Download Reproductive Ecology of the Green Treefrog (Hyla cinerea) in Northwestern Florida

Am. Midi. Nat. 155:321-328
Reproductive Ecology of the Green Treefrog
(Hyla cinerea) in Northwestern Florida
MARGARET S. G U N Z B U R G E R '
Department of Biological Science, Florida State University, Tallahassee
32306
AB.STR.M:I,—Information on life-histoi")' LraiLs is critical lo understanding population
dynamics of anurans. rhc objective of this study was to examine aspects of the reproductive
ecology of Hyla cinerea in northwestern Florida. Four breeding localities in Leon Count)'.
Florida, were sampled over three sea.sons (2001-2003), amplexed pairs were found as early as
12 April and as late as 12 AugusL Egg clutches were counted from 51 amplexed pairs and
adult .size was measured in 43 pairs. Average clutch size was 1214 ± 528 eggs (x ± SE, r a n g e 359-2658). Female H. cinerea were slightly larger than males (x ± SE tibiofibula length —
24,9 ± 2.2 and 24 ± 1.7 mm, respectively). Female size was significantly positively correlated
with clutch size and weakly correlated with size of the paired male. (Comparison of the
results of this study with previous research indicates ihat the reproductive ecology of
H. cinerea varies across its geographic range.
In order to vinderstand populatioti dynamics, information on growth, sui"vival and reproduction of each life-history stage is critical (Duellman and Tiiieb, 1994; Scott, 1994). For
many anurans, characteristics of the larval stage such as growth and sur\'ival have been
studied extensively (Wilbur, 1997). Life-histor^' characteristics of the adult stage, including
reproductive traits, are less well-known. Many studies of reproduction of treefrogs in the
southeastern United States focus on adult behaviors such as selection of breeding sites, mate
selection and male advertisement calls {Gerhardt e.t al, 1987; Resetarits and Wilbur, 1991;
Murphy and Gerhardt, 2002). However, a critical component for uiidei"slanding the lifehistory of anurans is the reproductive output of female.s (Williamson and Bull, 1995). Glutch
size in anurans tend.s to be positively correlated with female size in species with a similar
reproductive mode (Ritke et ai, 1990; Duellman and Trueb, 1994). Many pond breeding
anurans produce large clutches of small eggs and have low survival rates of both eggs and
larvae (Duellman and Trueb, 1994). Clutch size and breeding phenolog)' may vary over the
geographic range of a wide-ranging species which may lead to variation in population
dynamics (Ritke et ai, 1990; Morrison and Hero, 2003).
The green treefrog, Hyla cinerea, breeds in permanent ponds and lakes with emergent
vegetation and is the only species in the germs Hyta in the southeastern United States that
regularly breeds in habitats with large predatory fish (Mount, 1975; Wright and Wright,
1995). Hyla cinerea breeds throughout the spring and summer months, from April to August
(Garton and Brandon, 1975; McAlpine, 1993). This species occurs across a broad geographic range over which its reproductive ecology may vaiy Previous studies of reproductive
ecology have been condticted on this species in Illinois, South Garolina and Georgia
(Garton and Brandon, 1975; Perrill and Daniel, 1983; Gerhardt el ai, 1987; McAlpine,
1993). The objectives of this study were to describe the reproductive ecology of//. cinerea in
northwestern Florida by quantifying breeding season length, cititch size and male-female
' Present addre.ss: United States (leologital Survey, Florida liuegraied Science Geiiter, 7920 NW 71st
Street, Gainesville 32653. Telephone: (352) 264-3520; e-mail: [email protected]
321
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T H E AMERKIXN MIDLA.ND NATURALIST
TABLE 1.—Summary of sampling effort al four Hyla dnerea breeding localities in Leon County, Florida.
Number of visits is the total number of nights each .site was visited each season, and in parenthe.ses is the
number of visits in which amplexed pairs were collected. Not all pairs collected were used for data
analysis as some pairs and clutches were part of another study
# visits (w/pyirsl
# amplexed pairs collided
Lo<ali,v
2001
2002
2003
200!
2002
Chapman Pond
Harriman Pond
lnnov"ation Pond
Mej^ginnis Arm
7(1)
6(2)
0
11 (1)
8 (3)
16(4)
3 (0)
4(1)
10(6)
5(0)
6
12
NA
1
11
14
NA
5
0
11 (3)
0
2
20
0
size relationships. The results of this study will be compared to other previously published
studies from throughout the range of H. cinerea. This inloniiation will contribute to understanding the population ecology of this widespread and abundant species of treefrog.
METHODS
I sampled four localities in Leon County, Florida, over three Hyla cinerea breeding seasons
(2001-2003). Three of these localities (Chapman Pond, 1 ha; Harriman Pond, 0.56 ha; and
Innovation Pond, 1.4 ha) were suburban ponds surrounded by mowed grass, trees and
shnibs, with relatively sparse emergent aquatic vegetation. The fourth locality, Megginnis
Arm, is an 8 ha portion of Lake Jackson (1620 ha) characterized hy abundant emergent and
floating aquatic vegetation and thick growth of shrubs and trees around the edge.
Megginnis Arm was much more difficult to search for amplexed pairs due to the thick
vegetation and, although this site supported a much larger population of H. cinerea, I
suspect I missed many amplexed pairs. Sampling effort was not unifoiin for each site in all
three years and I did not visit each locality on each night of sampling (Table 1). To determine the length of the breeding season in each year I monitored the sites from March
through Octoher, hefore, during and after the suspected breeding season of H. cinerea.
At each \isit, I recorded whether male Hyla cinerea were calling. On visits after rain or on
very humid nighLs, I searched for amplexed pairs (Table 1). I visited sites from approximately 2000 to 0200 h, and most amplexed pairs were found hetween 2230 and 0030 h.
Typically I could only search one locality per night when I was collecting amplexed pairs,
thus my data underestimate the total numher of amplexed pairs at each site. After collection, I returned the amplexed pairs to the laboratory for egg collection. I placed each
amplexed pair in a separate, covered 19 1 hucket filled with well water to a depth of 10 cm.
For 43 of the amplexed pairs I measured size (tibiofibula length, TFL) of the adult frogs
tising a ruler or dial calipers. I used TFL as a measurement of body size because this measurement is correlated with snout-vent length (SVL) for treefrogs and is more accurate and
repeatahle than SVL (Murphy, 1994). Adults were measured and released at the site of
capture the day after collection. Because 1 did not mark adtilt frogs, I cannot determine
whether some females and males may breed more than once in a breeding season. It is
possible that some females and males were captured multiple times during one breeding
season or over several breeding seasons (Perrill and Daniel, 1983).
I counted the number of eggs in 51 clutches hy removing all eggs from the bucket using
a fine mesh aquarium net and placing the eggs in a single layer in shallow water in a small
white plastic container (110 X 105 mm). I photographed each clutch usitig a film (20012002) or digital (2002-2003) camera. Slides were converted to digital images and all images
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323
were printed out on standard computer paper. From these images, I counted all eggs in each
clutch using a transparency overlay to record which eggs had been counted. This counting
method was highly repeatable; I recounted four clutches and was within seven egg.s of the
original count in each case (average difference between counts < 0.004%). Eggs were maintained in the laboratory for tise in other studies (Gunzburger and Travis, 2004; Gunzburger
and Travis, 2005).
In order to determine if data could be pooled for analysis, I evaluated wbetber clutch
size and adult body size varied across tbe tbree breeding seasons or across tbe four
sampling localities. I used two separate analysis of variance (ANOVA) to first compare
mean clutch size for tbe tbree sampling years, and second compare clutcb size for tbe four
localities. I used a similar technique to evaluate variation in male and female size across
sites and years. I did not have stifficient replication across years and localities to perform
two-way ANOVAs witb year and locality as tbe factors for this analysis. Because I found no
differences across localities or over time, I pooled the data for tbe remaining analyses.
I evaluated wbether males and females differed in body size using ANOVA. I used linear
regression analysis to determine if clutcb size was correlated witb female size. I evaluated
wbether the size of the female and male in eacb amplexed pair were correlated using
linear regression analysis.
RESULTS
The breeding season of Hyla cinerea in tbis sttidy in northwestern Florida lasts from March
tbrough September. Tbe earliest date H. cinerea cboruses were observed was 19 March at
Megginnis Ann and tbe latest calling date was 2 September at Megginnis Arm. Hyla ciyierea
often called in mixed cboruses witb otber anuran species including Acris gryllus, Bu/o
terrestris, H. squirella, Gastrophryne cartdinensis, Rana catesbeiana, R. damitan.\, R- grylio and
R. sphenocephala. Hyla cinerea calling males were observed on almost every visit to each locality
during tbe breeding season. A total of 8] visits were made, and dtiHng 9 of those visits (11%)
no H. cinerea cborus was observed. Amplexed H. cinerea pairs were fotmd from 12 April
tbrougb 12 August; breeding .season lengtb, the period during wbich amplexed pairs
were found, ranged from 53 days in 2001 to 88 days in 2003.
Hyla cinerea dutiih size ranged from 359 to 2658 ( x ^ 1214 ± 529, N = 51). Clutcb sizes did
not differ across years (ANOVA, P =0.39, F.2,4fl = 0.96, N = 51) or across localities (ANOVA,
P = 0.35, F3,47= 1.1, N —51) so data were pooled for all localities and years for the retnainder
of tbe data analysis. There was no obvious trend in clutch size over the breeding season
(Fig. 1). Larger females tended to oviposit larger clutches, clutcb size was positively
correlated with female size (P < 0.001, F,,:,7 = 28, r" = 0.43, N = 39; Fig. 2). Female H. cinerea
TFL ranged from 21.1 to 30 mm (x = 24.9 ± 2.2, N = 43) and male TFL ranged from 19.6 to
27 mm (x = 24 1: 1.7, N = 43). Females were larger tban males, this size difference was
marginally significantly (ANOVA F| ^4 = 3.47, P = 0.066). Tbere was some evidence of sizeassortative mating, female size was correlated with male size (P = 0.02, Fi,4i = 6.2, r^ = 0.13,
N — 43; Fig. 3), but the coefficient of determination was low.
DISCUSSION
Hyla cirierea in northwestern Florida engage in a prolonged breeding season, witli males
calling from Marcb tbrougb September, and mating occuring at least from April tbrougb
August. Hyla cinerea females have been documented to oviposit mtiltiple cltitcbes in a single
season in a population in Georgia, but tbere were insufficient data to determine if clutch
size varies across multiple clutches in the same season or across seasons for the same female
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155(2)
T H E AMERKLAN MmiAND NATUKALIST
3000
O
n
A
O
Chapman Pond
Harriman Pond
Innovation Pond
Megginnis Arm
O D
2000o
O
.0
1000-
'
'
i
I
I
I
1
I
I
I
1
I
I
I
L
I
I
I
I
1
Date
¥[(... I.—Hylu cinen-a clutch si/c Dvcr the breeding season from four localities in Leon County, Florida,
across three breeding seasons (2001-2003)
(Penill and Daniel, 1983). In this sttuly I did not mark females so I cannot directly asses
whether or not females lay multiple clutches within a breeding season. For another species
of treefrog, H. chrysosceiis, within a single breeding season, initial clutch sizes tend to be
larger than later clutches (Ritke ei ai. 1990). Thus, it tniglit be expected that for species
whi(h oviposit multiple clutches in a breeding season, there would be a trend toward
smaller clutch sizes later in the season (Lips, 2001). In this study, I detected no trend in
H. cinerea clutch size throughout the breeding season (Fig. 1). However, le.ss than half the
variation in clutch size is explained by female body si/e (Fig. 2). Thus, it is possible that
H. cincrm in nothwestcrn Florida oviposit more than one clinch each season and some of
the smaller clutch sizes in this study are second or third clutches.
As is typical of other studies of Hyla cinerea, in this study males outnumbered females
during every visit to each site, and males chorused on nighls wheti no females were observed
and no amplexed pairs were lonnd (Clarton and Brandon, 1975; (ierhardt e( al., 1987). For
species of anurans with female choice of mates, females may choose mates based on advertisement call, male size, or other characteristics (Mur|)hy and Gerhardt, 2002). Some anurans possess size-assortative mating, in which females choose males similar to them in size,
possibly to increase fertilization success (Duellman and Tnteh, 1994). In this study there was
a trend lor larger female H. rinerea to mate with larger males, suggesting size-assortative
mating, but the correlation coefficient was low. This could be due to the limited sample size
2006
325
GUNZBURGER: TREEFRO(; REPROIX'CTION
3000
N
O
O
D
A
O
Chapman Pond
Harriman Pond
Innovation Pond
Megginnis Arm
2000
1000
22
24
26
28
30
Female TFL
Fic. 2.—Relaiioiiship
between clutch size ( m i n i b e r of eggs) a n d female size (libiofibula length in
m m ) at Hy Iti cinerea i'mm four localities in l.eon County, Florida ( P < 0.001. FI,M7 = 28, r"^ ^ 0.43. N = 39)
of this Study; a similar study of H. cinerea in (Icorgia with a much larger sample size failed to
detect any size-assortative mating in H. cinerea using snout-vent length as a measure of body
size (Gerhardt et ai, 1987). It is unlikely that these conflicting restilts stem from tise of
different body size metrics in these two sttidies because the ratio of snoui-tirostyle length to
TFL is similar across populations throughout the southeastern United States (Aresco, 1996).
Comparison of tbis study witb three otber studies indicates tbat Hyla cinerea reproductive
ecology varies across its geographic range. Clutcb size generally inct eased with a decrease in
latitude: average clutch si/e of //. cinerea was larger in this study (x = 1214 ± 529, N = 51)
than a population in southern Illinois (x — 700 ± 222, N = 5; Garton and Brandon, 1975)
and in coastal Georgia (x = 790 ± 262, N - 26; Perrill and Daniel. 1983), but smaller tban
a population in Soutb Carolina (x = 1472 ± 462, N = 82; McAlpine. 1993). Tbis result
contradicts tbe generalization tbat clutch size of anurans increases wiib latitude because
females at bigber latitudes tend to be larger and female body size is correlated witb clutcb
size (Morrison and Hero, 2003). However, tbere is evidence tbat the breeding season of
H. cinerea decreases witb increases in latitude (Morrison and Hero, 2003), the breeding
season of H. cinerea commences earlier in tbe year and continues later in tbe southern
portion of its range. In southern Illinois, at the northern limit of its range, H. cinerea breeds.
from early May to early August, with the peak of breeding activity occurring in ]une, and
amplexed pairs were observed from early Jtine tbrough late July (Garton and Brandon,
1975). In southern Georgia, calling male H. cinerea were observed from April 24 tbrougb
Aug. 13. with peak breeding activity from mid-May tbrougb mid-July (Wright, 2002). In this
326
155(2)
TFIE AMERICAN Mini AND NATL-RAIJST
D
P
o
D
26-
D
_
D
[T
A
24-
OA
^
D
O
CD
A
22-
0
°
t
A
0
20-
O
D
A
O
A
I
I
1
I
22
24
Chapman Pond
Harriman Pond
Innovation Pond
Megginnis Arm
i
26
28
30
Female TFL
FIG. 3.—Relationship between size (ribiofibula length in mm) of male and female Hyla cinerea {nund
in amplexed pairs from four localities in Leon Count\; Florida (P = 0.02, F|,4i -6.2, r"^ = 0.I3, N = 43)
study in northwestern Florida, choruses of //. cinerea were observed from March through
September, and amplexed pairs were found from April through August.
These geographic differences in reproductive season are probably due to differences in
temperature and precipitation across the range of this species. Both average maximum daily
temperature and rainfall increase as latitude decreases from Illinois to Florida (NCDC,
2003). Comparison of weather data to previous data on Hyla cinerea indicates that choruses
began during the first month of the year in which the mean temperature exceeded 16 C,
and peak breeding activity occurred during months with mean temperatures exceeding 22 C
(Carton and Brandon 1975: Wright. 2002; NCDC, 2003). Thus. H. cinerea breeding activity is
apparently triggered by similar temperatures throughout its range. Hyla cinerm populations
in Florida may have less need to build up fat reserves prior to the winter than more northern
populations, and thus may be able to extend breeding later in the year (Ritke et al., 1990). In
addition, average precipitation during the summer months (May-Sept.) increases from
about 10 cm/month in Illinois to 16 cm/month in northwestern Florida (NCDC, 2003).
Additional data on weather and H. cinerea reproductive ecology throughout iLs geographic
range could allow predictions of the potential effects of global climate change on //. cinerea
population dynamics.
Knowledge of stage-specific survival and reproductive characteristics is critical to our
understanding of antiran population dynamics. This study provides information on
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GUNZBURGER: TREEFROG REPRODUCTION
327
reproductive potential of Hyla cinerea in northwestern Florida, and comparison of tbe results
of this study with other studies of H. cinerea suggests that there is c(HisiderabIe variation in
reproductive ecology across tbe geograpbic range of tbis species. Variation in reproductive
success over time and across babitats may have important implications for population
dynamics (Richter et al., 2003). Previous research in northwestern Florida demonstrates that
predation rates on H. cinerea tadpoles vary strongly across the habitats in wbich it breeds
(Gunyburger and Travis, 2004). The influence of variation in reproductive otitptit and
tadpole survival to overall population dynamics should be tbe focus of further research in
tbis system.
Acknowledgments.—M.J. Aiescoand C. K. Dodd, Jr., provided helpful comments on earlier drafts ofthe
manu.script. I was supported dining this research project and preparation of the manuscript by the
National Science Foundation ihrougli Krani DF.B 99-03925 to Jo.spph Travis and the Amphibian
Research and Monitoring Initiative of the United States Geological Survey. This study was performed
under FSU ACUC Protocol # 01 l.'j.
LlTERATLRE CiTED
ARESCO, M. J. 1996. Geographic rariation in the morphology and lateral siripe of the green treefrog
{Hyla cinerea) in the .Siiiithcasteni United Siatrs, .\m. Midi Nat.. 135:293-298.
DutLLM.W, W. E. ANli L. TKL'K.B. 1994. Biology of amphibians. Jolins Hopkins University Pres.s, Baltimore,
Maryland, USA. 670 p.
GABTON, J . S. AM
. D R. A. BRANIXJN. 1975. Reproductive ecology of the green treefrog, Hyla cinerea. in
southern Illinois (Anura: Hyiidae). Hejpetologira. 31:150-161.
GERHARDT. H . C , R. E. DANIK.L, S. A. PERRIU AND S. SCHRAMM. 1987. Mating behavior and male mating
success in the green treefrog. An. Rehav., 35:1490-1503.
GuNZBLRGF.R. M. S. ANDJ. TRAVIS. 2004. Evaluating predation pressure on green treefrog larvae across
a habitat gradient. Oecologia, 140:422-429.
AND
. 2005. Effects of multiple predator species on green treefrog {Hyla cinerea) tadpoles.
Can.J. of Zool. 83:996-1002.
LIPS, K, R. 2001. Reproductive trade-offs and bet-hedging in Hyla calypsa. a Neotropical treefrog.
Oecolo^a. 128:509-518.
McALPtNE, S. 1993. Genetic heterozygosity and reproductive success in the green treefrog, Hyla rinerea.
Heredity, 70:553-558.
MoEiRisoN, C. A.M) J. HERO. 2003. Geographic variation in life-history characteristics of amphibians:
a review./ Antm. Kcol, 72:270-279.
MOUNT, R. H . 1975. The reptiles & amphibians of Alabama. Auburn Priming (Jo., Auburn, Alabama,
USA. 347 p.
MURPHY, C. G . 1994. Chorus tenure of male barking treefrogs, Hyla graliosa. An. Behav., 48:763-777.
AND
' H. C. GERHARDT. 2002. Mate sampling by barking treefrogs (Hyla gratiosa). Behav. Kcol,
13:472-480.
NATIONAI. Ci.iMATf DATA CKNTF.R ( N C D C ) . 2003. U. S. climate nomials 1971-2000. Asheville, Nortb
Carolina. Available on the intcnifi at liup://www.ncdc.noaa.gov/oa/ncdc.html.
PERRJI,!., S. A. A\!) R. K, DANit.i., 1983, Multiple egg clutches in Hyla regilla. H. rinerea. and H. gratiosa.
Copeia. 1983:513-516.
RESETARII?;, W.J., JR. AND H . M. Wn.BiJK. 1991. Calling site choice by Hyla chrysosceliz effects of predators,
competitors, and oviposition sites. Kcology, 72:778-786.
RICHTER, S. G., J. E. YOUNG, G. N.JOHNSON AND R. A. SIE(;EL. 2003. Stochastic variation in reproductive
success of a rare frog, Rana sevosa: implications for conservation and for monitoring amphibian
populations. BioL Cons., 111:171-177.
RiTKE, M. E.,J. G. BABB ,VM) M. K. RITKE. 1990. Life histoi-y variation of the gray treefrog (HyUi chrysoscelis)
in western Tennessee./ Herp.. 24:13.5-141.
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T H E AMERICAN MIDLAND NATLRALTST
155(2)
S<:oTT. D. F. 1994. The effect of laival density on adtih demographic traits in Ambystoma opacum. Ecology.
75:1383-1396.
WiLtiLiR, II. M. 1997. Fxperinu-ntal ecologj' of food web.s: complex systems in temporary' ponds. Ecology,
78:2279-230.3.
WILLIAMSON, I. .AND C:. M. Bt:i.L. 1995. Life-histoiy variation in a population of the Australian frog
Ranadella signifera: seasonal changes in clutch parameters. Copeia, 1995:10.5-113.
WRIGHT, A. H. 2002. Life-histories of the Frogs of the OkeHnokee Swamp, Georgia. North American
Salienta (Anura) No. 2. Cornell University Press, Ithaca, New York, USA. .509 p.
—
AND A. A. WRi(;m. 199.^. Handbook of frogs and toad.s of the United States and Canada. C:omstock Cl;vs.sic Handbook. Comstock Publi.shing Company, Inc., Ithaca, New York, USA. 640 p.
SUBMITTED 12 NOVEMBER 2004
ACCEPTED 27 SEPTEMBER 2005