Hyla Chrysoscelis Research Articles

Effects of enhanced lighting on the behaviour of nocturnal frogs

Abstract. Biologists studying anuran amphibians usually assume that artificial, visible light does not affect the behaviour of nocturnal frogs. This assumption was tested in a laboratory experiment. The foraging behaviour of grey treefrogs, Hyla chrysoscelis, was compared under four lighting conditions: ambient light (equivalent to bright moonlight, 0·003 lx), red-filtered light (4·1 lx), low-intensity 'white' light (3·8 lx), and high-intensity 'white' light (12·0 lx). The treatments were chosen to correspond to standard methods of field observation of frog behaviour. The foraging behaviour of frogs in the four treatments was observed using infra-red light that was invisible to the frogs. The ability of the frogs to detect, and subsequently consume prey was significantly reduced under all of the enhanced light treatments relative to the ambient light treatment. Thus, the use of artificial light, within the visible spectrum of the frogs' eyes, can influence the outcome of nocturnal behavioural observations. These results lead to the recommendation that anuran biologists use infra-red or light amplification devices when changes in frogs' visual capabilities may influence the conclusions drawn from a study.

Your Pond or Mine?

Eggs of the gray tree frog. During amplexus, the male deposits sperm on the eggs as the female lays them. Hours later, the protective jelly firms, shielding the developing embryos until they hatch as tadpoles. I t's a warm Saturday night in July, and things are literally hopping down at the pond. Hordes of male gray tree frogs have gathered at the water's edge, belting out guttural trills that fill the air with songs of frog love. Seduced by the dulcet tones, female frogs their eggs ripe for fertilization descend from the trees around midnight for their once-yearly night of romance. Each selects a male and pulls him onto her back in a mating clasp called amplexus. But instead of lingering to luxuriate in the embrace and release her eggs, she immediately hops away with her lover still clinging in search of a more perfect pond for pairing. Why, after being dragged around season after season for millennia, haven't the species' males evolved to call from sites that better suit the females? Ecologists who study the gray tree frog, Hyla chrysoscelis, have long been stumped by this odd gender gap. Although the phenomenon remains unresolved, new research shows that males and females of this nonterritorial species have somewhat different criteria for what constitutes the ultimate love puddle. William J. Resetarits Jr. and Henry M. Wilbur of Duke University in Durham, N.C., began studying the species' disjointed breeding behavior in 1985. As population ecologists, they sought to discover whether opposing natural forces kept the males and females out of sync. We want to understand how ecology affects the behavior of organisms, says Resetarits, now at the University of Missouri-St. Louis. We're really interested not only in the behavior of individual species, but what effect their behavior may have on the community In the June ECOLOGY, Wilbur, now at the University of Virginia in Charlottesville, and Resetarits report the results of 52 long, sweaty summer nights spent in the woods of North Carolina, monitoring the frogs' mating quirks at 45 pseudo-ponds.

Calling Site Choice by Hyla Chrysoscelis: Effect of Predators, Competitors, and Oviposition Sites

We examined the effect of predators, competitors, and conspecifics on the choice of calling sites by male gray treefrogs, Hyla chrysoscelis, and the correlations between choice of calling sites by males and choice of oviposition sites by females. The experiment was conducted in an array of wading pools and utilized a replicated block design for variance analysis. Pools were open to natural colonization by male and female treefrogs. The treatments consisted of controls containing no predators or competitors, four treatments containing one species of predator (either Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, or Tramea carolina larvae), and two treatments containing one species of competitor (either Rana catesbeiana or Hyla chrysoscelis). Male and female treefrogs avoided pools containing conspecific tadpoles and pools containing adult black—banded sunfish, Enneacanthus chaetodon. Females also avoided pools containing larval spotted salamanders, Ambystoma maculatum. Correspondence between calling sites and oviposition sites was examined in response to treatment, block (consisting of a location and a time), and location. Correspondence between male choices and female choices was weak. Number and location of calling males showed no significant correlation with the number and location of eggs along any of the axes of choice. Both males and females chose sites based on the species present, and both showed preferences in regard to location and time. The differences between male and female choices indicate that the potential choices have different values to each sex, or that different criteria are used to rank potential choices. Anuran reproductive behavior is not a simple consequence of the direct interactions between males and females, but is a result of complex interactions between the behavior of the two sexes and the environment in which their behavior is embedded.

Interactions Between the Salamander Siren Intermedia and the Keystone Predator Notophthalmus Viridescens

Is the ability to function as a keystone predator a property of a species or an emergent property of the community? We discussed this question in a temporary—pond community where the broken—striped newt Notophthalmus viridescens dorsalis is known to act as a keystone predator on larval anurans. We independently manipulated the initial density of adult Notophthalmus (two or four) and the presence or absence of one adult Siren intermedia in a set of 20 artificial ponds to determine if this additional predator affected the ability of Notophthalmus to function as a keystone predator. Each pond received a diverse assemblage of larval anuran prey: a spring—breeding assemblage of 275 Rana utricularia, 100 Pseudacris crucifer and 25 Bufo americanus tadpoles, and a summer assemblage of 200 Hyla chrysoscelis and 150 Gastrophryne carolinensis tadpoles. Two additional pond received neither predator, to assay the outcome of competition among the tadpoles. The effect of the salamander Siren on Notophthalmus survival and fecundity depended on the density of Notophthalmus. At low newt density Siren had no effect on survival, but at high newt density competition reduced the survival and growth rates of Notophthalmus. The density of Notophthalmus and the presence of Siren interacted to determine the fecundity of Notophthalmus. At low newt density Siren reduced Notophthalmus reproductive success by preying on larvae. At high newt density Siren indirectly enhanced reproductive success by reducing survival of adults, thus releasing larvae from intraspecific competition and cannibalism. Notophthalmus density had no effect on adult Siren survival or growth rate in this experiment, but other evidence indicates that competition with Notophthalmus reduces the growth rates of Siren. Notophthalmus acted as a keystone predator on the assemblage of spring—breeding anurans; it increased the number of metamorphs of the weak competitor Pseudacris crucifer by releasing them from interspecific competition. Siren preyed on tadpoles in a nonselective manner. An additive model was sufficient to describe the effects of Siren and initial density of Notophthalmus on the structure of the tadpole assemblage. In our system of experimental ponds, the strong interaction between Siren and Notophthalmus density did not extend its effects to lower trophic levels, and the direct and indirect effects of Siren did not alter the role of Notophthalmus as a keystone predator.

Effects of nonlethal injury and habitat complexity on predation in tadpole populations

Our purpose was to determine how nonlethal prey injury and habitat complexity mediate the dynamics of a predator–prey system. We assessed rates of predation by the crayfish Procambarus acutus acutus on Hyla chrysoscelis tadpoles with four levels of tail loss (0, 25, 50, and 75% total tail length removed), and in habitats of three levels of complexity (zero, low, and high density of screen) in a 4 × 3 factorial design. We also examined the effects of tail loss on tadpole sprint velocity and distance traveled. Tadpoles with 75% tail loss were preyed upon significantly more often than tadpoles in the other tail-loss treatments. Habitat complexity did not affect tadpole survival. In addition, there was no interaction between tail loss and habitat complexity. Tail loss significantly affected both tadpole swimming velocity and sprint distance traveled. Tadpoles with 75% tail loss had slower sprint speed and swam a shorter distance than tadpoles with 0 and 25% tail loss, and tadpoles with 50% tail loss had slower sprint speed and swam a shorter distance than tadpoles in the 0% tail loss treatment. Although tadpoles generally rely on short bursts of speed, generated by the tail, to escape predators, tail injury and apparently reduced swimming performance did not increase vulnerability to predation in a simple linear fashion.

Mating behavior and determinants of male mating success in the gray treefrog, Hyla chrysoscelis

We studied mating behavior and male mating success in a population of gray treefrogs (Hyla chrysoscelis) during 1987 and 1988 in western Tennessee. During 1988, individual males called from 1 to 7 nights ([Formula: see text]) and mated 0–4 times ([Formula: see text]). The number of nights that males called or achieved amplexus was not related to body size, but males that called on more nights had a relatively greater chance of mating. There was no significant difference in body size between mated and unmated males on any of the nights tested. Males that mated or called on a previous night were not more likely to achieve amplexus than males that had not mated previously or those that were new to the population. We have no strong evidence to suggest that assortative mating was characteristic of our population or that male–male aggression directly influences female choice.

CALL ANALYSIS OF TRIPLOID HYBRIDS RESULTING FROM DIPLOID-TETRAPLOID SPECIES CROSSES OF HYLID TREE FROGS

ABSTRACT Calls produced by hybrids resulting from laboratory crosses of tetraploid Hyla versicolor females and either diploid Hyla chrysoscelis (type I) or Hyla arborea (type II) males were induced through manipulation of environmental conditions. Type I hybrids produced trilled calls similar in note repetition to H. versicolor, but more similar in dominant frequency to H. chrysoscelis. Mean duration was shorter than in both parent calls. Type II hybrids produced calls which were longer in duration and lower in note repetition rate than H. versicolor, but shorter in duration and higher in note repetition rate than H. arborea. Dominant frequency of type II hybrids was lower than in H. arborea but not significantly different than in H. versicolor. Hybrid calls were not strictly intermediate, and may provide information regarding parental relationships.

Interactive Effects of Predators and Early Larval Dynamics of the Treefrog Hyla Chrysocelis

Ecologists need to determine the frequency and nature of higher order interactions and to predict when such interactions will be important to understand community dynamics. I experimentally tested for higher order interactions in a freshwater food web by independently manipulating the presence and absence of two predators (the salamander Notophthalmus viridescens and the crayfish Cambarus bartonii) and their tadpole prey (Hyla chrysoscelis) in a series of artificial ponds. By destructively sampling selected ponds early in the larval period (days 5 and 6), I hoped to predict whether the predators would have an interactive effect on tadpole survival and growth on day 30. Early destructive samples adequately predicted subsequent influences of interactions on the survival, but not on the size of Hyla. Early samples predicted that the predators would interact to determine tadpole survival and size. The two predators did not act additively to determine tadpole abundance: more survived in the presence of both predators than was predicted. This higher order interaction was most likely caused by physical interference between the predators and predator avoidance by the tadpoles. Early samples correctly predicted higher order interactions would be seen on tadpole growth, but completely failed to predict their direction because the biological interactions changed over time. Early in their larval period, tadpoles behaviorally avoided predators, which caused slower growth, but later, tadpoles in predator treatments grew faster due to diminished intraspecific competition. These results suggest that higher order interactions may be important in nature. Short—term experiments may prove useful in predicting when higher order interactions will occur, but detailed models may be required to predict their outcome accurately.

Effects of body size, sibship, and tail injury on the susceptibility of tadpoles to dragonfly predation

The effect of tadpole body size, sibship, and tail injury on the survival of treefrog tadpoles (Hyla chrysoscelis) in the presence of dragonfly larvae (Tramea lacerata) was examined in a three-way factorial experiment. Tadpole body size had a dramatic effect on survival; large tadpoles had higher survival than medium-sized or small tadpoles. The presence of tail injury simulating an unsuccessful predation attempt significantly reduced survival. Survival of control tadpoles without tail injury in the presence of a predator was almost twice as high as that of tadpoles with 75% tail loss. Tadpole sibship had no effect on survival and indicated that genetic differences in antipredator behavior or production of alarm substances and allelochemicals, independent of body size, were not apparent. There were no significant interactions between tail injury and body size or tail injury and sibship. These results indicate that increasing body size is an effective mechanism for reducing predation and that injury from an unsuccessful predation attempt reduces subsequent survival by increasing the risk of future predation.

An examination of multiple factors affecting community structure in an aquatic amphibian community

The potential effects of multiple factors structuring certain larval amphibian communities were studied using a pen experiment in a natural pond. Potential factors (predation and competition from other species) were allowed to act in a stepwise fashion such that their relative importance could be evaluated. Based on a previous study, it was hypothesized that predation by Ambystoma salamander larvae on other larval amphibian species would be the most important factor. Survival of Ambystoma jeffersonianum salamander larvae and Rana sylvatica tadpoles was significantly depressed only by Ambystoma opacum predation. Survival of Ambystoma maculatum salamander larvae was significantly greater in the absence of both A. opacum and A. jeffersonianum predators. The virtual elimination of Hyla chrysoscelis larvae in all treatments also can be largely attributed to Ambystoma predation. Thus, Ambystoma predation was the dominant factor determining larval survival of four amphibian prey species in the experimental communities.

Female choice of large males in the treefrog Hyla chrysoscelis: the importance of identifying the scale of choice

In this study, I detected the presence of female choice for larger males in the treefrog Hyla chrysoscelis. Mated males were larger than unmated males at the scales females could have assessed males: local groups (males found within 2 meters of a mated pair) and the nightly chorus. In addition, females were observed to initiate amplexus with the larger males in local groups. Mated males were larger than unmated males for two of the four seasons studied. Seasonal mating success was also analyzed with a multiple regression model that included size and chorus attendance (nights spent at the breeding site). The partial selection coefficients, which represent the relative magnitude of directional selection, were significant for size in only one season and for chorus attendance in all four seasons. Therefore, there are two possible advantages to being a large male: increased likelihood of obtaining a mate due to female preference, and increased likelihood of mating even if no female preference because of a greater number of opportunities to obtain a mating if matings occurred at random.

A mechanism for female choice of large males in the treefrog Hyla chrysoscelis

In this study, we combine both field and laboratory experiments to address the effects of female preference for certain call characteristics on a large-male mating advantage in the treefrog H. chrysoscelis. In laboratory-choice experiments, females always chose the call with the lower fundamental frequency when call rate and call intensity were held constant and the difference in frequency between the two calls was 15%. The lower frequency call was preferred by 8 out of 12 females when the difference in fundamental frequency was 7.4%. These results are consistent with field comparisons of the size of unmated males calling within 2 m of a mated male: Male body size was negatively correlated with fundamental frequency and the greater the size difference, the more likely that the larger male mated. In field choice experiments, females preferred males with higher call rates. Since size differences between males used in this experiment averaged only 2.3 mm, we would not expect the fundamental frequency of a male's call to be the best predictor of mating success. Laboratory results demonstrated that call rate could override female preference for the ‘low frequency’ call over the ‘high frequency’ call, while intensity could at least dilute this preference. However, individual males in the field varied both call rate and the call intensity as perceived by the female. We suggest that the interaction between call rate, male size and mating success should be studied further through the use of field-choice experiments.

Polymorphism in the location of the 18S and 28S rRNA genes on the chromosomes of the diploid-tetraploid treefrogs Hyla chrysoscelis and H. versicolor

The chromosomes of the diploid Hyla chrysoscelis and its tetraploid sibling species H. versicolor were studied with AgNO3 staining and in situ hybridization to determine the chromosome location of the 18S and 28S ribosomal RNA (rRNA) genes. A total of 236 Hyla chrysoscelis from 34 localities in 15 U.S. states and 100 H. versicolor from 15 localities in 12 states were examined. The rRNA gene sites were extremely variable in H. chrysoscelis, and also variable, but to a lesser extent, in H. versicolor. The most common rRNA gene site in both H. chrysoscelis and H. versicolor was on the short arm of chromosome 6. All of the rRNA gene locations seen in H. versicolor were also seen in H. chrysoscelis, supporting the hypothesis that the tetraploid H. versicolor arose from H. chrysoscelis. Although polymorphic rRNA gene sites in H. versicolor may reflect the positions of the rRNA genes in H. chrysoscelis ancestors, the origin of the extreme variability of such sites in H. chrysoscelis seems more obscure. Possible explanations include inversions, translocations, mobile genetic elements or a combination of some or all of these.

Effects of Parentage and Competitor Phenology on the Growth of Larval Hyla Chrysoscelis

Larvae of four sibships (a 2 x 2 factorial cross) of Hyla chrysoscelis were raised in three competition regimens. These were competitors (larval Rana catesbeiana) absent, competitors introduced simultaneously with Hyla, and competitors introduced 11 d after Hyla. Hyla body mass at day 41 of growth was influenced by level of competition, timing of competitor introduction, and effects of male and female parent. Rana introduced simultaneously with Hyla reduced Hyla mass at day 41 more than did Rana introduced 11 d later. Female parentage affected body mass at day 41 additively. The effects of male parentage depended on the timing of competitor introduction: when Rana were introduced simultaneously with Hyla, offspring of male 2 were larger at day 41 than offspring of male 1. When Rana were introduced 11 d after Hyla, offspring of male 1 were larger at day 41 than offspring of male 2. This interaction between the effects of male parentage and timing of competitor introduction suggests that some genotypes may exhibit tradeoffs between maximum growth rate and competitive ability, and that phenological variation may lead to selection among genotypes in the natural population.

Choice of Oviposition Site by Hyla Chrysoscelis: Role of Predators and Competitors

The role of predators and competitors in the choice of oviposition site by the treefrog Hyla chrysoscelis was examined in a randomized complete block experiment using 90 replicated experimental ponds. Control ponds containing neither predators nor competitors were contrasted with treatment ponds into which one of four species of predators (Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, Tramea carolina larvae) or one of two species of competitors (Rana catesbeiana, Hyla chrysoscelis) was added. Treatments had significant effects on the mean number of eggs deposited in ponds. Fewer eggs were laid in ponds with Ambystoma, Enneacanthus, or Hyla, as a result of fewer females laying eggs and fewer eggs laid per visit, compared with control ponds. Notophthalmus, Rana, and Tramea had no effect on the number of eggs laid. Ovipositing Hyla discriminated among potential oviposition sites based on the species present. Choice of oviposition site can determine the success of a female's reproductive investment, and it can be a mechanism affecting the structure of ecological communities as well. Our results emphasize the importance of oviposition site choice in the evolution of reproductive patterns and implicate species avoidance by ovipositing females as a mechanism important in generating variability in ecological communities.

Variation in Predator Phenology Affects Predator Performance and Prey Community Composition

Variation in predator phenology strongly influenced the performance of four species of anuran larvae in experimental temporary ponds, the composition of the assemblage of anuran metamorphs from those ponds, and the reproductive performance of the predator. The resolution of variance by a statistical model of these communities was greatly improved when the model incorporated phenological variation. These results suggest that the predictive power of more general models of species interactions could be improved by incorporating size— or stage—specific effects. The experimental communities contained larval Rana utricularia, Scaphiopus holbrooki, Bufo americanus, and Hyla chrysoscelis. Dates of introduction of these species to experimental ponds corresponded with their breeding in nature. The predators were adult broken—striped newts (Notophthalms viridescens dorsalis). The experiment was a 2 x 4 factorial design:two levels of initial anuran density (1 x and 4 x) crossed with four predation regimes (predators absent; initially present and removed after 9 d; initially present and removed after 51 d; always present). The predator—removal treatments simulated variability observed in the timing of departure of adult newts from a natural pond. At both initial anuran densities, competition and predation interacted in determining the composition of the metamorph assemblage. The contribution of competition was greater at high initial density than at low. Competition among the four anuran species did not fall into a simple hierarchy, but formed a more complex network of effects. The presence of predators during the first weeks of the larval period of any anuran species greatly reduced the number of metamorphs of that species. Predator removal after the first few weeks of species' larval periods did not affect their final abundance, indicating that they reached body sizes that reduced their vulnerability to predation. Later breeding anurans benefited when newts were present but were removed before they reproduced, reducing competition from earlier breeding species. When earlier breeding species were exposed to newt predation, they performed better when newts were present throughout the summer than when they were removed before later breeding species reproduced. The vulnerability of these early breeding species to newt predation had greatly decreased by the time of the late newt removal. Presence of newts past this time benefited them by reducing the competitive effects of late breeders. A statistical model of community composition that included the effects of predator presence or absence and anuran initial density explained 47% of the variation in composition of the metamorph assemblage. A model that explicitly considered predator phenology explained 76%, leaving less than half as much variation unexplained. The reproductive success of newts was affected by their own phenology and by the density of anuran larvae. Longer periods of adult newt residence increased the numbers of newt offspring while decreasing offspring mean body mass. There was strong correlational evidence that the body masses of newt offspring were limited by their own density. The final masses and numbers of newt offspring were also affected by both initial anuran density and the numbers of anuran metamorphs emerging from experimental ponds. Newt offspring responded more strongly to the density of anuran species with which they had greater temporal overlap.

Cryptic Species of Hylid Marsupial Frogs in Peru

Cryptic Species of Hylid Marsupial Frogs in Peru

Fish predation in size-structured populations of treefrog tadpoles.

The effects of tadpole body size, tadpole sibship, and fish body size on predation of gray treefrog tadpoles, Hyla chrysoscelis, were studied in laboratory and artificial pond experiments. Tadpole body size had a significantly positive effect on the survival of tadpoles in all experiments. The relationship between tadpole biomass eaten and biomass available suggested that fish were not satiated when consuming the largest tadpoles. Large tadpoles were probably better able to evade predators. A difference in survival among full sib families of tadpoles was only present in one family, suggesting that genetic differences in predator avoidance behavior or palatability were probably secondarily important to body size per se. Fish body size had a significantly negative effect on the survival of tadpoles. Larger fish consumed a larger number and proportion of tadpoles as well as greater biomass. These results indicate that environmental factors affecting the growth rate of tadpoles cand dramatically alter their vulnerability to gape-limited predators.

Regulation of Structure in Complex Systems: Experimental Temporary Pond Communities

Interactions among competition, predation, and disturbance in determining the abundances of four species of anurans were studied in a factoral experiment using 36 replicated experimental ponds. Hatchlings of the four species (Rana utricularia, Scaphiopus holbrooki, Bufo americanus, and Hyla chrysoscelis) were introduced at the same relative abundances at a low or high initial density. Competition determined survival, body size at metamorphosis, and length of larval period in high—density communities, which were food—limited. The predatory salamander Notophthalmus viridescens did not alter either the total number of metamorphs or their combined biomass in the high—density communities, but the relative abundance of metamorphs was shifted as Scaphiopus holbrooki, the competitive dominant, was selectively eaten. In low—density communities, predation decreased survival and biomass production of tadpoles, often to zero, in all species. For each of the four combinations of tadpole density and presence or absence of predators, ponds were drained at three rates to simulate different drying regimes. Competition slowed growth and thus increased the risk of desiccation in high—density populations in drying ponds. Predation ameliorated the effects of competition, allowing survivors to grow rapidly enough to metamorphose before ponds dried. Survival of tadpoles to metamorphosis, body size at metamorphosis, and the timing of metamorphosis were determined in a complex way by interactions among all of the treatment effects and the life history characteristics of the four species studied. Scaphiopus was the species least sensitive to tadpole density and was the competitive dominant in ponds without newts. It has a rapid growth rate and metamorphosed soon enough to escape desiccation. It suffered the greatest risk of predation and was eliminated from even some of the high—density communities. Rana was most successful in low—density communities without predators. No Rana survived in rapidly drying (50—d) ponds because of insufficient time to obtain a minimum size for metamorphosis. Rana were also eliminated from most populations exposed to predation. The effect of competition on Bufo in high—density populations, few or no survivors, was reversed by predation as newts selectively fed on Scaphiopus and Rana. This result was most striking in the tanks that dried most rapidly. Hyla did very poorly in all slowly drying (100—d) tanks compared with controls because of intense competition. It had moderate success in high—density communities where newts had removed most competitors. These results show that biological and environmental factors interact to determine the structure of anuran communities. Neither competition no predation is the single unifying force, but rather they interact to determine the different consequences of the date of drying of a pond to the success of each species.

Predator-prey interactions among fish and larval amphibians: use of chemical cues to detect predatory fish

Gravitational flow-through systems were used to determine whether larvae of two-lined salamanders, Eurycea bislineata, and Cope's grey treefrog, Hyla chrysoscelis, use chemical cues to detect predatory fish. Eurycea larvae were attracted to water conditioned with food, showed no response to water conditioned with green frog, Rana clamitans, tadpoles, and avoided water conditioned with a dangerous predator, the green sunfish, Lepomis cyanellus. Hyla tadpoles exposed to water conditioned with green sunfish spent significantly more time in refuges than did controls exposed only to water. In a comparable experiment performed at a natural breeding site, Hyla tadpoles significantly increased refuge use when exposed to water conditioned with fish. These data suggest that amphibian larvae use chemical cues in their natural habitats to minimize predation risk from fish.