Title: To eat or not to eat?
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Permanent Link: http://ufdc.ufl.edu/UF00102808/00001
 Material Information
Title: To eat or not to eat?
Physical Description: Book
Language: English
Creator: Freed, Arthur Nelson, 1952-
Copyright Date: 1982
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Bibliographic ID: UF00102808
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: ltuf - ABU4401
oclc - 08835271

Full Text








I wish to thank Martha L. Crump and John F. Anderson for the

interest and guidance they provided. M.L. Crump, J.F. Anderson, and

T.J. Walker kindly reviewed this entire manuscript many times. In

addition, P. Feinsinger, R.G. Jaeger, R.A. Merz, R.A. Alford, and

Y.B. Linhart critically reviewed various portions of this dissertation.

R.A. Alford developed the electivity equation presented in this work.

I am grateful to the following people who helped identify particular

invertebrates: J. Deisler and G. Goodfriend (Stylommatophora),

G.B. Edwards and J. Reiskind (Araneae), D.H. Habeck (Lepidoptera and

Coleoptera larvae), R.I. Sailer (Hemiptera), and T.J. Walker

(Orthoptera). I thank J. Stevenson of the Florida Department of

Natural Resources for permission to conduct this research on Paynes

Prairie State Preserve. The work presented here was partially

supported by a Sigma Xi Grant-in-Aid of Research and NSF Pre-Doctoral

Dissertation Grant DEB-8019502. Finally, I would like to thank my

parents for their support and my undergraduate education, which made

this dissertation possible.


ACKNOWLEDGEMENTS . . . . . . . . . . .

ABSTRACT . . . . . . .. . . . . .

OVERVIEW . . . . . . . . . . . . .




. . ii

S. . 1i

. . 3

Introduction . . . . . . .
Methods . . . . . . . . .
Results . . . . . . . . .
Discussion. . . . . . . . .


Introduction. . . . . ... . . . . . 30
Methods . . . . . . ... .. . . . . 31
Results . . . . . . .... . . . . . 33
Discussion. . . . . . ... . . . . . 43





LITERATURE CITED . . . . . . .... . . . . . 58

BIOGRAPHICAL SKETCH. . . . . . ... . . . . . 63

Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy



Arthur Nelson Freed

May 1982

Chairperson: Martha L. Crump
Major Department: Zoology

Frogs have long been considered opportunistic predators. In

order to determine if the green treefrog (Hyla cinerea) is a

non-selective forager, I studied the feeding ecology of this frog on

Paynes Prairie State Preserve, Florida, and performed prey selection

experiments in the laboratory. The comparison of prey from frog

stomachs with prey available in the habitat (determined by tack-trap

and sweep netting) indicated that prey selection was not associated

with prey availability. Larvae of the families Noctuidae and

Cantharidae were the two prey most frequently eaten by H. cinerea in

the field, but the most common insect families in the habitat were

Chloropidae, Cicadellidae, and Entomobryidae. In laboratory preference

experiments, the two former prey were ranked highest among the eight

natural prey offered to treefrogs. However, the house fly (Musca

domestic) was consistently selected over all natural prey. The house

fly was included in these experiments for comparative purposes, since

it readily elicits feeding behavior from H. cinerea in the laboratory.

Three genus-specific variables influenced prey selection by

Hyla cinerea: (1) prey length, (2) activity patterns exhibited by prey,

and (3) the proportion of time devoted by prey to specific behavior

patterns. The quality of feeding stimuli (estimated in captures/h)

varied within the behavioral repertoire of a genus as well as among

genera performing similar activities. Stimulus quality did not

influence the degree or speed of the response exhibited by the

treefrog, but only the probability of occurrence of that response.

Natural prey items that were consistently selected in the laboratory

possessed length/width ratios greater than 3:1. The average visual

angle subtended by prey used by treefrogs as a cue during selection

was 2.30 at an average distance of 22.5 cm. Crawling velocity of prey

was not an important selection parameter, but tended to be positively

associated with the stimulus value of crawling prey.


Hyla cinerea is a common green treefrog found throughout the

southeastern United States. In north-central Florida green treefrogs

are arboreal and at times semi-aquatic. HyZa cinerea is a nocturnal,

visually oriented, sit-and-wait pursuer (Schoener 1969) that feeds on

a variety of invertebrate prey. Foraging begins in the early evening

and continues until dawn. During the day, individuals rest in positions

concealed by vegetation, or bask (Freed 1980a) in exposed positions on

grasses and shrubs.

Field work was done on Paynes Prairie State Preserve, located

8 km south of Gainesville, Florida. Hardwood meso-xeric hammock

surrounds the study site that consists of three temporary sinkhole

ponds adjacent to Alachua Sink. In March, ponds begin drying up, and

the area is eventually overgrown by annual plants. Pigweed (Amaranthus

australis), bagpod (Sesbania vesicaria), and sicklepod (Cassia

obtusifolia) are the most plentiful annuals. Interspersed among these

weeds grow several common grasses: coast cockspur (EchinochZoa ualteri),

vaseygrass (Paspalum urvillei), and guineagrass (Panicium maximum).

The vegetation of an area is very important to a treefrog

population, since it provides cover, and influences the potential prey

available. All invertebrate prey are directly or indirectly associated

with these plants. Most of these invertebrates utilize one or more of

these plants as food. Stinkbugs (Euschistus and Oebalus) suck plant

jucies whereas caterpillars, flea beetles, and snails (Spodoptera,

Disonycha, and Polygyra respectively) chew or rasp leaves or stems.

Those invertebrates that do not rely on plants for food utilize

vegetation as foraging areas, and eat herbivorous insects associated

with these plants (e.g. ChauZiognathus and CZubiona). Treefrogs are

associated with the vegetation in a fashion similar to these predatory


This dissertation addresses the question of prey selection by the

green treefrog. The natural diet of the treefrogs found on Paynes

Prairie is presented in section one. The food items and their degree of

utilization pertain only to this particular population. Other

populations occupying nearby habitats may utilize different prey (see

Kilby 1945) mainly due to the differences in floral composition of that

area. Section one also includes data from laboratory preference

experiments designed to determine the degree of prey selectivity

exhibited by H. cinerea. These laboratory results are compared to data

gathered on prey selection under natural conditions.

In section two I examine the potential cues that are used by the

green treefrog in selecting prey. Consistent selection of particular

invertebrates from a variety of potential prey indicates that treefrogs

utilize certain visual cues. In this section I present those cues that

are most probably used by H. cinerea during prey selection.




Pulliam (1974) defined feeding strategy as the choices a predator

makes upon encountering different food items, i.e., whether to eat or

to ignore a prey. MacArthur and Pianka (1966) presented a model that

predicts the optimal diet of a predator based upon the availability of

prey species. Later models considered prey availability and also the

relative value of the prey to the predator. Emlen (1966), Rapport

(1971), Schoener (1971), Cody (1974), Pulliam (1974), Ellis et al.,

(1976), Estabrook and Dunham (1976), Pyke et al.,(1977), Hughes (1979),

Sih (1979), and Stenseth and Hansson (1979) defined an optimal diet as

one that maximizes energy per unit time. The importance of prey

nutrient value to the predator is also recognized (Marten 1973; Pulliam

1975; Rapport 1980). None of these models explains mechanisms allowing

for predator discrimination among prey items.

Amphibians in general, and ranids, bufonids, and hylids in

particular, are described as indiscriminate predators (Korschgen and

Baskett 1963; Johnson and Bury 1965; Klimstra and Myers 1965; Linzey

1967; Heatwole and Heatwole 1968; Hedeen 1972; Kramek 1972; Bury and

Martin 1973; Blackith and Speight 1974; Clark 1974; Tyler and

Hoestenbach 1979). Oplinger (1967), Johnson and Christiansen (1976),

and Labanick (1976), all working on hylids, concluded that availability

and abundance of prey rather than preference determine the type of

food consumed by these frogs. Conversely, Cott (1940), Sweetman (1944),

Bragg (1957), Turner (1959), Brower and Brower (1962), and Toft (1980)

suggested anurans are capable of discrimination among prey. Ingle and

McKinley (1978) and Ewert et al. (1979) showed that anurans respond

differentially to various artificial prey stimuli. I showed that

activity patterns of insect prey are differentially utilized as cues in

prey selection experiments with Hyla cinerea (Freed 1980b).

In behavioral studies, both Ewert (1968) and Ingle (1968) reported

frogs and toads show consistent size-selectivity in choice of

artificial prey. The configuration of a moving stimulus is an important

selection parameter (Ewert 1968; Borchers et al., 1978; Ingle and

McKinley 1978). Worm-like forms are better feeding stimuli for toads

than similarly moving square objects. However, small square stimuli

moving in a stepwise fashion elicit stronger feeding responses from

toads than squares moving at constant velocity (Borchers et al., 1978).

These experimental stimulus movements represent crude simplifications

when compared with the diversity and complexity of activity patterns of

natural prey (Borchers et al., 1978).

I combined a field study on treefrog feeding ecology with

laboratory preference experiments utilizing natural prey. Emphasis was

placed on the quality of the behavior patterns exhibited within and

among nine prey types. These activity patterns may be used as cues for

selection of prey by a foraging predator. The purposes of this study

were (1) to identify the prey most frequently eaten by Hyla cinerea in

the field, (2) to determine the degree to which prey discrimination is

developed in the green treefrog, and (3) to estimate the strength of the

behavioral cues utilized by the frog during prey selection.


Field work on hylid feeding ecology was conducted between May

1977 and May 1978 on Paynes Prairie. Stomach contents of 163 Hyla

cinerea were obtained via pumping (Legler and Sullivan 1979). Prey

abundance was estimated by using Tack Trap)placed on the ground and

0.6, 1.2, and 1.8 m above the ground. Each 315 cm2 plastic sheet of

Tack Trap was left overnight or for a 24 h period. Though a

significantly greater total number of prey were sampled on ground than

aerial traps (X2 = 518, df = 18, P < 0.001), samples were combined for

analysis since family representation at all levels was similar (Spearman

Rank Correlation, r8 = 1, P = 0, N = 8). Sweep net samples were taken

between 2100 and 2400 h.

Arthropods were identified to family, and where possible, to

genus and species. The rank of prey in the diet of Hyla cinerea was

determined by calculating the percent biomass-occurrence (%BO) for

each item:

SM. N.
%BO = xx 100 10(

where M. is the estimated mass of prey i, Mt is the estimated mass of

all prey items eaten by the sample population, N. is the number of

stomachs containing prey i, Nt is the total number of stomachs

containing food. The index %BO is the geometric mean of two common

indicies, % biomass and frequency of occurrence. This compound index

determines prey rank based on evenly weighting the potential value of the

prey and its frequency of utilization by the predator. It minimizes the

effect of extremes in the ranking process, such as abundant tiny prey or

rare large prey, either of which may be insufficient to meet the long

term energy requirements of the predator. The length of all whole prey

items was measured, and mass estimated by using length-mass regression

equations prepared by myself (Appendix 1) or taken from Zug and Zug

(1979). The %BO was calculated for the most frequently utilized prey

genera, and then divided by the summed %BO to adjust these values to 100

percent (Table 1).

Thirty-five (18 male, 17 female) Hyla cinerea (Snout-Vent-Length:

X SE = 4.74 0.06 cm, Range= 3.8 5.2 cm) were collected during

the summer of 1980. They were maintained at room temperature (240C,

light uncontrolled) in the laboratory 1 wk prior to experimentation.

The eight top ranked genera of prey as determined by %BO, and the house

fly (Musca domestica, were used as prey for the preference experiments.

Palthus was omitted from the top eight (Table 2) due to its similarity

to Spodoptera. The house fly was included for comparative purposes

since treefrogs preferred this insect over four mosquito species in

laboratory experiments (Freed 1980b). The eight prey were divided into

two subgroups, each including the house fly. The subgroups were made up

of all odd or all even ranked prey as determined by %BO. Clubiona was

switched with Oebalus to avoid having two pentatomids in subgroup 2.

Subgroup 1 consisted of Musca, Spodoptera (noctuid caterpillar),

Gryllus (cricket), Oebalus (stinkbug), and Polygyra (land snail).

Subgroup 2 included Musca, Chauliognathus (soldier beetle larva),

Euschistus (stinkbug), Clubiana (spider), and Disonycha (leaf beetle).

These two subgroups were used in 40 initial trials. Ten trials

involving each subgroup were run in daylight (20 trials), and the

remaining 20 trials were run at night under red light. The latter

Table 1. Percentage biomass-occurrence of the most frequently
utilized prey genera in the diet of Hyla cinerea.

Stomachs Est.

Genus Family Number with biomass Adj.

item (g) %BO

Spodoptera (larva)

ChauZiognathus (larva)

Gry ZZus








Carib atta


































































Table 2. Comparison of the percent abundance of prey families
represented by at least two individuals in the diet of Hyla
cinerea with relative abundance estimates derived from tack
trap and sweep net sampling. N equals the total number of
individuals obtained for each type of sample.

Family Stomach Tack trap Sweep net Tack + Sweep

Noctuidae 15.21 2.88 2.51 2.72

Cantharidae 10.74 0.24 0.68 0.44

Pentatomidae 8.29 0.24 5.03 2.35

Formicidae 6.49 1.38 7.01 3.86

Chrysomelidae 6.04 1.56 1.37 1.48

Curculionidae 5.82 0.60 2.44 1.41

Clubionidae 5.15 0.78 2.06 1.34

Alleculidae 4.70 0.06 0.08 0.07

Sminthuridae 3.58 17.66 0.23 9.97

Gryllidae 3.13 1.32 0.76 1.07

Phalangidae 2.68 0.30 0.00 0.17

Blattidae 2.24 0.00 0.23 0.10

Cicadellidae 2.01 13.27 12.18 12.79

Lycosidae 2.01 0.30 2.51 1.28

Polygyridae 1.79 0.00 0.76 0.34

Languriidae 1.79 0.00 0.00 0.00

Reduviidae 1.79 0.00 0.91 0.40

Chloropidae 1.57 25.05 45.70 34.15

Araneidae 1.57 1.62 4.64 2.96

Table 2. Continued.

Family Stomach Tack trap Sweep net Tack + Sweep

Forficulidae 1.34 0.00 0.76 0.34

Carabidae 1.34 0.54 0.23 0.40

Scarabaeidae 1.34 0.00 0.15 0.07

Tetrigidae 1.34 0.36 0.30 0.34

Entomobryidae 0.89 28.65 0.00 16.02

Flatidae 0.89 0.00 0.00 0.00

Porcellionidae 0.89 0.00 0.00 0.00

Salticidae 0.89 0.30 1.29 0.74

Tetragnathidae 0.67 0.06 0.61 0.30

Dictynidae 0.67 0.00 0.00 0.00

Ichneumonidae 0.67 0.24 0.00 0.13

Cerambycidae 0.67 0.00 0.00 0.00

Staphylinidae 0.45 1.80 0.15 1.07

Lygaeidae 0.45 0.06 0.91 0.44

Tettigionidae 0.45 0.12 6.32 2.85

Phloeothripidae 0.45 0.60 0.15 0.40

447 1665 1313


experiments were an attempt to determine if prey selection was affected

by time of day. A total of ten prey in equal ratios (2 prey/genus)

were available during each run. Twenty frogs were each used in two

experimental trials, once with subgroup 1, the other with subgroup 2.

The three most preferred prey from each subgroup were used in the final

15 trials; a different frog was used each time. Musca and Spodoptera

were obtained from the United States Department of Agriculture Insect

Research Laboratory, Gainesville, Florida. All other prey were

collected on Paynes Prairie.

The frogs were tested in a gridded plexiglass chamber (28 x 28 x

28 cm) with white paper covering three walls of the cube; the front and

top were left uncovered to permit observation. The white wooden floor

of the chamber had a sliding panel containing two small chambers

recessed on opposite sides outside of the cube (see Figure 1, Gardner

1964). This allowed introduction of the frog into the feeding chamber.

Ten prey (2/genus) were placed into the chamber 5 min prior to the

release of the predator. During the 30 min experiment the following

were recorded: order of prey capture, estimated distance of prey from

frog at recognition, and behavior of prey just prior to capture.

The following electivity equation (ei) was developed to estimate
predator preferences. This equation produces an index that varies from

0, indicating that the prey item is completely ignored by the predator,

to 1, indicating that the predator specializes only on that prey:

e. ( )Ri 1N (2)
I 1
j3i niU)/N

where ni(j) is the number of individuals of prey species i present just

prior to capture of individual j of the species i, N is the total

number of all individuals of all prey types present just prior to the

capture of individual j, and R. is the ordinal rank of individual j

captured by the predator.

None of the measures of preference presented in the literature

(Ivlev 1961; Rapport and Turner 1970; Chesson 1978; Cock 1978) considers

the order of prey capture. These equations were designed for

application to open systems in which the predator encounters only one

or few prey types at any given time. In such a system, choice is

limited either to pursuing or not pursuing prey coming within reach. In

my experiments the predator was simultaneously presented with ten prey

items of several prey species. Discretion here included the ability to

focus on one prey species in the presence of other, potentially

distracting, prey species. Ranking allows assignment of higher values

to each successive member of a prey type captured within a rank, thus

emphasizing the predator's capacity for type-selection as the absolute

abundance of the preferred prey type decreases. Therefore, if a series

of one prey type was captured in sequence, the same rank (R.) was

assigned to all individuals in that set. In other words, if the first

three prey items consumed out of ten possible items (3 different

species) were the same kind, each of the three would carry a rank of 1,

since any of them could have been captured first. In addition to prey

rank, the relative abundance of prey types was calculated at each

predation. With the closed system utilized, prey replacement is

eliminated and rank order of prey capture becomes extremely important.

If prey selection were random, a single predation could almost halve the

chance of a second successive predation on the same species, an obvious

departure from natural conditions. The value e. is relative, and like

all measures of preference its value for a particular item will be

greatly influenced by alternative choices available at the time of

selection. Consequently, mean values of e. were used for comparing the

electivity of the various prey species.

An activity index was developed by recording the behavior of each

prey several seconds prior to capture. The following activities were

ranked 0 through 5 on the basis of increasing levels of activity:

0--motionless, 1--antennae waving, 2--grooming, 3--searching,

4--crawling, and 5--flying. Searching behavior of prey was defined

as vertical and/or lateral waving of the anterior third of the body.

If a prey was observed performing several behaviors simultaneously, it

was given the highest appropriate ranking. This index is the mean rank

of all recorded pre-capture and first attempted capture activity for a

particular prey genus. Uneaten prey were assigned a 0 in order to

avoid bias that results in an increase in index values for genera that

characteristically spend long periods of time motionless and thereby

escape predation.

An activity budget was developed for each prey genus by recording

the time spent on each characteristic behavior during a 2 min time

period. Depending upon the number of behavior patterns commonly

displayed by a genus, a total of 6 to 12 min were spent on each

individual to record all activity. The behavior of ten individuals

from each genus was recorded during daylight and an equal number for

each genus was recorded at night under red light. This time budget was

for isolated prey and differs from the activity index in that the latter

involved interacting individuals and included predator disturbance.

The time budget in conjunction with the level of predation

recorded for each activity pattern of each genus allowed the

determination of the strength of feeding stimulation (s) associated

with prey behavior. The following index provides an estimation of the

stimulus strength (in predations/h) for each behavior pattern exhibited

by prey:
Pi(x) (3)

i (x) Ti

where P. is the number of predations and/or attempted captures on
prey genus i performing behavior x, ti(x) is the proportion of time

normally spent performing behavior x by prey genus i in the absence of

the predator, and T. is the total number of hours prey i was exposed to

the predator.

Lastly, the difficulty of capture (I) for each prey was

calculated by dividing the number of misses of prey i by the number of

capture attempts recorded for that prey genus.


Families of prey represented in the sample population by at least

two individuals in the stomach contents of Hyla cinerea are presented in

Table 2. A list of all identified prey is included in Appendix 2.

Spearman's rank correlation was used to compare the proportions of prey

from stomachs with those of potential prey available in the habitat.

Tests performed using prey families with a minimum of five items in

stomachs (N = 7) showed that no significant correlation exists between

prey eaten by treefrogs and prey available for consumption. However, a

significant correlation was found when all 35 prey families from

Table 2 were compared to the proportions of prey obtained from sweep

net (rs = 0.41, P = 0.02) and combined (rs = 0.34, P = 0.05) samples.

Depending on the number of prey families included in the statistical

analysis, it is possible to conclude that either the frog was a

non-selective feeder or a discriminating forager. Naturally rare prey

families were rarely included in the frogs' diet. Therefore, by adding

many infrequently utilized prey families to the Spearman rank analysis,

the coefficient increasingly approaches significance.

Since no significant difference (ANOVA, F = 0.01, P = 0.91) was

found between the e.'s generated by frogs under the two different light

regimes, data from light and dark experiments were combined. Musca was

the most preferred prey item in laboratory experiments (Table 3).

Spodoptera and ChauZiognathus larvae ranked respectively second and

third in the initial trials, but were reversed in the standings of the

final preference experiments. This is not unexpected since both the

latter two prey were highly favored in the field by treefrogs. A

comparison of initial e.'s with field ranks determined by %BO of the

eight natural prey genera revealed a significant correlation between

these independent indicators of prey preference (Table 4A).

Prey selection significantly correlates with prey activity

(Tables 4B and 5) and length (Tables 4C and 6) but not mass (Table 4D).

A parametric test for partial correlation between predator selectivity,

prey length, and prey activity indicated that 42% (r = 0.65, P < 0.1)

and 55% (r = 0.74, P < 0.05) of the variability observed in ei are

accounted for by prey activity, and prey length, respectively.

Substituting mass for the estimate of size, the variation in e.

Table 3. Mean electivity coefficients (e.) for nine prey genera.
N equals the number of preference experiments performed.
The number of each genus used equals 2N.

Prey Genus Initial Final Combined

Musca 0.416 0.04 40 0.408 0.06 15 0.414 0.05 55

Spodoptera 0.340 0.05 20 0.137 0.03 15 0.253 0.05 35

Chauliognathus 0.269 0.03 20 0.209 0.06 15 0.243 0.05 35

Gryllus 0.136 0.03 20 0.131 0.03 15 0.134 0.03 35

Euschistus 0.075 0.04 20 0.003 0.01 15 0.045 0.03 35

Oebatus 0.074 0.02 20

Disonycha 0.071 0.02 20

Clubiona 0.049 0.01 20

Polygyra 0.000 0.00 20

Table 4. Spearman's coefficient of rank correlation, r for
predator-prey variables (two-tailed test). N equals the
number of prey genera.

Correlation N r P

A. Prey rank (%BO) vs selectivity 8 0.79 0.021

A. Prey rank (%BO) vs prey activity 8 0.79 0.021
B. Prey rank (%BO) us prey activity 8 0.75 0.031

C. Prey rank (%BO) vs prey length (stomach) 8 0.76 0.028

D. Prey rank (%BO) vs prey mass (stomach) 8 0.57 0.139 (NS)

E. Electivity vs prey activity 9 0.83 0.006

F. Electivity vs prey length 9 0.42 0.265 (NS)

G. Electivity vs prey mass 9 0.00 1.000 (NS)

H. Electivity us capture difficulty 9 -0.37 0.332 (NS)

I. Electivity vs medium cm from frog 9 -0.45 0.223 (NS)

J. Electivity vs % prey within 10 cm 9 0.29 0.452 (NS)

K. Electivity vs % prey within 5 cm 9 0.67 0.047

L. Prey activity vs % prey within 5 cm 9 0.90 0.001

Table 5. Indicators of relative activity for nine prey genera. The
activity index is calculated as the mean rank of all recorded
precapture activity as well as activity prior to the first
unsuccessful capture attempt. The activity scale runs from
0 to 5, 5 representing the greatest activity level. I equals

Prey Genus Index of Prey Activity Capture Difficulty

Musca 3.3 0.2 110 0.47 (68/145)

Spodoptera 2.1 0.2 70 0.02 ( 1/ 43)

ChauZiognathus 2.2 0.2 70 0.23 (10/ 44)

Gryllus 2.0 0.2 70 0.49 (28/ 57)

Euschistus 1.4 0.2 70 0.85 (23/ 27)

Oebalus 1.0 0.3 40 0.20 ( 2/ 10)

Disonycha 0.6 0.2 40 0.00 ( 0/ 8)

Clubiona 1.7 0.3 40 0.52 (11/ 21)

PoZygyra 1.0 0.3 40 1.00 (13/ 13)

Table 6. Size of nine prey genera. Widest shell diameter was used for measurements of Polygyra.
Measurements of shell width/extended length for five Polygyra resulted in the addition of 65%
of the shell diameter to the mean shell width for the estimate of extended length shown in

Live Measurements Estimated Measurements (stomach)

Prey Genus Mass (mg) Body length (mm) Mass (mg) Body length (mm)

Musca 10 12 10 6.4 0.1

Spodoptera 20 158 10 24.9 1.0 20 102 30 17.0 2.0

Chauliognathus 20 16 10 10.6 0.5 46 17 10 10.8 0.6

Gryllus 20 244 50 14.7 0.9 7 149 50 10.4 1.0

Euschistus 20 76 10 12.1 0.4 12 36 10 9.5 0.5

Oebalus 20 32 10 9.7 0.2 9 31 10 9.6 0.3

Disonycha 20 18 10 6.9 0.1 22 14 10 6.3 0.1

Clubiona 20 13 10 6.0 0.2 18 8 10 4.0 0.3

Polygyra 18 60 10 5.8 (9.6) 0.1 7 68 20 5.5 (9.1) 0.7

accounted for by activity and mass is 51% (r = 0.71, P < 0.05) and

0.01% (r = 0.009, P > 0.05) respectively. When considering all nine

prey items including Musca, prey selection is significantly associated

with prey activity (Table 4E) but not with size (Tables 4F and G).

Partial correlations indicated that activity accounts for 77% (r = 0.88,

P < 0.05) of the variation observed in e whereas length accounts for

only 21% (r = 0.46, P > 0.05) of the observed variation. Using mass as

an estimator of size, mass accounts for 0.25% (r = -0.05, P > 0.05) of

the observed variation of e whereas activity accounts for 74%

(r = 0.86, P < 0.05) of the observed variability of the electivity


The activity time budget for each genus is presented in Table 7.

There was no significant difference (t tests, P > 0.05) in prey

behavior between day and night observations, and therefore the data were

combined. The activity index may be considered circular, since it

relies on predator response as a signal for recording prey behavior.

Nevertheless, this index best represents the observed activity of prey

during an experiment. The prey-specific activity indices are

significantly correlated (rs = 0.59, P = 0.048, one-tailed test) with

activity ranks obtained from time budget data. These ranks were

derived by multiplying the numerical value (0-5) of each type of

behavior by the proportion of time devoted to that behavior (Table 7).

Therefore, the activity index does correlate with the activity time

budget, an independent estimator of the mean level of prey activity.

The activity patterns displayed by the prey and the proportion of

time devoted by the prey to each behavior influence prey selection.

The strength of the feeding stimulus associated with each prey genus

Table 7. Activity time budget and summary of predations associated with each activity pattern of nine prey.
Note: a = value for palp drumming. b = value for spinning silken retreat.

Behavior Pattern

Genus Variable N Motion- Antennae
less wave Groom Search Crawl Fly




activity time (X SE)


missed behavior noted

not eaten

activity time (X SE)


missed behavior noted

not eaten

activity time (X SE)


missed behavior noted

not eaten

8 3

- 40 4

- 5

- 2

47 6


- 50 4

- 64

- 12

6 2 22 3


26 5




3 3


5 1 12 8


79 3



3 0



0 0

Table 7. Continued.

Behavior Pattern

Genus Variable N Motion- Antennae
less wave Groom Search Crawl Fly

activity time (X SE)


missed behavior noted

not eaten

activity time (X SE)


missed behavior noted

not eaten

activity time (X SE)


missed behavior noted

not eaten

41 6


31 6

54 6

7 2



4 2



19 3


23 2


0 0 18 3

33 5



41 6



27 6



Gry lus



0 0


0 0

Table 7. Continued.

Behavior Pattern

Genus Variable N Motion- Antennae
less wave Groom Search Crawl Fly

activity time (X SE)


missed behavior noted

not eaten

activity time (X SE)


missed behavior noted

not eaten

activity time (X SE)


missed behavior noted

not eaten

31 4

55 8

6 1


0 0a

36 8

40 3


22 5


- -

5 2 19 7b

21 4



24 3 40 7



0 0


while performing a specific behavior is presented in Table 8. Behavior

patterns exhibited within a prey genus vary tremendously in their

ability to elicit feeding behavior in Hyla cinerea. For example,

grooming behavior in the cricket (GrytZus) is associated with a level

of predation of 0.2 captures/h whereas crawling elicits 2.9

predations/h. In addition, similar behavior patterns of different

genera vary in their stimulus strength. To the treefrog, crawling flea

beetles (Disonycha) represent a feeding stimulus 61% less effective than

crawling crickets, which in turn represent a feeding stimulus 27% less

effective than crawling spiders (Clubiona). Since 3.9 predations/h are

associated with spider crawling, it is surprising that CZubiona was

ranked only eighth in overall preference. This low rank is due to the

proportion of time the spider remained motionless (54%), greater than

any of the other prey included in this experiment. Musca's high rank is

due to the combination of two powerful frog orienting stimuli, flight

and crawling. The flight-crawl behavioral sequence accounts for 53% of

the fly's activity time budget and resulted in 8.1 predations/h.

Linkage between searching and crawling behavior patterns, associated

with 2.7 and 2.3 predations/h respectively, probably accounts for the

high ranking received by Spodoptera. Conversely, Chauliognathus

crawling at 1.3 captures/h is a mildly effective stimulus for a

highly ranked prey type, but the soldier beetle larvae spent 80% of

their observed time performing this behavior. The combination of

crawling and searching behavior (0.7 captures/h) accounts for 92% of

the beetle larvae's activity time budget.

Finally, prey selection is not affected by the difficulty

involved in capturing a prey (Tables 4H and 5), or the distance of prey

Table 8. Strength of feeding stimulus (s) associated with prey
behavior. Values in the table are for s = captures/h
(see text). a = values for palp drumming. b = value
for spinning silken retreat.

Prey Genus Behavior

Motionless Antennae wave Groom Search Crawl Fly

Musca 0.0 0.3 2.8 5.3

Spodoptera 0.1 0.0 2.7 2.3 -

Chauliognathus 0.9 0.0 0.7 1.3 -

Gryllus 0.1 2.6 0.2 2.9 -

Euschistus 0.0 1.4 0.1 1.4 2/0

Oebalus 0.0 0.0 0.0 1.9 0.0

Disonycha 0.0 1.6 0.1 1.1 -

Clubiona 0.0 0.0a 0.0 0.3b 3.9 -

Polygyra 0.3 0.6 1.0 -

Table 9. Percentage of predation experienced by each genus within a
specified distance from the treefrog. Included are estimated
median distances of frogs from prey just prior to pursuit
and capture.

Prey Genus Distance from Predator

N 0-5 cm 0-10 cm Median (cm)

Musca 84 71 92 4.0

Spodoptera 39 46 72 7.5

Chauliognathus 37 62 76 5.0

GryZlus 36 44 50 10.3

Euschistus 22 41 46 11.5

Oebalus 16 38 50 10.3

Disonycha 8 38 63 8.0

Clubiona 8 44 50 10.3

Polygyra 9 44 78 7.5

from the predator prior to attack (Tables 41, J and 9). However, there

is a significant association between ei and the percentage of predations

occurring within a 0 to 5 cm distance from the frog (Table 4K). In turn,

the percentage of predations between 0 and 5 cm is highly correlated

with prey activity, emphasizing the greater probability of predator

encounter associated with more active prey (Table 4L).


Spodoptera and ChauZiognathus were the two prey items most

frequently consumed by HyZa cinerea in the field. Both are worm-like,

a configuration shown to be a strong stimulus in eliciting feeding

(Ewert 1974; Ingle 1976). However, Musca was the most consistently

preferred food item offered in laboratory experiments in which prey were

equally available at the beginning of each trial. Previous work

(Freed 1980b) showed that Musca was most preferred, although the mean

ei generated from that study was 0.745 versus the present ei of 0.416.

Electivity is relative, and can be greatly influenced by the

alternative choices available at the time of selection. The lower e.

for Musca in this study reflects the greater stimulus value of

alternative prey. Mosquitoes were the only alternate prey available in

the previous study; they are almost completely ignored by treefrogs in

their natural habitat.

Hyla cinerea is capable of consistently selecting specific prey.

This high degree of discrimination is emphasized by similar ranks

generated in laboratory preference experiments and ranks based on

stomach content analysis of the sample population. Three prey-specific

variables influence selection under laboratory conditions: prey size,

prey activity, and the frequency of occurrence of various attack-

inducing behaviors. Prey mass, unlike length, may not be easily

discernible at a distance, and is not an important parameter. Prey

length does influence prey selection in the field. However, this

association does not exist in laboratory experiments that include house

flies. This indicates that even though prey length is an important cue

utilized by the frog, it could be overshadowed by the cues provided by

prey activity. Hyla cinerea differentially selects prey in relation to

the proportion of time a prey species remains active, as well as the

types of activity most often displayed. An increase in the proportion

of a specific prey activity results in a subsequent increase in the

functional density of that prey (Werner and Hall 1974), thus resulting

in greater predation. Since the various behavior patterns within a prey

genus are not equally stimulating to frogs, the stimulus value of prey

is not fixed, but (in accordance with its activity time budget) will

fluctuate over time. Similar behaviors shared among different prey

also vary in their ability to elicit feeding in H. cinerea. This

variation is most likely related to genus-specific differences in

configuration, and in the case of crawling and flight, differences in

velocity (Luthardt and Roth 1979a). Hyla cinerea select prey that most

frequently display the behaviors of greatest stimulus value. Thus if

Clubiona partitioned their activity time in a manner similar to Musca or

Chauliognathus, these spiders would probably rank equal to or higher

than house flies or soldier beetles in the diet of the treefrog.

The difficulty involved in capturing each prey does not influence

prey choice; however, several interesting observations were noted.

First, the humeral spines located on the prothorax of Euschistus are an

effective predator deterrent. Treefrogs repeatedly rejected stinkbugs

after lodging these spines between their jaws. Second is the

unexpected difficulty encountered by frogs attempting to capture the

pulmonate land snail Polygyra. The adhesion of the snail to the

substratum by mucus secretion interrupted the timing of the frog.

This resulted in the jaws closing slightly before the snail was

completely in the mouth, forcing the snail off the frog's tongue.

Finally, Chauliognathus larvae, assumed to be distasteful due to the

presence of cantharidin (Carrel and Eisner 1974), were consistently

eaten by some individuals and completely ignored by others. The

propensity of some individuals to select ChauZiognathus could be

attributed to learning (Luthardt and Roth 1979b) or possibly to

polymorphism (Arnold 1977) in the hylid population.

In conclusion, the great variety of behavior patterns exhibited

by the different prey form a complex of cues that differentially

influence feeding behavior in Hyla cinerea. Behavioral patterns vary

within and among prey genera in quality, i.e., in the ability to elicit

orientation and prey capture by the frog. Selection is closely related

to the time devoted by each prey to specific behaviors. Size, as

measured by length, is also a cue used in the selection of prey but can

be overshadowed by prey behavior. Behavior modification is probably a

major evolutionary process affecting the interactions between visually-

oriented predators and their prey. Predators that utilize only cues

that represent large proportions of the prey's activity time budget

should have a selective advantage. Conversely, a reallocation of the

prey's available activity time away from high risk behavior patterns or

changes in the level of activity thus altering its stimulus value are


mechanisms that may function over ontogenetic as well as evolutionary





Discrimination is a prerequisite for prey selection. Upon

encountering potential prey, predators evaluate the suitability of that

item. This evaluation results in the decision to pursue or ignore the

prey. Behavioral studies of different species of anurans demonstrated

that size (Ewert 1968, 1970; Ingle 1968, 1971; Ingle and Cook 1977),

configuration (Borchers et al., 1978; Ewert and Kehl 1978; Ewert et al.,

1979; Ingle and McKinley 1978), velocity (Ewert 1968, 1970; Ewert

et al., 1979) and movement pattern (Borchers et al., 1978; Ingle 1975)

of artificial prey stimuli are important parameters in eliciting

feeding behavior. Since no biological relationship exists between the

natural food items and the dummy prey stimuli used, data that are the

product of predator encounters with artificial prey lack ecological


In Section I, I showed that green treefrogs (Hyla inerea) select

natural prey according to the behavior patterns displayed by the prey,

and the proportion of time devoted to each prey-specific behavior (also

see Freed 1980b). Prey length was also an important selection

parameter, but its value as a cue may be overshadowed by prey activity.

The following hypotheses were formed based on experimental observations:

(a) preferred prey items elicit larger turns during orientation by

H. cinerea than prey items of lower preference, (b) the time involved

for the predator to recognize and attack prey, as well as the latency

period separating these two events, varies inversely with prey rank in

the frog's diet, and (c) highly preferred prey elicit predator

recognition at greater distances than lower ranked prey. The purposes

of this study were (1) to test these hypotheses and determine if these

variables are suitable indicators of predator preference, and (2) to

quantify the natural cues utilized by the green treefrog during the

process of prey selection.


Ten adult (6 males, 4 females) Hyla cinerea (SVL: X SE = 4.6

0.1 cm, Range: 3.8 4.9 cm) were collected in Gainesville, Florida.

Treefrogs were maintained in 1 L containers, each housing three or four

frogs. Upon capture, individuals were fed once with house flies. Frogs

were then starved four days prior to and between experimental trials to

minimize any effect of satiation. The frogs were fed only during the

experimental trials. The experimental chamber was the plexiglass cube

described previously. The top and the front plexiglass walls were left

uncovered to allow lighting and filming of the predator-prey encounter.

A digital stopwatch was positioned in the lower left corner of the

chamber allowing timing of sequences.

Previous experiments in Section I allowed treefrogs to choose from

a variety of prey; the present study focused on one-on-one predator-prey

encounters. The same nine invertebrates (Table 3) described in Section

I were used in these trials. Frogs were placed in the experimental

chamber a minimum of 15 min prior to the beginning of the experimental

trial. After a frog assumed a resting position, a prey item was placed

in the chamber at the farthest point away from and directly behind the

frog. An aspirator was used to position the prey item. At the time the

prey was placed in the chamber, the stopwatch and the movie camera were

started simultaneously. Since the fly was known to be a highly

preferred food item, it was always used to start a session, testing the

readiness of the frog. After capturing the prey, the next trial did not

begin until the frog reassumed a resting position. If a prey was

ignored by the treefrog, the trial was terminated at the end of 5 min.

Predator-prey interactions were recorded on 16 mm Kodak black and

white Tri-X reversal film using a motorized Bolex H-16 Rex movie camera

with a 10 mm wide angle lens. A remote controlled Viewlex M-16 TA 16 mm

stop-frame projector allowed single frame analysis of predator and prey.

The following data were recorded directly from the film: time to

recognition of prey, time to attack, prey behavior at the time of

recognition, prey behavior just prior to prey capture, prey length, prey

velocity, and coordinate location of frog and prey at the time of prey

recognition by the predator. Recognition was defined as the first

visible movement of the frog in response to the prey. This movement

varied from a head twitch to the initiation of orientation towards the

prey item. Since prey width could not always be measured accurately

from the film, it was estimated from measurements of prey length using

various length-width regression equations developed for each prey

(Appendix 3). The period of latency separating recognition and attack

was calculated as the difference between these variables.

Location data were used to recreate the positions of predator and

prey at the time of prey recognition by the frog. This was done by

using a gridded cardboard box that was identical in size to the feeding

chamber. Two walls were removeable to facilitate measurements: a model

frog and prey were attached to the walls in their respective positions,

and a string was tightly strung from the prey to a point between the

model frog's eyes. Three dimensional orientation (Mardia 1972) by the

treefrog was determined by measuring the angle of rotation about the

horizontal axis of the frog and the frog's angle of vertical elevation

towards the prey. The distance between the predator and the prey item

was also measured. The angle of the frog's visual field subtended by

prey (e) was calculated by incorporating the size of the prey (length

and width) and the distance at the time of recognition:


Since Ingle and Cook (1977) showed that within 15 cm, Rana pipiens

feeding behavior is dependent on real object size and visual angles are

only utilized at farther distances, visual angles are presented only for

prey that were recognized by the frog at distances greater than 15 cm.


The mean spherical direction in which Hyla cinerea oriented toward

each prey is presented in Figure 1 and Table 10. A Watson and Williams

multi-sample test (Mardia 1972) indicated that a significant difference

exists (F = 2.18; df = 8, 100; P < 0.05) among the mean angles rotated

by treefrogs toward the various prey types. However, Spearman's test

for rank correlation showed that no significant correlation exists

between prey ranks (e.) generated in laboratory preference experiments

(Table 3) and the frog's mean directional rotation towards a prey genus

(Table 11A).


\ -4


2 \ 9
S 135


Schematic illustration showing mean rotational orientation
towards each of nine prey. The frog is located at the center
of the sphere and the prey, represented by dots, are located
on the surface of the sphere. Numbers 1 through 9 refer to
prey listed in Table 10. Concentric circles represent
degrees latitude that define vertical location of prey.

Figure 1.

Table 10.

Mean spherical direction (Mardia 1972) of the orientation
of Hyla cinerea towards various prey genera. S = the
spherical variance. N = the number of predator-prey
encounters analyzed.

Prey Genus Horizontal Angle Vertical Angle S N
(degrees) (degrees)

1. Musca 86 37 0.3 18

2. Spodoptera 155 45 0.2 16

3. Chauliognathus 164 16 0.1 9

4. Gryllus 122 39 0.2 14

5. Euschistus 132 42 0.3 13

6. Oebalus 142 20 0.1 10

7. Disonycha 147 25 0.1 8

8. CZubiona 134 26 0.2 9

9. Polygyra 140 26 0.1 12

The average time used to identify and subsequently attack each

prey, the latency period separating these two events, and the mean

distance of the prey from the predator at the time of recognition are

presented in Table 12. A Kruskal-Wallis one-way analysis of variance

(Siegel 1956) indicated that time between prey introduction and

recognition by the frog does not significantly differ among the nine

prey (X2 = 13.6, df = 8, P = 0.09). No significant correlation exists

between predator electivity (e.) and the frog's time to recognition of

prey (Table 11B). However, when considering only the eight natural

prey, a significant negative correlation was found between the time to

predator recognition and the average crawling velocity of the prey

(Table 11C). The correlation for all prey, including the house fly, is

also negative but not significant (Table 11C). A significant difference

did exist among the treefrog's latency periods separating the times for

recognition and attack (X2 = 31.5, df = 8, P = 0.0001). However, no

association was found between predator electivity and these latency

periods (Table 11D). Similarly, there are significant differences in

the amount of time preceding predator attack (X2 = 25.7, df = 8,

P = 0.001), but there is no significant correlation between e. and

the time to attack for each prey genus (Table 11E). In addition, the

average distance at the time of recognition for each prey does not

significantly differ (X2 = 9.9, df = 8, P = 0.27), and no significant

correlation is observed when rank order of prey selection is compared

to these distances (Table 11F).

Significant differences exist for both length (X2 = 81.6, df = 8,

P = 0.0001) and width (X2 = 80.6, df = 8, P = 0.0001) among the nine

prey genera (Table 13). A significant correlation with prey selection

Table 11. Spearman's coefficient of rank correlation, r for predator-prey variables from one-on-one
encounters of Hyla cinerea with prey (two-tailed test).

Correlation Natural Prey (N = 8) Prey (N = 9)
r P r P

Electivity vs horizontal angle of orientation

Electivity Vs time to predator recognition

Time to predator recognition vs prey velocity

Electivity us predator latency to attack

Electivity vs time to attack

Electivity vs prey distance at recognition

Electivity vs prey length

Electivity us prey width

Electivity vs length/width ratio of prey

Electivity vs visual angle of prey length at

distances > 15 cm





































0.23 0.546(NS)

0.76 0.028

Table 11. Continued.

Correlation Natural Prey (N = 8) Prey (N = 9)
r P r P
8 8

K. Electivity us visual angle of prey width at

distances > 15 cm -0.41 0.320(NS) -0.43 0.244(NS)

L. Electivity ve prey velocity (crawling) 0.17 0.693(NS) 0.37 0.332(NS)

M. Stimulus of crawling prey vs predator latency -0.69 0.058(NS) -0.70 0.036

N. Stimulus of crawling prey vs prey velocity 0.67 0.071(NS) 0.73 0.025

0. Electivity us stimulus of crawling prey 0.29 0.493(NS) 0.33 0.381(NS)

Table 12. Response times of Hyla cinerea for nine prey genera.
Distance was recorded at the time of prey recognition.

Time to Time to

Prey Genus Recognition Latency Attack Distance N

(sec) (sec) (sec) (cm)



Chau liognathus















































































exists for length of natural prey but not when Musca is included in the

analysis (Table 11G). This indicates that other cues (e.g., activity

level and behavior) may be more important during selection than prey

size. When prey width was substituted as the variable describing prey

size, no significant correlation with e. was found (Table 11H).

However, length/width ratios approach a significant correlation with

selection of natural prey (Table 11I).

The mean visual angles subtended by each prey at distances greater

than 15 cm from the frog's eye (Table 13) are significantly different for

both prey length (X2 = 45.4, df = 8, P = 0.0001) and prey width (X2

54.3, df = 8, P = 0.0001). The mean angles subtended in the visual

field of the frog by the lengths of the eight natural prey are

significantly correlated with prey selection (Table llJ). Including

the house fly in this comparison eliminates this association (Table llJ).

Again, the activity level and behavior that characterize Musca may have

compensated for the fly's small visual angle and induced the frog to

select it over prey subtending larger angles. Likewise, no correlation

was found relating prey selection to the visual angles subtended by the

width of prey as perceived by the frog (Table 11K).

Even though significant differences exist among the crawling

velocities of the nine prey types (X2 = 58.8, df = 8, P = 0.0001), no

significant association was found that relates these velocities to the

selection of invertebrate food items by Hyla cinerea (Table 11L).

The stimulus value of crawling in the eight natural prey approaches

a significant negative correlation with the latency period separating

predator recognition and attack (Table 11M). This negative association

is significant when all nine prey items are considered (Table 11M).

Table 13. Prey cues possibly utilized by Hyla cinerea during the
process of prey selection.

Prey Size Size Visual Angle Visual Angle Crawling

Genus Length Width Length Length Velocity

> 15 cm > 15 cm

(mm) (mm) (degrees) (degrees) (cm/sec)



Chau i ognat hus

Gry lus





























































































Table 13. Continued.

Prey Size Size Visual Angle Visual Angle Crawling

Genus Length Width Length Length Velocity

> 15 cm > 15 cm

(mm) (mm) (degrees) (degrees) (cm/sec)

Disonycha X 6.3 3.0 1.6 0.8 0.9

SE 0.2 0.1 0.3 0.2 0.2

N 7 7 5 5 6

Clubiona X 6.9 2.2 1.5 0.5 4.9

SE 0.6 0.2 0.2 0.0 0.5

N 9 9 6 6 9

Polygyra X 7.9 5.2 1.9 1.3 0.1

SE 0.6 0.4 0.2 0.1 0.0

N 12 12 12 12 4

The stimulus value of crawling prey is positively correlated with the

characteristic crawling velocity of that prey (Table l1N). However,

the stimulus value of crawling prey is not significantly associated with

prey selection (Table 110).


Prey/non-prey discrimination at the level of the retina and the

optic tectum of the brain in anurans precedes the behavioral motor

response (Schurg-Pfeiffer and Ewert 1981). Toads and frogs express

initial interest for a prey item by orienting towards the location of

a stimulus (Ewert and Burghagen 1979). My results showed that orienting

movements directed towards the stimulus location are not useful

indicators of the stimulus value of natural prey in Hyla cinerea.

Spodoptera and Chauliognathus larvae are highly preferred by H. cinerea

both in the laboratory and the field (Table 4). The time utilized in

the recognition-attack sequences for these insects does not distinguish

these prey from less preferred items. However, prey with high crawling

stimulus values do tend to shorten the period of latency separating

recognition from attack. Since the configurational meaning of an

artificial stimulus is independent of distance in AZytes obstetricians

and Bufo bufo, and the discrimination ability has common components in

A. obstetricans, B. bufo, Bcmbina variegata, Hyla arboria, and H. cinerea

(Ewert and Burghagen 1979), it is surprising to find that H. cinerea

does not recognize and pursue preferred prey at greater distances than

for prey of lower rank. The desirability of the prey item apparently

does not influence the degree or alacrity of the response exhibited by

the treefrog, but only the probability of the occurrence of that

response. Similarly, Heatwole and Heatwole (1968) found that the

motivational state of Bufo fowleri (i.e., degree of satiation) does not

affect the speed with which responses are given when they occur, but

rather whether they occur at all.

Ingle (1968) showed that Rana pipiens respond to stimuli on the

basis of real object size. Ewert and Burghagen (1979) found that

worm-like objects 6 to 12 mm long were particularly attractive to Hyla

cinerea. Optimal prey catching response occurred when 8 mm long black

rectangles were moved at a constant velocity of 200/s (1 cm/s). This

size range approximates the data generated in this study using

invertebrate prey whose mean was 10.2 mm (SE = 1.5, N = 9). However,

the average length of the most preferred natural prey item (Spodoptera)

was approximately 21 mm, which is 2.6 times larger than the optimal

artificial stimulus cited above.

Prey width alone is not an important selection parameter. However,

there is an indication that wild treefrogs select prey that possess

length/width ratios greater than 3:1. Bufo bufo discriminates between

prey and non-prey when dummy stimuli with length/width ratios greater

than 3:1 are presented (Ewert 1968; Ewert et al., 1979). Ewert (1976)

stated that square objects represent neutral or indifferent stimuli and

fall between horizontally (i.e., prey) and vertically (i.e., enemy)

oriented rectangles in ability to elicit prey catching behavior in

toads. Even so, prey items that are almost "square" (e.g., Euschistus)

are frequently eaten by treefrogs.

Actual prey size is utilized by frogs within predator-prey

distances of 15 cm. Beyond this distance the visual angle cue is

involved in prey choice (Ingle 1968, Ingle and Cook 1977). Preferred

stimulus angles at various distances have been reported for a variety

of amphibians (Ewert 1968, 1970; Ewert et at., 1979; Himstedt 1967;

Ingle 1968, 1971; Ingle and Cook 1977; Roth 1976). Mean visual angles

utilized by Hyla cinerea varied from 1.50 to 4.60. The average for

all prey genera utilized was 2.30 (SE = 0.3, N = 9) at an average

distance of 22.5 cm (SE = 1.2, N = 9). This value is identical to that

recorded by Ingle and Cook (1977) for Rana pipiens. The functional

stimulus angles utilized by H. cinerea varied from a minimum of 0.80 for

Disonycha at 42.5 cm, to a maximum of 6.10 recorded for Spodoptera at

20 cm. If frogs did cue on visual angles of prey less than 15 cm

distant, the maximum angle utilized was 54.50 for Musca at 0.5 cm.

Although Hyla cinerea does not always select prey items that

characteristically display high crawling velocities, the time to

recognition of natural prey is shorter for fast moving invertebrates.

Fast crawling prey also possess high stimulus values for treefrogs.

Increasing stimulus angular velocity in a range from 5/s to 400/s

(0.7 cm/s to 4.9 cm/s at 7 cm) increased the overall prey-catching

activity of toads towards artificial prey objects (Ewert et al., 1979).

Roth (1978) found that the efficacy of a stimulus in eliciting feeding

activity in salamanders increases with greater velocity from 0.5 cm/s to

3.1 cm/s. Roth (1976) noted that anurans, unlike the salamander

Hydromantes italicus, do not show sensitivity to fast movement.

Hydromantes italicus optimally responds to angular velocities of 4.80/s

to 720/s and a maximum velocity of 1720/s (6 cm/s) at a distance of 2 cm.

The response of Bufo bufo decreases significantly at 900/s (11 cm/s at

7 cm) when compared to its optimal response for stimuli moving at 200/s

(2.4 cm/s) (Ewert et al., 1979). In addition, neurophysiological

studies on Rana esculenta showed that responses of class-2-ganglion

cells ("bug detectors") decrease sharply when stimulus velocity exceeds

1000/s and almost cease at 1400/s (Finkelstein and Grusser 1965).

However, treefrogs do orient toward flying Musca. Flies have an average

flight velocity of 93.8 cm/s (SE = 6.18, N = 3). The estimated angular

velocity for a fly in flight 18 cm (Table 12) away from the frog exceeds

3000/s. Musca commonly walks within 2 cm of resting treefrogs. Average

crawling speed at this distance approximately equals an angular velocity

of 1400/s; this corresponds to the point at which class-2-ganglion cease

responding in Rana esculenta.

In addition to prey size and velocity, activity patterns displayed

by prey are also important parameters in eliciting feeding response in

amphibians. Small squares, known to be poor stimuli for eliciting prey

capture in Bufo bufo, are more attractive if moved in a step-wise manner

rather than in a continuous fashion (Borchers et al., 1978). Ingle

(1975) noted that discontinuous motion facilitates prey capture in Rana.

In Hydromantes, step-wise movement is more effective than continuous

moving stimuli for eliciting fixation and approach (Roth 1976, 1978).

Luthardt and Roth (1979a) stated that stimuli of certain orientation

must move in a specific manner and at specific velocities in order to

elicit optimal prey catching behavior in Salamandra salamandra. Prey

selection by Hyla cinerea is influenced by the activity patterns exhibited

by the prey and the proportion of the time each prey genus devotes to each

behavior pattern (Section 1, p. 19). Analysis indicated that 62% of all

prey were crawling at the time of recognition and 67% were performing

the same behavior just prior to predation. Even so, treefrog preference

is not associated with the stimulus value of crawling prey, since these

values are time-specific for each prey genus. If each prey spent equal

time crawling, the stimulus value for each prey genus would reflect the

rank of that prey in the diet of the green treefrog.

The effect of activity patterns on prey selection is best

illustrated by Musca domestic. Though its configuration provides a

poorer stimulus than many of the more elongate natural prey, it was the

most often eaten in laboratory experiments. This is due to the fly's

characteristic crawling pattern and the linkage of this behavior with

flight (Section 1, p. 19). The combination of these two behaviors

produces a very discontinuous or jerky activity pattern (see Ingle 1975

and Roth 1978) and accounts for a large proportion of the fly's activity

time budget (Table 7).

In conclusion, preference is expressed as the greater probability

that orientation and attempted prey capture will occur, not the degree

or speed at which these processes take place. The cues utilized by

Hyla cinerea during the selection of natural prey are prey size,

configuration, and activity pattern (which includes the variable prey

velocity). This is in close agreement with the findings of experiments

using artificial prey stimuli, though H. cinerea uses a broader range of

some of these parameters with natural prey items. Although the

experiments with artificial stimuli are useful in defining cues utilized

by frogs during the prey selection process, natural prey elicit

responses not predicted from the analysis of anuran encounters with prey



This work possesses certain methodological flaws that are inherent

in behavioral studies. A major departure from reality exists since prey

activity time budgets lack time periods for foraging behavior. The

difficulty of providing food for the various prey and the subsequent

decrease in prey visibility (e.g., prey hidden by food) precluded the

placing of food in the experimental chamber during time budget analysis.

Ideally, time budgets should be developed in the field; however,

locating and observing invertebrates in dense vegetation at night is a

formidable task. The omission of foraging time from prey activity

budgets may be of insignificant consequence since these time budgets

were complete for the artificial environment in which the preference

testing took place. Conversely, the time prey spend feeding may affect

the probability of capture in the field; however, the agreement found

between field and laboratory determinations of prey diet rank indicated

that this is not the case. A possible explanation for this is that

feeding behavior time may already be included in one of the recorded

time budget periods. For example, prey may characteristically remain

motionless while eating (e.g., Euschistus and Oebalus) making periods of

rest and feeding indistinguishable. The time spent foraging by

predatory prey (e.g., Chauliognathus and Clubiona) is included in the

search period; however, the time spent motionless may be underestimated

since actual feeding time could add to this period.

Another bias is found in the closed system in which treefrogs

were forced to forage. One could argue that prey should be replaced

after being eaten, thus maintaining a constant relative prey abundance

for predators to choose from. However, natural habitats presumably are

not characterized by a constant relative abundance of prey. In addition

the electivity equation was specifically designed to analyze prey

capture in a closed system. By differentially weighting the order of

prey captured in the ranking process, the electivity equation assigns

the highest electivity values to prey types that are captured both early

and consecutively. Prey that are captured late in the experiment and/or

non-consecutively will have low ranks due to the increasing probability

of being eaten as total prey abundance decreases.

Finally, the identification of prey taxa may not be important in

the foraging ecology of treefrogs. Prey items could be classified as to

the presence of important physical and behavioral components utilized by

predators during the selection process. Prey with similar morphology,

behavior patterns, and size may be classified by the predator as one

prey type. Many insects that look and move similarly belong, not only

to different genera, but to different orders (e.g., Spodoptera and

Chauliognathus larvae) and, depending on the predator's discriminative

ability, may be regarded as one prey (ranked 2,3 in the initial trials

and 3,2 in the finals, respectively). In addition, the fifth and sixth

ranked prey in this experiment may also be considered a single prey

type; though belonging to different genera (Euschistus and Oebalus) they

belong to the same family and exhibit many similar morphological and

behavioral features. I assumed that I was offering treefrogs a choice

among nine different prey genera, but in reality, frogs may have


recognized only seven distinct types of prey. I consider this and the

above criticisms valid, but of minor consequences; they do not alter

the conclusions concerning the discriminative ability of Hyla cinerea

or the visual cues used during the selection process.


Power function model: y = bxm. Length is in mm, mass in mg. Polygyra
size range is for shell diameter, not extended length.

Genus Size Range b m r N

(mm) (bX10-2)

Spodoptera (larva) 5.5-33.5 2.1 2.8289 0.99 35

ChauZiognathus (larva) 6.9-14.0 0.6 3.2725 0.97 20

Gryllus 7.0-23.0 14.5 2.6319 0.88 24

Euschistus 8.9-14.0 6.6 2.8041 0.90 20

Oebalus 7.5-11.6 76.9 1.6334 0.83 20

Disonycha 6.0- 7.9 8.8 2.7496 0.95 20

Clubiona 4.3- 7.9 3.8 3.1804 0.96 20

Polygyra 5.2-10.8 28.9 3.0294 0.99 20


Prey identified from the stomachs of 163 Hyla cinerea. L = larva

Prey Item Total No. Est. Biomass No. of Adjusted

of items (g) Frogs %BO


Coleoptera 158 2.766 105 21.56

Alleculidae 21 0.356 5 1.91

Cantharidae (L) 46 0.793 32 7.21

Cantharidae 2 0.048 2 0.45

Carabidae 6 0.124 5 1.13

Cerambycidae 3 0.130 3 0.90

Chrysomelidae 27 0.387 15 3.45

Curculionidae 26 0.239 21 3.21

Elateridae 1 0.016 1 0.19

Hydrophilidae 1 0.002 1 0.06

Languriidae 8 0.307 6 1.94

Scarabaeidae 6 0.235 3 1.20

Staphylinidae 2 0.035 2 0.38

Tenbrionidae 1 0.094 1 0.44

Unknown 8 8 -

Collembola 21 13 -

Dermaptera 6 0.059 6 0.75

APPENDIX 2. Continued.

Prey Item Total No. Est. Biomass No. of Adjusted

of items (g) Frogs %BO


Diptera 16 0.009 13 0.45

Hemiptera 49 1.161 41 8.73

Alydidae 1 0.050 1 0.32

Coreidae 1 0.095 1 0.44

Lygaeidae 2 0.015 1 0.17

Pentatomidae 37 0.806 30 7.04

Reduviidae 8 0.195 8 1.79

Homoptera 14 0.194 13 2.01

Cicadellidae 9 0.114 8 1.37

Cercopidae 1 0.055 1 0.33

Flatidae 1 0.025 1 0.22

Unknown 3 3 -

Hymenoptera 41 0.150 35 2.90

Formicidae 29 0.053 23 1.57

Ichneumonidae 3 0.088 3 0.73

Platygasteridae 1 0.0001 1 0.02

Unknown 8 8 -

Lepidoptera 128 8.223 104 36.99

Gelechiidae 3 0.007 1 0.12

Geometridae 6 0.269 5 1.63

Noctuidae (L) 66 6.402 54 26.61

APPENDIX 2. Continued.

Prey Item Total No. Est. Biomass No. of Adjusted

of items (g) Frogs %BO



Noctuidae 2 0.433 2 1.34

Nolidae (L) 2 0.006 1 0.12

Psychidae (L) 1 0.003 1 0.08

Zygaenidae (L) 4 0.029 2 0.34

Unknown (L) 31 0.569 25 5.30

Unknown 13 0.505 13 3.60

Odonata 1 0.109 1 0.41

Orthoptera 41 2.044 40 11.43

Acrididae 1 0.220 1 0.67

Blattidae 10 0.418 9 2.77

Gryllidae 14 1.041 14 5.46

Tetrigidae 6 0.352 6 2.08

Tettigoniidae 2 0.013 2 0.23

Unknown 8 8

Thysanoptera 3 3 -

Unknown 67 49


Acarina 55 5 -

Araneae 72 0.712 62 8.40

Araneidae 7 0.083 5 0.92

APPENDIX 2. Continued.

Prey Item Total No. Est. Biomass No. of Adjusted

of items (g) Frogs %BO



Clubionidae 23 0.168 19 2.56

Dictynidae 3 0.003 3 0.14

Linphiidae 2 0.006 2 0.16

Lycosidae 9 0.275 7 1.99

Mimetidae 1 0.001 1 0.05

Pisauridae 2 0.019 2 0.28

Salticidae 4 0.041 3 0.50

Tetragnathidae 3 0.025 3 0.39

Unknown 18 0.092 17 1.80

Opiliones 12 0.285 12 2.34


Lithobiomorpha 1 1


Isopoda 4 0.035 4 0.47


Stylommatophora 9 0.592 9 2.92

Polygyridae 8 0.526 8 2.94

Unknown 1 0.066 1 0.36


Squamata 1 0.252 1 0.64

APPENDIX 2. Continued.

Prey Item Total No. Est. Biomass No. of Adjusted

of items (g) Frogs %BO



Iguanidae 1 0.252 1 0.72

Total (Order) 699 16.592 100.00

Total (Family) 513 16.095 99.63


Logarithmic function model: y
y = bxm. Length and width are
0.652 x length (see Table 6).

= b + mlnx. Power function model:
in mm. Polygyra width estimate =

Genus Model b m r N

Musca Log -5.31470 4.04408 0.92 10

Spodoptera (larva) Power 0.06713 1.22141 0.99 14

ChauZiognathus (larva) Power 0.20221 0.98155 0.99 9

Gryllus Power 0.25731 1.04462 0.99 17

Euschistus Log -16.64649 10.10852 0.80 10

Oebalus Power 0.68178 0.84693 0.97 10

Disonycha Log -3.88884 3.76397 0.85 10

Clubiona Power 0.33842 0.96167 0.96 10

Polygyra -


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On 14 March 1952, Arthur Nelson Freed was born in the city of

brotherly love. His formative years were spent on the streets of

Philadelphia and in the woods of Fairmont Park (the largest city park in

the world). He graduated from Northeast High School but decided not to

enter Temple University (considered by many to be Northeast's 13th

grade). Instead, he headed west to study biology at Indiana University

of Pennsylvania (Pennsylvania's largest state owned university). After

four glorious years in the mountains of west-central Pennsylvania,

Arthur returned to Philadelphia with one blue Alfa Romeo 1750 Spider

Veloce and a B.S. in biology. He entered the Intern Teaching Program

for College Graduates at Temple University (it was his destiny) and

after two years earned an M.Ed. During those two years, he experienced

the tragic loss of a Porsche 911E Targa, but bravely continued teaching

seventh grade life science at Upper Moreland Junior High School, Upper

Moreland, PA. He then decided not to devote his life to educating

children, bought a Lotus Elan Sprint, and headed with elan south to the

University of Florida in pursuit of the elusive Ph.D. He was half-

heartedly accepted into the zoology graduate program, not being offered

support until his second year. After two frustrating and three

invigorating years he finally reached the top of the graduate student

pecking order and tried vainly to apply for tenure. Seeing no hope of

attaining Nirvana, he threw up his hands in disgust, and graduated.

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.

M. L. Crump, Chairperson
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.

F. Arnders-n
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.

T. J./ alker
Prof sor of Entomology and

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 Council, and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.

May 1982
Dean for Graduate Studies
and Research

3 1 262 IIII 1111I II1
3 1262 08553 4294

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