Stream Noise,Hybridization, and Uncoupled Evolution of Call Traits in Two Lineages of Poison Frogs: Oophaga histrionica and Oophaga lehmanni

According to the acoustic adaptation hypothesis, communication signals are evolutionary shaped in a way that minimizes its degradation and maximizes its contrast against the background noise. To compare the importance for call divergence of acoustic adaptation and hybridization, an evolutionary force allegedly promoting phenotypic variation, we compared the mate recognition signal of two species of poison frogs (Oophaga histrionica and O. lehmanni) at five localities: two (one per species) alongside noisy streams, two away from streams, and one interspecific hybrid. We recorded the calls of 47 males and characterized the microgeographic variation in their spectral and temporal features, measuring ambient noise level, body size, and body temperature as covariates. As predicted, frogs living in noisy habitats uttered high frequency calls and, in one species, were much smaller in size. These results support a previously unconsidered role of noise on streams as a selective force promoting an increase in call frequency and pleiotropic effects in body size. Regarding hybrid frogs, their calls overlapped in the signal space with the calls of one of the parental lineages. Our data support acoustic adaptation following two evolutionary routes but do not support the presumed role of hybridization in promoting phenotypic diversity.     

Eavesdropping on the Neighbourhood: Collared Pika (Ochotona collaris) Responses to Playback Calls of Conspecifics and Heterospecifics

The acoustic environment, composed in part by the vocalizations of sympatric animals, is a major source of information and can be used to fine-tune behavioural decisions. Active assessment of alarm calls within and between mammal species is not fully understood. We explored the behavioural responses of collared pikas to con- and heterospecific vocalizations, in order to determine whether they selectively attend to these calls. Pikas increased their vigilance after playback of alarm calls of heterospecific mammals (marmots and ground squirrels), but responded most strongly to conspecific calls. While responses to playback calls of their own, of neighbours and of a stranger did not differ, pikas did discriminate between individual callers in a habituation-discrimination experiment. The ability to make use of information from different sources in their acoustic environment likely facilitates pikas' behavioural decisions that affect foraging, predator avoidance and nepotism.     

The effect of body size and habitat on the evolution of alarm vocalizations in rodents

When confronted with a predator, many mammalian species emit vocalizations known as alarm calls. Vocal structure variation results from the interactive effects of different selective pressures and constraints affecting their production, transmission, and detection. Body size is an important morphological constraint influencing the lowest frequencies that an organism can produce. The acoustic environment influences signal degradation; low frequencies should be favoured in dense forests compared to more open habitats (i.e. the ‘acoustic adaptation hypothesis’). Such hypotheses have been mainly examined in birds, whereas the proximate and ultimate factors affecting vocalizations in nonprimate mammals have received less attention. In the present study, we investigated the relationships between the frequency of alarm calls, body mass, and habitat in 65 species of rodents. Although we found the expected negative relationship between call frequency and body mass, we found no significant differences in acoustic characteristics between closed and open‐habitat species. The results of the present study show that the acoustic frequencies of alarm calls can provide reliable information about the size of a sender in this taxonomic group, although they generally do not support the acoustic adaptation hypothesis.     

Abiotic noise,call frequency and stream-breeding anuran assemblages

Environmental noise can be an important selective force modulating signal evolution in species with acoustic communication. Many anuran species breed alongside streams; hence, the sound produced by the flowing water is an important source of noise for acoustic communication. Since calling is physiologically very expensive in anurans, and communication is essential for reproduction, we expected adaptations that reduce environmental masking effects and allow acoustic communication in streamside breeders. This basic assumption of the acoustic adaptation hypothesis has not been yet evaluated at a large phylogenetic scale. We combined ahistorical and phylogenetic methods to test whether anuran species that breed alongside streams call at higher frequencies than species that breed away from streams. We compiled primary and secondary data on body size, breeding habitat, and the dominant frequency of the advertisement call for 110 species; 40 of them breed alongside streams and 70 away from streams. Call frequency was slightly higher and body size was significantly smaller in streamside breeding species. After controlling for the effects of body size and phylogenetic signal, only differences in body size persisted between species breeding at both kinds of habitats. Our data suggest that habitat filtering rather than acoustic adaptation explains the high call frequency of stream breeders. Species with large body size, pleiotropically constrained to utter low-frequency calls, would have succeeded less often in establishing viable populations alongside streams, due to the masking effect of low-frequency noise. Thus, small species calling at relatively high frequencies would be more common there. Although our data do not preclude adaptations to noisy habitats in some anuran species, they do not provide support for the acoustic adaptation hypothesis at a wider phylogenetic scale.     

Habitat type and density influence vocal signal design in satin bowerbirds

1. This study provided a thorough test of the acoustic adaptation hypothesis using a within-species comparison of call structure involving a wide range of habitat types, an objective measure of habitat density and direct measures of habitat-related attenuation. 2. The structure of the bower advertisement call of the satin bowerbird was measured in 16 populations from throughout the species' range and related to the habitat type and density at each site. Transmission of white noise, pure tones and different bowerbird dialects was measured in five of six habitat types inhabited by satin bowerbirds. 3. Bowerbird advertisement call structure converged in similar habitats but diverged among different habitats; this pattern was apparent at both continent-wide and local geographical scales. Bowerbirds' call structures differed with changes in habitat density, consistent with the acoustic adaptation hypothesis. Lower frequencies and less frequency modulation were utilized in denser habitats such as rainforest and higher frequencies and more frequency modulation were used in the more open eucalypt-dominated habitats. 4. The white noise and pure tone transmission measurements indicated that different habitats varied in their sound transmission properties in a manner consistent with the observed variation in satin bowerbird vocalizations. 5. There was no effect of geographical proximity of recording locations, nor was there the predicted inverse relationship between frequency and body size. 6. These findings indicate that the transmission qualities of different habitats have had a major influence on variation in vocal phenotypes in this species. In addition, previously published molecular data for this species suggest that there is no effect of genetic relatedness on call similarity among satin bowerbird populations.     

Evolution of intraspecific variation in the advertisement call of a cricket frog (Acris crepitans, Hylidae)

Advertisement calls of the cricket frog, Acris crepitans , show statistically significant variation among populations in all call variables measured. Call variables show strong clinal variation resulting in calls of lower frequency, longer duration and slower call rates produced by A. c. blanchardi in open habitat in the west of the range, and calls of higher frequency, shorter calls and faster call rates produced by A. c. crepitans in the pine forests in the eastern part of the range. This clinal variation does not result from pleiotropic effects of body size or any other morphological characters we measured.
The two subspecies usually reside in different habitats, but some A. c. blanchardi reside in an isolated pine forest in central Texas. By comparing the calls of this subspecies in open and forest habitat, and by statistically removing the effects of clinal variation for all populations, we determined that habitat explains some of the variation in call structure; this is not true of subspecies.
Our data reject several hypotheses that purport to explain the evolution of mate recognition signals. (1) We reject the notion of Paterson and others that there is strong stabilizing selection on species-specific mate recognition signals. (2) There is no support for the hypothesis that call variation is primarily due to pleiotropic effects of body size or other morphological characters over the geographic range we examined. (3) There is no evidence for reproductive character displacement. (4) Our data, as well as experimental studies of habitat acoustics, support the hypothesis that some differences in calls among habitats result from environmental selection on call structure to enhance call transmission. We suggest that the latter hypothesis does not explain the strong clinal component of call variation. This might result from the passive effects of gene flow between populations at the extremes of the range under selection generated by habitat acoustics.     

Divergence of Acoustic Signals in a Widely Distributed Frog: Relevance of Inter-Male Interactions

Divergence of acoustic signals in a geographic scale results from diverse evolutionary forces acting in parallel and affecting directly inter-male vocal interactions among disjunct populations. Pleurodema thaul is a frog having an extensive latitudinal distribution in Chile along which males'' advertisement calls exhibit an important variation. Using the playback paradigm we studied the evoked vocal responses of males of three populations of P. thaul in Chile, from northern, central and southern distribution. In each population, males were stimulated with standard synthetic calls having the acoustic structure of local and foreign populations. Males of both northern and central populations displayed strong vocal responses when were confronted with the synthetic call of their own populations, giving weaker responses to the call of the southern population. The southern population gave stronger responses to calls of the northern population than to the local call. Furthermore, males in all populations were stimulated with synthetic calls for which the dominant frequency, pulse rate and modulation depth were varied parametrically. Individuals from the northern and central populations gave lower responses to a synthetic call devoid of amplitude modulation relative to stimuli containing modulation depths between 30–100%, whereas the southern population responded similarly to all stimuli in this series. Geographic variation in the evoked vocal responses of males of P. thaul underlines the importance of inter-male interactions in driving the divergence of the acoustic traits and contributes evidence for a role of intra-sexual selection in the evolution of the sound communication system of this anuran.     

Patterns of mating call preferences in túngara frogs, Physalaemus pustulosus

We examine acoustic mating preferences of a focal population at four different scales of divergence: within the population, between populations in the same genetic group, between populations in different genetic groups and between different species. At all scales there is substantial genetic divergence, variation in mating signals and preferences are influenced by signal variation. There is, however, no support for the hypothesis that mating preferences accumulate predictably with genetic distance. Females preferred the local conspecific call to the foreign conspecific call in about one-third of the experiments, and preferred the local call to all of the heterospecific calls tested. But there was no significant relationship between the variation in the strength of preference and genetic distance either among conspecific populations, or among heterospecific species. Thus, in this study macroevolutionary patterns are not apparent at the microevolutionary scale.     

Sensory Drive Mediated by Climatic Gradients Partially Explains Divergence in Acoustic Signals in Two Horseshoe Bat Species,Rhinolophus swinnyi and Rhinolophus simulator

Geographic variation can be an indicator of still poorly understood evolutionary processes such as adaptation and drift. Sensory systems used in communication play a key role in mate choice and species recognition. Habitat-mediated (i.e. adaptive) differences in communication signals may therefore lead to diversification. We investigated geographic variation in echolocation calls of African horseshoe bats, Rhinolophus simulator and R. swinnyi in the context of two adaptive hypotheses: 1) James’ Rule and 2) the Sensory Drive Hypothesis. According to James’ Rule body-size should vary in response to relative humidity and temperature so that divergence in call frequency may therefore be the result of climate-mediated variation in body size because of the correlation between body size and call frequency. The Sensory Drive Hypothesis proposes that call frequency is a response to climate-induced differences in atmospheric attenuation and predicts that increases in atmospheric attenuation selects for calls of lower frequency. We measured the morphology and resting call frequency (RF) of 111 R. simulator and 126 R. swinnyi individuals across their distributional range to test the above hypotheses. Contrary to the prediction of James’ Rule, divergence in body size could not explain the variation in RF. Instead, acoustic divergence in RF was best predicted by latitude, geography and climate-induced differences in atmospheric attenuation, as predicted by the Sensory Drive Hypothesis. Although variation in RF was strongly influenced by temperature and humidity, other climatic variables (associated with latitude and altitude) as well as drift (as suggested by a positive correlation between call variation and geographic distance, especially in R. simulator) may also play an important role.     

Genetic population structure and call variation in a passerine bird, the satin bowerbird, Ptilonorhynchus violaceus

Geographic variation in vocalizations is widespread in passerine birds, but its origins and maintenance remain unclear. One hypothesis to explain this variation is that it is associated with geographic isolation among populations and therefore should follow a vicariant pattern similar to that typically found in neutral genetic markers. Alternatively, if environmental selection strongly influences vocalizations, then genetic divergence and vocal divergence may be disassociated. This study compared genetic divergence derived from 11 microsatellite markers with a metric of phenotypic divergence derived from male bower advertisement calls. Data were obtained from 16 populations throughout the entire distribution of the satin bowerbird, an Australian wet-forest-restricted passerine. There was no relationship between call divergence and genetic divergence, similar to most other studies on birds with learned vocalizations. Genetic divergence followed a vicariant model of evolution, with the differentiation of isolated populations and isolation-by-distance among continuous populations. Previous work on Ptilonorhynchus violaceus has shown that advertisement call structure is strongly influenced by the acoustic environment of different habitats. Divergence in vocalizations among genetically related populations in different habitats indicates that satin bowerbirds match their vocalizations to the environment in which they live, despite the homogenizing influence of gene flow. In combination with convergence of vocalizations among genetically divergent populations occurring in the same habitat, this shows the overriding importance that habitat-related selection can have on the establishment and maintenance of variation in vocalizations.     

Geographic Variation in Male Sexual Signals in Strawberry Poison Frogs (Dendrobates pumilio)

In this paper, we compare the advertisement calls of 207 neotropical strawberry poison frogs (Dendrobates pumilio) collected in 21 localities along a transect from northern Costa Rica to western Panama. Populations varied most in call duration and call rate, while pulse rate and duty cycle were less variable. Multivariate analyses showed that call variation followed a cline with higher call rates, shorter calls, lower duty cycles and higher pulse rates in the southeast. Body size decreased towards the southeast and explained most variation in dominant frequency, as well as some residual variation in call rate. We conclude that a combination of geography and morphology is largely responsible for call variation within this species. Two inferred bio‐acoustic groups were roughly in accordance with two genetic groups, geographically separated in central Costa Rica. However, genetic distances among populations did not co‐vary with call dissimilarity after correction for geographic distances. Thus, differences in calls between genetic groups are probably mainly a result of clinal variation. These findings agree with the general observation that bio‐acoustic variation is often not (highly) associated with genetic divergence. Moreover, colour polymorphism observed among Panamanian populations was not reflected in a higher variability in call parameters relative to the monomorphic Costa Rican populations.     

Habitat-dependent advertisement call variation in the monkey frog Phyllomedusa nordestina

The acoustic adaptation hypothesis (AAH) predicts that acoustic signals are selected to propagate more efficiently in the habitat where they are normally transmitted. Several studies corroborated the AAH for primates and birds, but evidence for frogs is contentious: While most studies failed to support the AAH, recent studies have shown that within-species variation conforms to the predictions of the AAH. Herein, we test the AAH by comparing advertisement calls of Phyllomedusa nordestina from two contrasting habitats (Atlantic Rainforest and Caatinga) and by testing the influence of the amount of vegetation around calling sites on acoustic parameters of frog calls. The interval between pulses was significantly different between individuals from the Atlantic Rainforest and from the Caatinga, and the number of pulses was correlated with the amount of vegetation around calling sites. Hence, our results indicate that multiple evolutionary forces may act simultaneously on the advertisement calls of frogs.     

Geographic variation in advertisement calls of a Microhylid frog – testing the role of drift and ecology

Acoustic signals for mating are important traits that could drive population differentiation and speciation. Ecology may play a role in acoustic divergence through direct selection (e.g., local adaptation to abiotic environment), constraint of correlated traits (e.g., acoustic traits linked to another trait under selection), and/or interspecific competition (e.g., character displacement). However, genetic drift alone can also drive acoustic divergence. It is not always easy to differentiate the role of ecology versus drift in acoustic divergence. In this study, we tested the role of ecology and drift in shaping geographic variation in the advertisement calls of Microhyla fissipes. We examined three predictions based on ecological processes: (1) the correlation between temperature and call properties across M. fissipes populations; (2) the correlation between call properties and body size across M. fissipes populations; and (3) reproductive character displacement (RCD) in call properties between M. fissipes populations that are sympatric with and allopatric to a congener M. heymonsi. To test genetic drift, we examined correlations among call divergence, geographic distance, and genetic distance across M. fissipes populations. We recorded the advertisement calls from 11 populations of M. fissipes in Taiwan, five of which are sympatrically distributed with M. heymonsi. We found geographic variation in both temporal and spectral properties of the advertisement calls of M. fissipes. However, the call properties were not correlated with local temperature or the callers' body size. Furthermore, we did not detect RCD. By contrast, call divergence, geographic distance, and genetic distance between M. fissipes populations were all positively correlated. The comparisons between phenotypic Q_st (P_st) and F_st values did not show significant differences, suggesting a role of drift. We concluded that genetic drift, rather than ecological processes, is the more likely driver for the geographic variation in the advertisement calls of M. fissipes.     

Sexual selection in female perceptual space: how female túngara frogs perceive and respond to complex population variation in acoustic mating signals

Female preferences for male mating signals are often evaluated on single parameters in isolation or small suites of characters. Most signals, however, are composites of many individual parameters. In this study we quantified multivariate traits in the advertisement call of the túngara frog, Physalaemus pustulosus. We represented the calls in multidimensional scaling space and chose nine test calls to represent the range of population variation. We then tested females for phonotactic preference between calls in each pair of the nine test calls. We used statistics developed for paired comparisons in such "round robin" competitions to evaluate the null hypothesis of equal attractiveness, and to examine the degree to which females responded to calls as being different from or similar to one another in attractiveness. We then examined the attractiveness of each test call relative to all other test calls as a function of their location in multivariate acoustic space (the acoustic landscape) to visualize sexual selection on calls. Finally, we used methods from cognitive psychology to illustrate the females' perception of call attractiveness in multivariate space, and compared this perceptual landscape to the acoustic landscape of quantitative call variation. We show that correlations between individual call characters are not strong and thus there are few biomechanical constraints on their independent evolution. Most call variables differed among males, and there was high repeatability of call characters within males. Females often discriminated between pairs of calls from the population, and there were significant differences among calls in their attractiveness. Female preferences for calls were not stabilizing. The region of the acoustic landscape that was most attractive to females included the mean call but was not centered around it. The females' perceptual or preference landscape did not correlate with the call's acoustic landscape, and female perception of calls decreased rather than enhanced call differences.     

Geographic Variation in the Long Calls of Male Orangutans (Pongo spp.)

Relatively few data exist on population differences in the vocal behavior of mammals. Geographic variation in calls is of special interest because of the implications for resolving evolutionary and behavioral questions. For example, information on geographic variation in vocalizations complements morphological and molecular data used to infer phylogenetic relationships and provides evidence for the mechanisms underlying call development. A quantitative acoustic analysis of orangutan long calls was undertaken, comparing flanged adult males from four geographically distinct sites across Borneo and Sumatra, revealing consistent differences among the calls of individuals. Long calls produced by orangutans from the four sites in Borneo and Sumatra differ in quantitative acoustic measures. Discriminant function analysis reveals that acoustic variables can be used in combination to assign calls to the correct individual, site and island at rates higher than that expected by chance. Specifically, four acoustic parameters proved reliable for distinguishing among the individuals, between sites, and across the two islands that arguably represent populations from separate species or subspecies. Although Bornean and Sumatran long calls share a repetitive structure and show similar call rates (0.100–0.500 LCs/h) and maximum frequency bands (0.400–1.500 kHz), they differ significantly in the number of pulses per call, call speed, call duration, bandwidth, pulse duration, and dominant frequency. Strong consistency in these acoustic parameters is also seen among males within sites and the observed variation may allow for individual recognition. Individual identification by call structure presumably benefits dispersed orangutans, where individuals characteristically forage independently and both encounters and interactions with signaling males are highly variable and largely dependent on context. Acoustic recognition of callers facilitates the choice of which males to join or avoid, thus allowing receivers to manipulate potential costs and benefits of association.     

Divergent echolocation call frequencies in insular rhinolophids (Chiroptera): a case of character displacement?

Aims Because rhinolophids have been hypothesized to use echolocation call frequency to recognize conspecifics, sympatric species calling at similar frequencies should be subject to acoustic character displacement, i.e. a drift in frequency values to minimize the risk of misidentification of conspecifics. However, it has been proposed that insufficient geographical separation between populations in sympatry and allopatry may counter the establishment of frequency differences by character displacement. Here we tested the hypothesis that insular populations should exhibit acoustic divergence, and this should be revealed by comparing call frequencies with those observed in mainland, allopatric populations of conspecifics. We also tested whether the evolutionary pressure towards acoustic divergence should be especially strong at sites where rhinolophid species emitting similar call frequencies roost together in order to minimize interspecific frequency overlap. Location Sardinia and southern Italy (Campania, Lazio, Abruzzo). Methods Time‐expanded echolocation calls and body size were recorded from Sardinian populations of Rhinolophus mehelyi Matschie, Rhinolophus hipposideros (Bechstein) and Rhinolophus euryale Blasius. Both call frequencies and forearm length of insular R. hipposideros and R. euryale were compared with those of populations from mainland areas of Italy where R. mehelyi is absent, to explore the hypothesis that the presence of the latter species (which calls at frequency values intermediate between the other two) may determine acoustic divergence in the other species. For Sardinian R. mehelyi and R. euryale, we also carried out intraspecific comparisons of call frequencies between bats from monospecific colonies and those from mixed colonies. Results As hypothesized, Sardinian R. hipposideros and R. euryale called at frequencies higher and lower, respectively, than in the peninsula. In this way, overlap with R. mehelyi is avoided. Body size showed no difference between insular and peninsular populations, i.e. frequency differences are not a by‐product of difference in body size determined by insularity. Frequency values in Sardinian R. euryale from monospecific colonies did not differ from those of bats roosting together with R. mehelyi. However, R. mehelyi showed frequency values significantly higher when associated with R. euryale, possibly to minimize the risk of species misrecognition. Main conclusions At least under geographical isolation, character displacement may be a causal mechanism for shifts in call frequency of sympatric rhinolophids. Species recognition and facilitation of intraspecific communication (with possible implications for mate recognition) constitute the best candidate factors for the phenomenon we observed.     

Geographical variation in the high-duty cycle echolocation of the cryptic common mustached bat Pteronotus cf. rubiginosus (Mormoopidae)

The use of bioacoustics as a tool for bat research is rapidly increasing worldwide. There is substantial evidence that environmental factors such as weather conditions or habitat structure can affect echolocation call structure in bats and thus compromise proper species identification. However, intraspecific differences in echolocation due to geographical variation are poorly understood, which poses a number of issues in terms of method standardization. We examined acoustic data for Pteronotus cf. rubiginosus from the Central Amazon and the Guiana Shield. We provide the first evidence of intraspecific geographic variation in bat echolocation in the Neotropics, with calls significantly differing in almost all standard acoustic parameters for the two lineages of this clade. We complement our bioacoustic data with molecular and morphological data for both species. Considerable overlap in trait values prevents reliable discrimination between the two sympatric Pteronotus based on morphological characters. On the other hand, significant divergence in the frequency of maximum energy suggests that bioacoustics can be used to readily separate both taxa despite extensive intraspecific variability in their echolocation across the Amazon. Given the relative lack of barriers preventing contact between bat populations from the Central Amazon and French Guiana, the documented acoustic variation needs to be further studied in geographically intermediate locations to understand the potential isolation processes that could be causing the described divergence in echolocation and to determine whether this variation is either discrete or continuous.     

Genetic divergence is more tightly related to call variation than landscape features in the Amazonian frogs Physalaemus petersi and P. freibergi

Behavioural isolation from divergence in male advertisement calls and female preferences is hypothesized to cause genetic divergence and speciation in the Amazonian frogs Physalaemus petersi and P. freibergi, yet the importance of call variation and landscape features in genetic divergence is unresolved. We tested for correlations between genetic divergence at microsatellite loci and (1) call variables; and (2) landscape variables among 10 populations of these frogs. Genetic divergence was not correlated with geographical distance, rivers or elevation. There was a strong positive relationship, however, between genetic divergence and inter‐population differences in one call variable, whine dominant frequency. Effective population sizes varied among sites (range = 15–846) and were often small, suggesting that genetic drift could influence call evolution. Evidence for fine‐scale genetic structure within sites was also found. Our results support the hypothesis that behavioural isolation from divergence in male calls and female preferences causes genetic divergence and speciation.     

Call Transmission Efficiency in Native and Invasive Anurans: Competing Hypotheses of Divergence in Acoustic Signals

Invasive species are a leading cause of the current biodiversity decline, and hence examining the major traits favouring invasion is a key and long-standing goal of invasion biology. Despite the prominent role of the advertisement calls in sexual selection and reproduction, very little attention has been paid to the features of acoustic communication of invasive species in nonindigenous habitats and their potential impacts on native species. Here we compare for the first time the transmission efficiency of the advertisement calls of native and invasive species, searching for competitive advantages for acoustic communication and reproduction of introduced taxa, and providing insights into competing hypotheses in evolutionary divergence of acoustic signals: acoustic adaptation vs. morphological constraints. Using sound propagation experiments, we measured the attenuation rates of pure tones (0.2–5 kHz) and playback calls (Lithobates catesbeianus and Pelophylax perezi) across four distances (1, 2, 4, and 8 m) and over two substrates (water and soil) in seven Iberian localities. All factors considered (signal type, distance, substrate, and locality) affected transmission efficiency of acoustic signals, which was maximized with lower frequency sounds, shorter distances, and over water surface. Despite being broadcast in nonindigenous habitats, the advertisement calls of invasive L. catesbeianus were propagated more efficiently than those of the native species, in both aquatic and terrestrial substrates, and in most of the study sites. This implies absence of optimal relationship between native environments and propagation of acoustic signals in anurans, in contrast to what predicted by the acoustic adaptation hypothesis, and it might render these vertebrates particularly vulnerable to intrusion of invasive species producing low frequency signals, such as L. catesbeianus. Our findings suggest that mechanisms optimizing sound transmission in native habitat can play a less significant role than other selective forces or biological constraints in evolutionary design of anuran acoustic signals.