Proximate developmental causes of limb length variation between Hyla cinerea and Hyla gratiosa (Anura: Hylidae)

Hyla cinerea and H. gratiosa are closely related treefrogs that differ at metamorphosis in the length of the hind limb relative to snout vent length. A developmental series of larvae of the two species raised under uniform laboratory conditions reveals that H. cinerea have proportionately longer legs at metamorphosis than H. gratiosa for the following reasons: 1) Hyla cinerea initiate limb growth at a smaller body size than H. gratiosa, 2) H. cinerea have a faster rate of limb growth, and 3) body growth rates in H. cinerea and H. gratiosa are nearly identical until just before metamorphosis.     

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.     

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     

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.     

Functional distance and establishment of non-native species with complex life cycles

More than 80% of animals have complex life cycles and undergo distinct changes in ecology and morphology during development. The strength and type of factors regulating each life-stage may differ as an organism may occupy different niches during ontogeny. We examined the functional distance at larval and adult life-stages of two non-native anurans (Green Tree Frog [Hyla cinerea] and Bullfrog [Lithobates catesbeianus]) that have established in a Chihuahuan Desert anuran assemblage in Big Bend National Park. Both life stages of both non-native species occupied niche space outside of the native assemblage. At the larval stage, the ability of the tadpoles to utilize permanent aquatic habitats and coexist with predatory fishes differentiated the non-native species from the majority of the native species that are restricted to temporary pools. At the post-metamorphic life stage, each species appears to have established by exploiting unoccupied habitat and trophic niches in the recipient community. The arboreal habits of H. cinerea may enable it to utilize resources in microhabitats that are otherwise not used by native species because arboreal frogs are absent from this native assemblage. The large body size of post-metamorphic L. catesbeianus may enable it to utilize larger food resources that are otherwise unavailable to the smaller-bodied natives. Separate comparison of larval and adult functional traits between non-natives and the native community may help predict their potential establishment or invasion success as well as aid in the development of stage-specific control or eradication efforts.     

Thermal reaction norms in two Coenagrion damselfly species: contrasting embryonic and larval life-history traits

1. We studied the temperature‐dependence of important life‐history traits both at the embryonic (egg hatching success, embryonic development time and hatchling size) and the larval stage (larval growth rate, larval survival and larval size after 100 days) using full‐sib families of two congeneric damselflies, Coenagrion hastulatum and Coenagrion puella, that differ in latitudinal distribution. Larvae were reared in the laboratory from the egg stage at four temperatures (12, 17, 22 and 27 °C). 2. The observed patterns of thermal plasticity in embryonic traits showed that the northern species was more successful than the southern species at lower temperatures, in line with the pattern of temperature adaptation in thermal reaction norms. 3. At the larval stage, we found no consistent pattern of latitudinal compensation. The thermal family reaction norms indicate, however, the potential for latitudinal compensation to evolve. We observed an ontogenetic shift in thermal optima for larval growth rate, with a higher optimal temperature for growth rate during the first 2 weeks of the larval stage. 4. This is the first indication of the existence of latitudinal compensation at the interspecific level in an invertebrate; it is stage‐specific, being present only in the embryonic stage. We argue that compensation in the embryonic stage may be much more likely than in the larvae and stress the importance of including more then one life‐history stage when drawing conclusions about the adaptiveness of patterns in thermal reaction norms.     

Machine learning identifies specific habitats associated with genetic connectivity in Hyla squirella

The goal of this study was to identify and differentiate the influence of multiple habitat types that span a spectrum of suitability for Hyla squirella, a widespread frog species that occurs in a broad range of habitat types. We collected microsatellite data from 675 samples representing 20 localities from the southeastern USA and used machine‐learning methodologies to identify significant habitat features associated with genetic structure. In simulation, we confirm that our machine‐learning algorithm can successfully identify landscape features responsible for generating between‐population genetic differentiation, suggesting that it can be a useful hypothesis‐generating tool for landscape genetics. In our study system, we found that H. squirella were spatially structured and models including specific habitat types (i.e. upland oak forest and urbanization) consistently explained more variation in genetic distance (median pR² = 47.78) than spatial distance alone (median pR² = 23.81). Moreover, we estimate the relative importance that spatial distance, upland oak and urbanized habitat have in explaining genetic structure of H. squirella. We discuss how these habitat types may mechanistically facilitate dispersal in H. squirella. This study provides empirical support for the hypothesis that habitat‐use can be an informative correlate of genetic differentiation, even for species that occur in a wide range of habitats.     

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.     

Influence of priority effects and pond location on invaded larval amphibian communities

In Florida, the Cuban Treefrog (Osteopilus septentrionalis) is a superb colonist and appears to be a significant driver of amphibian community dynamics. Decline of native anurans has been linked to possible competition with adult O. septentrionalis but interactions during the larval stage are largely unknown. Rearing O. septentrionalis tadpoles along with two native anurans, the Squirrel Treefrog (Hyla squirella) and the Southern Toad (Bufo terrestris) in both experimental artificial ponds and laboratory aquaria, the role of competition as the mechanism driving the dynamics of invaded amphibian communities in Florida was examined. Also examined was the role of priority effects and variation between pond locations in altering interactions between O. septentrionalis and native anuran larvae. Interspecific competition was strong during the larval stage; the presence of O. septentrionalis reduced larval performance and survival of native anurans. Pond location alone had little effect on interspecific interactions, but priority effects were strong. Pond location and priority effects acted together to influence species interactions. The selective influence of different interaction modifiers acted to increase or decrease the impacts of exotic species on native taxa.     

Development of two forensically important blowfly species (Chrysomya megacephala and Chrysomya rufifacies) (Diptera: Calliphoridae) at four temperatures in India

The development of the Oriental latrine fly, Chrysomya megacephala (Fabricius), and hairy maggot blowfly, C. rufifacies (Macquart) (Diptera: Calliphoridae), was studied at four different temperatures (22°C, 25°C, 29°C and 31°C) in order to draw correlations between larval age, body length and body dry weight. The mean larval body length increased steadily from a minimum of 1.4 mm for C. megacephala and 1.8 mm for C. rufifacies to a maximum of 17.4 mm for C. megacephala and 15.9 mm for C. rufifacies at different temperatures. Similarly, the mean dry weight increased steadily from a minimum of 0.0007 g for C. megacephala (second instar) and 0.0008 g for C. rufifacies (second instar) to a maximum of 0.0290 g for C. megacephala and 0.0270 g for C. rufifacies at different temperatures. Entomological evidence is often used to estimate the minimum postmortem interval (mPMI) and both of these species are important from a forensic point of view. Graphs of age of larvae vs. body length and age of larvae vs. dry body weight at different temperatures can be used to estimate the larval age of these two species.     

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.     

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.     

Searching for constraints by cross‐species comparison: reaction norms for age and size at maturity in insects

An evolutionary explanation should consider the balance between environmentally‐based selective pressures, and the resistance of the organism's phenotype to adaptive evolution, with the latter being captured by the concept of constraint. The limited attention to non‐adaptive explanations in evolutionary ecology is at least partly caused by methodological difficulties with respect to identifying and quantifying constraints. As an example of an experimental approach evaluating a constraint‐based explanation, we present a cross‐species comparison of the shape of reaction norms for size and age at maturity. Instar‐ and sex‐specific development times and final sizes were recorded for two distantly‐related species of insects (Lepidoptera), with larval growth rates being manipulated by means of refined starvation treatments. We found that (1) the ‘classical’ L‐shaped reaction norms for final size and development time are characteristic also of individual larval instars; (2) these responses show a high degree of quantitative similarity across the species, different larval instars, and sexes within species; and (3) the similarity among species and sexes is higher for the penultimate than for the final instar. The high degree of similarity suggests that some physiological mechanisms determining such reaction norms are evolutionarily conservative. An alternative explanation (i.e. quantitative similarity of ecologically based selective pressures) appears less likely. The results of a previous study on a third lepidopteran species not only support our general conclusions, but also provide a clear case of adaptive evolution in some aspects of such reaction norms. The present study shows one way how the data required to measure evolutionary conservatism in reaction norms for body size can be obtained empirically. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 296–307.     

Amphibian phenotypic variation along a gradient in canopy cover: species differences and plasticity

The distributions of many freshwater organisms correlate with a gradient in canopy cover, ranging from sunny wetlands to closed woodland ponds. Little is known about mechanisms that exclude species from some sections of the gradient while allowing persistence in others. I addressed this question by manipulating shading in 740‐l outdoor mesocosms and measuring several ecologically‐relevant traits in three species of amphibian larva (Rana temporaria and Triturus alpestris, generalists occupying the entire gradient; and Hyla arborea, a specialist in open habitats). Shading caused delayed development, but had no effect on survival and increased the growth rate of R. temporaria. Body and tail color were darker in the shade. Plasticity in morphological shape, consisting of reduced gut width and increased tail size under shaded conditions, may reflect poor food availability and low dissolved oxygen. The canopy generalist R. temporaria increased activity in the shade, spent more time basking in shallow water, and maintained high larval performance. Unexpectedly, the specialist H. arborea was also highly plastic. These results describe extensive phenotypic plasticity induced by shade, and highlight traits that may influence performance along the canopy gradient.     

Ladybird population dynamics in potato: comparison of native species with an invasive species, Harmonia axyridis

Following the detection of the harlequin ladybird, Harmonia axyridis, in 2003 in potato crops in Belgium, a study was carried out between 2004 and 2006 on the phenology of this species compared to native species in potato. The results confirmed the success of H. axyridis, with high population levels in 2004 and 2005. In 2006, aphid populations were very low and no H. axyridis larvae were sampled in potato, but the indigenous species Coccinella septempunctata and Propylea quatuordecimpunctata were detected. A species by species comparison of the date of first larvae detection, the larvae population peak, and the difference between this peak and the aphid population peak was performed. Results showed a clear correlation between C. septempunctata and P. quatuordecimpunctata and potato aphids, with a delay of 3.5 and 6.5 days between the aphid and ladybird population peaks for the two native species. H. axyridis arrived 7–8 days after the two indigenous species and the larval peak population occurred 15.8 days after the aphid population peak. This meant that H. axyridis had to complete its larval development with very low aphid populations or even with no aphids at all. The reason for its late arrival and the possible food resources used by H. axyridis larvae are discussed.     

Estimating occupancy dynamics in an anuran assemblage from Louisiana,USA

Effective monitoring programs are designed to track changes in the distribution, occurrence, and abundance of species. We developed an extension of Royle and Kéry's (2007) single species model to estimate simultaneously temporal changes in probabilities of detection, occupancy, colonization, extinction, and species turnover using data on calling anuran amphibians, collected from 2002 to 2006 in the Lower Mississippi Alluvial Valley of Louisiana, USA. During our 5-year study, estimates of occurrence probabilities declined for all 12 species detected. These declines occurred primarily in conjunction with variation in estimates of local extinction probabilities (cajun chorus frog [Pseudacris fouquettei], spring peeper [P. crucifer], northern cricket frog [Acris crepitans], Cope's gray treefrog [Hyla chrysoscelis], green treefrog [H. cinerea], squirrel treefrog [H. squirella], southern leopard frog [Lithobates sphenocephalus], bronze frog [L. clamitans], American bullfrog [L. catesbeianus], and Fowler's toad [Anaxyrus fowleri]). For 2 species (eastern narrow-mouthed toad [Gastrophryne carolinensis] and Gulf Coast toad [Incilius nebulifer]), declines in occupancy appeared to be a consequence of both increased local extinction and decreased colonization events. The eastern narrow-mouthed toad experienced a 2.5-fold increase in estimates of occupancy in 2004, possibly because of the high amount of rainfall received during that year, along with a decrease in extinction and increase in colonization of new sites between 2003 and 2004. Our model can be incorporated into monitoring programs to estimate simultaneously the occupancy dynamics for multiple species that show similar responses to ecological conditions. It will likely be an important asset for those monitoring programs that employ the same methods to sample assemblages of ecologically similar species, including those that are rare. By combining information from multiple species to decrease the variance on estimates of individual species, our results are advantageous compared to single-species models. This feature enables managers and researchers to use an entire community, rather than just one species, as an ecological indicator in monitoring programs. © 2011 The Wildlife Society.     

A treefrog's menu: Selection for an evening's meal

Summary Spodoptera (Lepidoptera) and Chauliognathus (Coleoptera) larvae are the two prey most frequently consumed by Hyla cinerea in the field, whereas Musca (Diptera) is the most consistently preferred food item offered in laboratory experiments in which prey are equally available. Three genus-specific variables influence prey selection: (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) vary within the behavioral repertoire of a prey genus as well as among different prey performing similar activities.