Comparing bivariate reaction norms among species: time and size at metamorphosis in three species of Hyla (Anura: Hylidae)

Summary Univariate and bivariate methods for comparing norms of reaction among species are discussed and illustrated with an example using North American hylid treefrogs. Norms of reaction for size at metamorphosis (SM) and length of larval period (LP) were compared among treefrog species raised at different food levels (Hyla cinerea vs H. gratiosa) and at different temperatures (H. cinerea vs H. gratiosa vs H. squirella). Hyla cinerea and Hyla gratiosa show parallel norms of reaction across food levels and temperatures. Across temperatures, H. squirella shows a much smaller change in SM relative to change in LP than do H. cinerea and H. gratiosa. This difference in shape of reaction norms may reflect different histories of selection resulting from these species' use of different larval habitats.     

Interaction of pH,density, and priority effects on the survivorship and growth of two species of hylid tadpoles

Summary We examined the interactions of an abiotic factor (pH) and a biotic factor (density) on the survival and growth of two species of anuran larvae (Hyla gratiosa and Hyla femoralis) in outdoor tanks. Three levels of pH (4.3, 4.6, or 6.0) and three levels of density (0, 30 or 60 embryos) were arranged in a blocked design and replicated three times for Hyla gratiosa. At the end of this experiment the effects of pH (4.3, 4.6, or 6.0), density of H. femoralis (30 or 60), and prior use by H. gratiosa (at 0, 30, or 60 larvae per tank) on the survival and growth of H. femoralis, were examined. Higher density increased larval period and decreased size at metamorphosis of H. gratiosa. Lower pH decreased survival rate and also decreased size at metamorphosis. Body sodium concentrations were lowest at the low pH values. Lower pH increased the susceptibility of H. gratiosa tadpoles to the adverse effects of higher densities. For H. femoralis higher density decreased survival, increased larval period and decreased size at metamorphosis. Hyla femoralis also had lower survivorship at low pH and exhibited decreased size at metamorphosis. However, unlike the results with Hyla gratiosa, there were no interactive effects between pH and density for any of the life-history traits studied. The effect of previous colonization by H. gratiosa on H. femoralis survival was facilitative. Body sodium concentrations of H. femoralis were lowest at the highest pH value. Metamorphs of the same size had much lower levels of sodium in H. femoralis than H. gratiosa. In general, H. femoralis was less affected by pH variation than H. gratiosa. These results demonstrate that abiotic factors can interact strongly with biotic effects such as density and they suggest that interspecific interactions can be strongly modulated by the background abiotic environment.     

Mid-frequency suppression in the green treefrog (<Emphasis Type="Italic">Hyla cinerea</Emphasis>): mechanisms and implications for the evolution of acoustic communication

Advertisement calls of green treefrogs (Hyla cinerea) have two spectral peaks centered at about 1 kHz and 3 kHz. Addition of a component of intermediate frequency (1.8 kHz) to a synthetic call reduced its attractiveness to females relative to an alternative lacking this component. This mid-frequency suppression occurred over a 20-dB range of playback levels. Addition of other intermediate frequencies had weak effects on preferences at some playback levels, in some localities, and at lower-than-normal temperatures. These effects correlate well with the response properties of a population of low-frequency-tuned auditory neurons innervating the amphibian papilla. Males of a closely related species (H. gratiosa) produce calls with emphasized frequencies within the range of suppression in H. cinerea; however, suppression also occurred in localities well outside the area of geographical overlap with this species. Thus, previous speculation that mid-frequency suppression evolved to enhance species discrimination is probably incorrect. This phenomenon is more likely to reflect a general sensory bias in anurans and other vertebrates, tone-on-tone inhibition. Such negative biases, and other inhibitory mechanisms, almost certainly play an important role in the evolution of communication systems but have received far less attention than positive biases that enhance signal attractiveness.     

Sound localization behavior of the green treefrog (Hyla cinerea) and the barking treefrog (H. gratiosa)

Summary Female treefrogs (Hyla cinerea andH. gratiosa) can accurately localize a sound source (playback of male mating calls) if both ears are intact. When the sensitivity of one eardrum is attenuated, by coating it with a thin layer of silicone grease, females no longer can locate the sound source. This study demonstrates that female anurans rely on interaural cues for localization of a calling male. The neural basis for an anuran's sound localization ability presumably involves binaural convergence on single cells in the central auditory nervous system.This work was supported by research grants from the National Science Foundation and the U.S. Public Health Service. The assistance of Anne J.M. Moffat in measuring the directional characteristics of the loudspeaker is gratefully appreciated.     

Effect of changes in resource level on age and size at metamorphosis in Hyla squirella

Recent experiments suggest that timing of metamorphosis is fixed during development in some anurans, insects, and freshwater invertebrates. Yet, these experiments do not exclude a growth rate optimization model for the timing of metamorphosis. I manipulated food resources available to larvae of squirrel treefrogs (Hyla squirella) to determine if there is a loss of plasticity in duration of larval period during development and to critically test growth rate models for the timing of metamorphosis. Size-specific resource levels for individual tadpoles were switched from low to high or high to low at three developmental stages spaced throughout larval development. The effects of changes in resource availability on larval period and mass at metamorphosis were measured. Switching food levels after late limb bud development did not significantly affect larval period in comparison to constant food level treatments. Therefore, developmental rate in H. squirella is better described by a fixed developmental rate model, rather than a growth rate optimization model. The timing of fixation of developmental rate in H. squirella is similar to that found in other anuran species, suggesting a taxonomically widespread developmental constraint on the plasticity of larval period duration. Mass at metamorphosis was not significantly affected by the timing of changes in food levels; the amount of food available later in development determined the size at metamorphosis. Larval period and mass at metamorphosis were negatively correlated in only one of two experiments, which contrasts with the common assumption of a phenotypic trade-off between decreased larval period and increased mass at metamorphosis. Received: 19 August 1996 / Accepted: 20 June 1997     

Influence of density and salinity on larval development of salt‐adapted and salt‐naïve frog populations

Environmental change and habitat fragmentation will affect population densities for many species. For those species that have locally adapted to persist in changed or stressful habitats, it is uncertain how density dependence will affect adaptive responses. Anurans (frogs and toads) are typically freshwater organisms, but some coastal populations of green treefrogs (Hyla cinerea) have adapted to brackish, coastal wetlands. Tadpoles from coastal populations metamorphose sooner and demonstrate faster growth rates than inland populations when reared solitarily. Although saltwater exposure has adaptively reduced the duration of the larval period for coastal populations, increases in densities during larval development typically increase time to metamorphosis and reduce rates of growth and survival. We test how combined stressors of density and salinity affect larval development between salt‐adapted (“coastal”) and nonsalt‐adapted (“inland”) populations by measuring various developmental and metamorphic phenotypes. We found that increased tadpole density strongly affected coastal and inland tadpole populations similarly. In high‐density treatments, both coastal and inland populations had reduced growth rates, greater exponential decay of growth, a smaller size at metamorphosis, took longer to reach metamorphosis, and had lower survivorship at metamorphosis. Salinity only exaggerated the effects of density on the time to reach metamorphosis and exponential decay of growth. Location of origin affected length at metamorphosis, with coastal tadpoles metamorphosing slightly longer than inland tadpoles across densities and salinities. These findings confirm that density has a strong and central influence on larval development even across divergent populations and habitat types and may mitigate the expression (and therefore detection) of locally adapted phenotypes.     

Effects of parentage on competitive ability and vulnerability to predation in Hyla chrysoscelis tadpoles

Four sibships of Hyla chrysoscelis larvae were used to examine the effects of parentage on mass at day 23 of growth and on vulnerability to predation. The H. chrysoscelis larvae were raised alone, in competition with Rana clamitans larvae, and in competition with siblings. Vulnerability to predation by adult Notophthalmus viridescens dorsalis was evaluated for Hyla that had been raised in competition with siblings. Sibships differed in body mass and vulnerability to predation. Competition with Rana or with siblings resulted in a major reduction in body mass. There were no interactions between the effects of parentage and level of competition on body mass. Vulnerability to predation by newts appeared to be determined partially by body mass and partially by parental influences on factors other than body mass. If the differences observed between factors determining body mass and vulnerability to predation reflect negative genetic correlations among these determinants, tradeoffs between selection for increased competitive ability and reduced vulnerability to predators may partially account for the existence of genetic variation for growth rate in larval Hyla.     

The cause of correlations between nightly numbers of male and female barking treefrogs (Hyla gratiosa) attending choruses

The number of males displaying in a lek or chorus each day isoften positively correlated with the number of females visitingor mating in the aggregation. I tested hypotheses that mightexplain such correlations in a study of the barking treefrog(Hyla gratiosa). Experimental reduction of the number of callingmales did not reduce female visitation, ruling out the hypothesisthat such correlations are owing to female preference for, orpassive attraction to, larger choruses. Separate regressionof the numbers of males and females on 13 environmental variablesexplained 45–74% of the variance in the nightly numbersof males and females. Partial regression coefficients for mostof the 13 variables were not significantly different betweenthe sexes, and the relative importance of variables in explainingvariation in the numbers of individuals was similar for bothsexes. These results support the hypothesis that positive correlationsbetween nightly numbers of males and females are owing to asimilar response by both sexes to the same environmental variables.Thus, it appears that the intense sounds emanating from chorusesof H. gratiosa do not function in long-range communication andmay instead be an epiphenomenon of intense, short-range vocalcompetition for females.     

Evaluating predation pressure on green treefrog larvae across a habitat gradient

The effect of a predator on the abundance of a prey species depends upon the predators abundance and its ability to capture that prey. The objectives of this research were to evaluate the community structure of predators of green treefrog (Hyla cinerea) tadpoles across habitat types and evaluate the effectiveness of individual predators on H. cinerea tadpoles. Correspondence and cluster analyses of predator frequencies across 23 aquatic habitats indicated that the majority of variance in predator communities was due to a division between permanent and temporary habitats. Experimental work demonstrated that survival of the smallest H. cinerea tadpoles was significantly lower than survival of medium and large tadpoles with the most effective predators, indicating that H. cinerea tadpoles attain a refuge from predation at larger body sizes. We combined the effectiveness of predators in experiments with the abundance of each predator species from the predator community survey to demonstrate that predation pressure on H. cinerea tadpoles is higher in temporary ponds. This pattern may explain in part why this species generally breeds successfully only in permanent habitats. It also confirms that discussions about an increasing gradient of predation pressure from temporary to permanent aquatic habitats should be restricted to individual prey species for which such a gradient has been demonstrated.     

THE EFFECTS OF THE MATING SYSTEM ON PROGENY PERFORMANCE IN HYLA CRUCIFER (ANURA: HYLIDAE)

We performed a controlled mating experiment to determine whether genetic variation in larval traits in Hyla crucifer was predictable on the basis of mating status or body size of male parent. Larval growth rate was predictably related to body size of the sire. Males from the upper half of the body-size distribution sired offspring with 6% higher growth rates than those of offspring sired by males from the lower half of the body-size distribution. Offspring sired by males that obtained mates in nature had 3% higher growth rates than their half-siblings sired by males that did not mate in nature. Genetic variation for larval-period duration and size at metamorphosis was detected; however, neither mating status nor body size of sire could be used to predict values of these traits in the progeny. Although all three larval traits can affect fitness, there was no evidence that the offspring of some sires would always outperform the offspring of others in all three traits. The predictable association between adult male size and larval growth rate means that the H. crucifer mating system would have a directional effect on larval growth rate if male body size influences the outcome of male-male competition or female choice.     

MORPHOLOGICAL VARIABILITY IN GENETICALLY DEFINED CATEGORIES OF ANURAN HYBRIDS

Hybridization phenomena in anurans have traditionally been studied through morphological comparisons, under the assumption that various hybrids (e.g., F₁‘s, backcrosses) are predictably intermediate to parental species. We critically evaluate this assumption by examining morphology in genetically categorized hybrids between the treefrogs Hyla cinerea and H. gratiosa. A total of 202 frogs from a hybridizing population in Alabama were assayed for allozyme and mitochondrial DNA genotype and for a large suite of osteological characters. Discriminant analyses demonstrated distinct morphological separation between the genetically “pure” parental species. Morphometric analyses of genetically identified hybrids showed: 1) an extreme range of phenotypic expression within F₁ and backcross classes, and 2) no apparent directional parental bias on the F₁ phenotype. Had morphology alone been used as a guide, over 40 percent of the individuals with known hybrid ancestry would have been misclassified as “pure” parental species, and about 25 percent of the backcross individuals would not have been distinguished from F₁‘s. These results exemplify the utility of joint comparisons of morphology and genotypic constitution in studies of natural hybridization, and they emphasize the limitations inherent in describing hybrid classes solely by morphological criteria.     

Energy accumulation and amphibian metamorphosis

Summary Timing of metamorphosis and size at metamorphosis were examined for Hyla crucifer under two densities in the laboratory. Results agree with previously known relationships for ranids and bufonids: at higher densities developmental time is longer, mean size at metamorphosis is smaller, and both parameters have greater variance than at lower densities. Density also affects energy accumulation: under ad libitum feeding conditions, an average individual raised at a higher density is able to accumulate less energy before metamorphosis than an average individual raised at a lower density. The suggestion is made that, in addition to growth and differentiation rates, energy accumulation may be important in determining when amphibian metamorphosis occurs.     

Genetic differentiation of the Iberian tree frogs Hyla arborea molleri and Hyla meridionalis (Amphibia: Anura)

Genetic differentiation of Hyla arborea molleri and H. meridionalis was studied through allozyme electrophoresis and through essays of artificial hybridization. Samples were taken at several localities in Portugal, where these species occur either in sympatry or allopatry, and at one locality in France. Of the 21 presumptive loci analyzed, 6 were monoallelic, while 9 loci were discriminative between both species. A relatively high level of genetic differentiation was found between the two species (NEI's D = 0.725), contrasting with their morphological similarity. Remarkably low levels of genetic variability were found within species, with greater differentiation found among samples of H. meridionalis than of H. a. molleri. Although the capacity for interspecific hybridization was low, hybrid embryos resulted from both reciprocal crosses, but with much greater success in the cross ♂H. a. molleri x δH. meridionalis. Hybrids showed greater similarity with the male parent, and showed heterozygote allozymic phenotypes for discriminative loci. Results are compared with those of similar studies on the same and different species of Hyla, and differentiation of H. a. molleri and H. meridionalis is discussed in relation to their taxonomic status and origin.     

The separate and combined effects of harmonic structure, phase, and FM on female preferences in the barking treefrog (Hyla gratiosa)

In natural advertisement calls of the barking treefrog, Hyla gratiosa, a small amount of incoherent frequency modulation (FM) is present. Incoherency in the FM of a call creates inharmonicity and phase changes between its frequency components. In this study, the combined and separate effects of the harmonic structure, phase spectrum, and FM of an advertisement call on female choice were tested. The harmonic structure of a call can have a direct effect on female preference; females showed a significant preference for static-inharmonic calls over static-harmonic calls. Neither differences in phase or FM alone conferred a preference in two choice tests. However, when FM is present in both calls it does influence female preference for harmonic structure -namely harmonic calls become preferable to inharmonic calls. This reversal of female preference for inharmonicity in a call by the presence of FM suggests that call parameters may interact, and thereby effect mate choice.Abbreviations AP amphibian papilla - BP basilar papilla - FM frequency modulation - PM phase modulation - HS harmonic structure - GB Gaussian Band     

Osteological development of Cope's Gray Treefrog,Hyla chrysoscelis

Hyla chrysoscelis, Cope's Gray Treefrog, is a generalized treefrog found throughout much of east‐central North America. Although it is a model for many behavioural and ecological studies, little is known of its skeletal morphology or development. Herein, we describe the postembryonic skeletal development and adult osteology of H. chrysoscelis. The adult skull is well ossified with slight dermal ornamentation, the postcranial and tadpole skeletons are fairly non‐distinct with no obvious novel morphologies, and the Gosner stage by which bony elements first appear varies. We compare the rank order sequence of ossification to that of its sibling species Hyla versicolor and use examples from this study to demonstrate current complications with conducting ossification sequence meta‐analyses.     

The thermal physiology of two sympatric treefrogs Hyla cinerea and Hyla chrysoscelis (Anura; Hylidae)

Metabolic rates, temperature acclimation, lipid deposition and temperature tolerance were investigated in two species of hylid treefrogs, the green treefrog (Hyla cinerea) and the coastal plain (Cope's) gray treefrog (Hyla chrysoscelis). The rate of oxygen consumption at rest differed between the two species only at 30 degrees C; there was no difference in respiratory metabolism at lower ambient temperatures. Hyla cinerea generally completed metabolic acclimation earlier than H. chrysoscelis, particularly at high temperatures; both species appeared to be fully acclimated in 6 days or less. The gray treefrog is less tolerant of high ambient temperatures than the green treefrog; mean upper lethal temperature was 41.5 degrees C for Hyla chrysoscelis and 43.7 degrees C for H. cinerea. Metabolized energy was higher at high ambient temperatures (i.e. 29 degrees C) for H. chrysoscelis than H. cinerea, while the reverse was true at 19 degrees C. The coefficient of utilization (100 X metabolized energy/gross energy intake) did not vary significantly between species or within species over the ambient temperature range of 19-24 degrees C; H. chrysoscelis had a significantly higher efficiency at 29 degrees C. Lipid reserves were generally similar in the two species throughout the summer. Differences in behavior, seasonal variation in activity and timing of reproduction are all related to thermal physiology and may play a role in determining the distributional limits of the two species.     

Embryonic development of the tree frog, Hyla arborea

A staged series with time data and original photographs of the embryonic development of the tree frog, Hyla arborea are presented, following Gosner’s generalized table. To contribute to species specific embryological studies of anurans, the external features of development of this species were described. Duration of embryonic development, from fertilization to 25^th stage, is about 9 days at 20 ± 1 °C. Cleavage is holoblastic and unequal. Unlike the typical anuran development, neurular rotation was not observed in stage 15 or in any other stages. Other developmental stages resulted in a normal-looking. Embryos of H. arborea hatched at stage 20 or 21, which varied individually. Hatching occurred in the 4^th or 5^th day after fertilization. The mean total length of hatchlings was calculated as 6.38 mm. The mean size of ovum was estimated 1.4 mm in diameter.