had been
exposed to females since their fifth instar. Arenas consisted
of a plastic deli container (11.5 cm top diameter, 9.0 cm
bottom diameter, 14.0 cm height) holding one potted O.
humifusa cladode with a fruit positioned on top of the
cladode. We first marked each male's pronotum with a
paint pen for individual identification and then transferred
the males to the arenas. Markings were made with Zebra
All-Purpose Markers (Neon Orange), Elmer's Opaque
Paint Markers (Lime Green), or Sharpie Permanent Marker
(Metallic).
After 24 hours, we placed a three- to five-week-old
adult virgin female N femorata into each arena. The
greenhouse temperature was approximately 29 �C during
behavioral trials. We allowed insects fifteen minutes to
adjust to the new circumstances before beginning
observations. We recorded observations for intervals of one
minute every 11 minutes, for a total of ten time intervals
(110 minutes total). Both the location of the males and
their behaviors were recorded out of the 29 total
competitions run. Descriptions of each behavior recorded
are listed in Table 1.
We measured males from containers where mating
occurred (n=8). These live insects (n=16) were measured
within 0.01mm with Mitutoyo digital calipers. Based on
behavioral interactions among insects in this species and
related species (Miller, 2007), we considered male hind
legs to be weapons. Hind femur width was determined by
taking the mean of the measurements of both hind legs at
the third distal spine. This measurement includes both the
width of the femur itself and the length of the spine. We
used the widest part of the male femur as the measurement
point, which contacts the abdomens of rival males during


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





SEASONAL DIFFERENCES IN NUTRIENT ALLOCATION OF ARCTIC TUNDRA VEGETATION


Table 1: Behaviors recorded over each observation period.

Behavior Description
Male rapidly approached and
Attacking contacted the other male
Male walked around arena
Walking without apparent direction
Feeding Male fed on 0. humifusa
Sedentary Male did not move
Mating Male mated with the female
Male was in physical contact
Touching with the female
Male walked directly and rapidly
Approaching female towards the female
Mounting female Male mounted the female
Male mounted a copulating
Mounting pair female


escalated male-male competitions (CW Miller, personal
observation). We used pronotum width as a metric of body
size because it is easy to measure accurately and is a
common proxy for body size in insects.


Statistical analysis
We tested for an effect of the experimental treatment
(males reared with or without cactus fruit for the final stage
of development) on insect morphology with analyses of
variance (SPSS 15.0). Next, we examined the effect of
treatment on the scaling relationship allometryy) between
body size and hind femur width using ANCOVA with
pronotum width as the covariate. We first performed a
separate ANCOVA for hind femur width to test for effects
of experimental treatment on the slope of the scaling
relationship between body size and hind femur width,
indicated by a significant interaction with pronotum width.
If slopes did not differ according to pronotum width, we
examined effects of experimental treatment on the intercept
of the scaling relationship (i.e. trait size when controlling
for body size). The effect of experimental treatment on
developmental timing, weight at fifth instar, adult weight,
and weight gain between early fifth instar and adulthood
were tested using ANOVA (SPSS 15.0).
We employed multiple logistic regression analyses in
mixed models using the GLIMMIX macro of SAS to look
for associations between the natal experimental treatment
and the occurrences of each behavior. For probability of
mating, we used a paired t-test because GLIMMIX models
did not converge.


RESULTS


Effects of natural dietary differences on weight
and development
Treatment had a significant effect on development time
between insects from the two treatments. Insects raised in
the fruit treatment matured from the fifth instar to
adulthood in 9.5 days on average, while insects raised in
the no-fruit treatment took on average 13.5 days to mature.
Surprisingly, we did find a marginal difference in the
fifth instar weights of males and females selected for the
fruit treatment versus those selected for the no-fruit
treatment (F1,249 = 3.722, P = 0.055, Figure 1). Though
juveniles were randomly selected, those intended for the
fruit treatment weighed more in the fifth instar than those
intended for the no-fruit treatment. This initial difference is
small in comparison to the weight difference of adults
reared in the alternate treatments. Both males and females
reared for their final instar with access to fruits were much
heavier than those reared without access to fruits over this
developmental stage (F1,181 = 192.011, P < 0.001, Figure
1). Males with access to fruits gained on average 122%
more mass in their final instar than those deprived of fruits,
which gained only 50% more mass on average. Overall,
females gained more weight in their final instar than males.
Females with access to fruit gained an average of 129%
more mass in their final instar than those raised without
fruits, which gained 60% more mass on average in their
last instar.


0.09

0.08

0.07
05)
S0.06
_E
0)
S0.05

0.04

0.03

0.02


--- Females without fruit
-0-- Females with fruit
-- Males without fruit
A Males with fruit


Fifth instar


Adult


Developmental stage
Figure 1: Fifth-instar nymphs randomly chosen for the fruit treatment
initially weighed slightly more than nymphs deprived of fruits. During
the fifth instar, insects provided with cactus fruits gained substantially
more weight than insects deprived of fruits.


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





FAE NAGEON DE LESTANG


Effects of natural dietary differences on
competitive and mating behaviors
We observed matings in eight of the 29 containers, and
males reared with cactus fruits were significantly more
likely to mate (Table 2). In fact, not a single male reared
without cactus fruits mated during the observation window
(Figure 2). Surprisingly, no males attempted to mount a
mating pair in any of the trials, and only three males out of
all 29 trials attacked the other male during observation.
During our observation period, males reared with
cactus fruits were not more likely to be sighted on cactus
fruits than males raised on just cactus cladodes; nor were
they more likely to attack other males. However, the males
reared with cactus fruits had a greater activity level. They
were more often observed to be walking and were more
likely to approach females. Regardless, the probability of
males contacting and mounting females did not differ
across the treatments.

Effects of natural dietary differences on body size
and weapon size
In contests in which a male mated with the female,
males that completed development with cactus fruits
available were larger than males deprived of cactus fruits
in both pronotum width (body size; F1,13 = 141.02, P <
0.001) and mean hind femur width (weapon size; F1,13 =
200.49, P < 0.001, Figure 3). The slopes of the scaling
relationships between hind femur width and pronotum
width were not significantly different according to
experimental treatment (F1,11 = 0.98, P > 0.50). Therefore,
we proceeded to look for effects of the environmental
treatment on scaling intercept. We found a significant
effect of treatment on the scaling intercept between body
size and weapon size (Fl,12 = 7.96, P =0.015, Figure 3).
Males reared with cactus fruits during the final stage of
development had wider hind femurs for a given body size.





Table 2: Results of GLIMMIX models for competitive and courting
behaviors. Mating behavior was analyzed with a paired t-test.

Behavior d.f For t***
insect on fruit 1,28 <0.001
attacking male 1, 28 1.23
walking 1, 28 6.38**
approaching female 1, 28 4.94*
contacting female 1,28 0.1
mounting female 1, 28 0.22
mating 1, 28 2.05**
*P < 0.05, **P <0.02


Fruit


Without fruit


Environmental treatment
Figure 2: Males reared with fruit for the final stage of development
(grey bar) had a greater probability of mating during our observation
period than males reared without fruit.


1.5
E 1.4
1.3
1.2
S1.1
1.0

C 0.8
0.7


0.0






-r<
0


i 0



o800

2.8 3.0 3.2
P


3.4 3.6 3.8 4.0
ronotum width (mm)


4.2 4.4 4.6


Figure 3: Males achieving matings during our observation period
(closed circles) were larger and had a greater scaling intercept between
pronotum width (body size) and hind femur width (weapon size) than
males that did not achieve matings (open circles). All males that
achieved matings had been reared with cactus fruits for the final stages
of development. Cactus illustrations feature cladodes with and without
cactus fruits. Illustrations by David Tuss.

DISCUSSION

Weight and development
Males raised with fruit gained considerably more
weight over the fifth instar than males in the no-fruit
treatment (Figure 1). Juvenile males sorted into the fruit
treatment weighed more than males in the no-fruit
treatment initially, even though insects were randomly
selected for treatment. While this weight difference is
unusual and unexpected, treatment had such a profound


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





SEASONAL DIFFERENCES IN NUTRIENT ALLOCATION OF ARCTIC TUNDRA VEGETATION


effect on adult weight that the small initial disparity does
not likely account for the large difference in adult weights.
Males raised in the fruit treatment developed signif-
icantly faster than males raised in the no-fruit treatment.
Insects were sorted into the two treatments for only the
final stage of development; thus, effects of food treatment
would have likely been much more pronounced had food
treatment lasted for their entire development (Nageon de
Lestang & Miller, 2009).

Competitive and mating behaviors
Treatment had no effect on attack behavior in males
(Table 2). However, it is possible that males had already
established dominancy roles within the container prior to
observation, making the likelihood of observing an attack
improbable (Crespi, 1986). We found that males raised on
the fruit treatment were more likely to approach females
(P= 0.0345) but not more likely to contact or mount
females over the observed period (P= 0.7554) (Table 2).
There are a few possible explanations for this phenomenon.
It is possible that males from the fruit treatment were more
active overall than males from the no-fruit treatment due to
their more nutrient-rich diet (Sillanpaa, 2008; Low et al.,
2009). Males from the fruit treatment may have already
established dominancy within the arena, discouraging
males from the no-fruit treatment from approaching the
female and risking a potentially energy-intensive fight.
Males raised without 0. humifusa fruit were introduced to
the arenas 24 hours before any observations were taken and
may have been feeding on the fruit during this period.
Thus, males from the no-fruit treatment could have been
less active overall because they were digesting the fruit
they had previously ingested. Males raised with a more
complete diet may have had heightened sensory
capabilities, therefore making them more likely to locate
and contact females within the arena. Many studies have
shown a relationship between quality of diet and sensory
capabilities in insects (Chapman & Lee, 1991; Bernays &
Chapman, 1998; Triseleva & Safonkin, 2006).
Ultimately, males from the two treatments were
equally likely to mount a female within the given
observation period, yet only males raised with access to
cactus fruit successfully copulated with females (n=8,
Figure 2). This surprising result indicates the considerable
impact of food treatment and, more broadly, natural
environmental variation on competitive success for males
in this species. Given this information, we can assume that
female preference may have played a large role in the
results obtained (Hunt et al., 2009). Nevertheless,
measuring mating success provides us with a strong
indicator of reproductive success and, ultimately,
competitive success of the individual.


Body and weapon size
Male N femorata that mated with females during the
competitions were all from the fruit treatment and had a
significantly larger body size based on pronotum width
(Figure 3). The allometric relationship between femur
width and body size also differed across treatments. Males
from the fruit treatment had proportionately larger femurs
than males in the no-fruit treatment when comparing femur
width to body size. Because male femurs are used as
weapons and may also be ornaments, these results indicate
that males from the fruit treatment may have a considerable
advantage over males from the no-fruit treatment. This
evident disparity between insects from the two treatments
could be due to the higher quality of nutrition insects from
the fruit treatment experienced. Males raised with access to
0. humifusa fruit were presumably able to allocate more
energy and/or resources to developing this secondary sex
trait, indicating the importance of developmental environ-
ment for morphology in this species. The correlation we
found in body and leg size was not only due to apparent
success in male N femorata, but also reflects the effects of
treatment on the morphology of the insects. A random
sample of measurements from males raised on the two food
treatments showed similar morphological differences (CW
Miller, unpublished data).
We found environmental variation to have a significant
effect on the morphology, weight, development, and
behavior of male N femorata. Though we tested our
hypotheses by administering two different food treatments
to the N. femorata test population, it is important to realize
that these treatments represent two drastically different
environments to the species. Moreover, it is extremely
likely that juveniles living in close proximity will develop
in one of these two environmental conditions.
Other studies investigating effects of environmental
heterogeneity have found natural environment variation to
have a significant impact on male competitive success in a
broad spectrum of organisms (Eberhard & Gutierrez, 1991;
Folstad & Karter, 1992; von Schantz, 1999; Garant et al.,
2001). Such results indicate that environmental variation
may have far-reaching implications in the field of sexual
selection.
In conclusion, we found natural environment variation
to have staggering effects on the morphology, weight,
development, and competitive success of male N femorata.
These results suggest alternate environments may affect the
process of sexual selection for the species. More research is
still needed to further examine the impact of environmental
variation on the many aspects of sexual selection

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University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





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University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010