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Sustained perfect synchrony of signal production by animals is unrealistic, but even near-synchronous signalling is extremely rare. Near-synchronous signalling has been documented in some orthopteran insects and fireflies, and one kind of frog. This study provides observations and analyses of sustained bouts of impressive near-synchronous calling by a terrestrial breeding frog from Australia, the hip-pocket frog (Assa darlingtoni). Males called in scattered clusters of several individuals from the rainforest floor of northern New South Wales. In eight of nine pairs of semi-isolated males, there was sustained near-synchronous calling in bouts consisting of 16–20 calls and lasting 5–10 min. There was extensive overlap of the pulsed calls, and calls of a lagging male began overlapping that of a leading male after 2–5 pulses of a leader’s call note. In five pairs, one male’s calls were usually in the leading position; in three pairs, leadership frequently switched between males. In the latter interact...

Interspecific aggression has important consequences for ecological processes and the evolution of behavioral strategies. We examined interspecific aggressive interactions in the 2 gray tree frog species, Hyla chrysoscelis and Hyla versicolor. These species call side by side in the same ponds, and acoustic interference occurs because of the similar spectral characteristics of their vocalizations. The aggressive calls of these 2 species, although very similar, were statistically distinguishable: calls of H. chrysoscelis were more strongly amplitude modulated than those of H. versicolor. We used playbacks and staged interactions to characterize the behaviors of males exposed to heterospecific competitors or their signals. Males readily responded with aggression to heterospecific individuals and playbacks of their calls. These behaviors were qualitatively similar in both intraspecific and interspecific interactions, but there were some significant differences. First, males of H. chrysoscelis were more aggressive toward playbacks of conspecific advertisement calls than toward those of H. versicolor. There were no significant differences in this respect in H. versicolor. Second, interspecific interactions were usually more escalated than intraspecific interactions and more likely to end with the loser moving away from its opponent. Although neither species had an advantage in staged interactions, behavioral responses were asymmetrical because H. versicolor was more likely than H. chrysoscelis to initiate physical contact. Previous studies showed that H. versicolor suffers from a greater reduction in attractiveness than H. chrysoscelis in the presence of heterospecific call overlap. Thus, the asymmetries in aggressive behavior between the 2 species may be related to differential costs of heterospecific competition.

As animal contests escalate, variation in the performance of aggressive signaling behaviors can give important insights into contest dynamics. In anuran amphibians, males of numerous species utilize distinctive aggressive vocalizations during disputes over calling spaces. Little is known, however, about the causes and consequences of variation in aggressive-call characteristics. We analyzed recordings of calls made during staged aggressive interactions between male gray tree frogs, Hyla versicolor, to determine how variation in a key aggressive-call characteristic, dominant frequency, was affected by increasing contest escalation. We found that dominant frequencies of aggressive calls were lower than those of advertisement calls used to attract females. Furthermore, we found that males lowered their aggressive-call frequencies with increases in escalation. Winners tended to have lower-frequency aggressive calls than losers. We conclude that aggressive calls in H. versicolor are similar to the graded aggressive calls that have been described in several other species. This gradation may allow males to balance the energetic costs of producing lower-frequency calls with the benefits of efficiently repelling rival males. Other processes related to motivation and the physiological effects of participating in contests may also be responsible for the observed variation in aggressive-call frequency with contest escalation. Our results demonstrate that detailed experimental studies of aggressive calling behavior in anurans, which to this point have rarely been performed, are feasible and generate important insights relating to general problems in animal contest behavior and animal communication.

Call timing is an important component of the behavioral repertoire of many chorusing species that compete acoustically for mates. The costs and benefits of particular call-timing patterns may vary with factors in the social environment, yet few studies have examined the possibility of socially mediated plasticity in call timing. We studied the effects of competition on advertisement-call timing in the gray tree frog, Hyla versicolor, by manipulating intermale distances in staged interactions. We measured phase angles and the proportion of calls that overlapped as interacting pairs of males were moved closer to one another, simulating an increase in acoustic competition. We observed a shift in phase angle resulting in substantially higher levels of overlap as males were moved so that they were calling at extremely close range. At close range, males often engage in intense agonistic contests over the calling space and we found that call timing may play a role in these interactions. Although there were not major differences in the call-timing behavior of winners and losers of contests, we suggest that call overlap may be a general signal of aggressive motivation. Changes in call timing with changes in intermale distance may, therefore, be related to changes in the intended audience as competition escalates. Key wor ds: aggressive signal, anuran, call timing, competition, overlap. [Behav Ecol]

The canyon treefrog, Hyla arenicolor, is a wide-ranging hylid found from southwestern US into southern Mexico. Recent studies have shown this species to have a complex evolutionary history, with several phylogeographically distinct lineages, a probable cryptic species, and multiple episodes of mitochondrial introgression with the sister group, the H. eximia complex. We aimed to use genome wide AFLP markers to better resolve relationships within this group. As in other studies, our inferred phylogeny not only provides evidence for repeated mitochondrial introgression between H. arenicolor lineages and H. eximia/H. wrightorum, but it also affords more resolution within the main H. arenicolor clade than was previously achieved with sequence data. However, as with a previous study, the placement of a lineage of H. arenicolor whose distribution is centered in the Balsas Basin of Mexico remains poorly resolved, perhaps due to past hybridization with the H. eximia complex. Furthermore, the AFLP data set shows no differentiation among lineages from the Grand Canyon and Colorado Plateau despite their large mitochondrial sequence divergence. Finally, our results infer a well-supported sister relationship between this combined Colorado Plateau/Grand Canyon lineage and the Sonoran Desert lineage, a relationship that strongly contradicts conclusions drawn from the mtDNA evidence. Our study provides a basis for further behavioral and ecological speciation studies of this system and highlights the importance of multi-taxon (species) sampling in phylogenetic and phylogeographic studies.

Performance limitations on signal production constrain signal evolution. Variation in signaling performance may be related to signaler quality and therefore is likely to be a salient aspect of communication systems. When multiple signal components are involved in communication, there may be trade-offs between components, and performance can be measured as the degree to which signalers approach the upper limits of the trade-off function. We examined vocal performance in the gray tree frog Hyla versicolor, in which females prefer values of call duration and rate exceeding the usual range of variation within and among males. We recorded interactions between pairs of males calling on mobile platforms that allowed us to manipulate intermale distance and place males in highly competitive environments. We found that, although there was a clear upper boundary on the ability of males to maximize call duration and call rate simultaneously, call effort did not remain constant in this highly competitive situation. Our estimates of an upper limit to vocal performance were corroborated by analyses of calling behavior in the context of close-range mate attraction. We discuss potential constraints on signaling performance and the relevance of this measure of performance for both intrasexual and intersexual communication.

Even simple biological signals vary in several measurable dimensions. Understanding their evolution requires, therefore, a multivariate understanding of selection, including how different properties interact to determine the effectiveness of the signal. We combined experimental manipulation with multivariate selection analysis to assess female mate choice on the simple trilled calls of male gray treefrogs. We independently and randomly varied five behaviorally relevant acoustic properties in 154 synthetic calls. We compared response times of each of 154 females to one of these calls with its response to a standard call that had mean values of the five properties. We found directional and quadratic selection on two properties indicative of the amount of signaling, pulse number, and call rate. Canonical rotation of the fitness surface showed that these properties, along with pulse rate, contributed heavily to a major axis of stabilizing selection, a result consistent with univariate studies showing diminishing effects of increasing pulse number well beyond the mean. Spectral properties contributed to a second major axis of stabilizing selection. The single major axis of disruptive selection suggested that a combination of two temporal and two spectral properties with values differing from the mean should be especially attractive.

To understand the variation in aggressiveness and factors important for contest outcome, we quantified and compared agonistic interactions of four field cricket species in eastern North America: Gryllus fultoni (Orthoptera; Gryllidae), G. vernalis, G. pennsylvanicus, and G. rubens. The most aggressive behavior that we observed, the grapple, was frequently displayed in agonistic trials of G. pennsylvanicus and G. rubens. By contrast, we never observed this behavior in trials involving G. fultoni and G. vernalis. Consequently, species was the only significant variable affecting the variation in aggressiveness, whereas size difference, age difference, and contest duration had no significant effect. In species with high levels of aggressiveness, G. pennsylvanicus and G. rubens, the factors that might be indicative of resource holding potential such as size or age difference seemed to be significant for contest outcome. In species with low levels of aggressiveness, G. fultoni and G. vernalis, however, there were indications that contest outcomes were determined by aggressiveness itself rather than the size and age differences between opponents. Markov chain analyses revealed that the difference in aggressiveness between species with high and low levels of aggressiveness lay in a sequence of escalating behaviors: antennal fencing, mandible flare, and grapple. The escalated state of aggressiveness characterized by this behavioral sequence in G. pennsylvanicus and G. rubens seems to be the ancestral state in the North American Gryllus phylogeny. We argue that the loss of a tendency to use burrows in G. fultoni and G. vernalis might be related to low levels of aggressiveness in these species.

Signal-timing adjustment is common in communally signaling species, and there is large variation in signal-timing formats found in nature. We conducted a survey of geographic variation in female signal-timing preferences and male signaling behavior of a tree frog to test predictions of two hypotheses about the sources of selection acting on signal-timing behavior. We found that female preferences are important in shaping male signal timing, affecting the absolute placement of signals relative to the temporal limits of female preferences. Variation in other signal characters, particularly signal period, also affects signal timing, albeit in a more complicated pattern: the influence of signal period on signal timing varied between populations. Overall, our findings indicate that the evolution of male signal-timing behavior is strongly influenced by female preferences and by an interaction with other aspects of male signaling behavior such as male signal period.

SUMMARYFrogs have two inner ear organs, each tuned to a different range of frequencies. Female treefrogs (Hylidae) of three species in which males produce calls with a bimodal spectrum (Hyla chrysoscelis, H. versicolor, H. arenicolor) preferred alternatives with a bimodal spectrum to alternatives with a single high-frequency peak. By contrast,females of H. avivoca, in which males produce calls with a single,high-frequency peak, preferred synthetic calls with a single high-frequency peak to calls with a bimodal spectrum. These results are consistent with the expectations of the matched-filter hypothesis and run counter to the predictions of the pre-existing bias hypothesis. At moderate to high playback levels (85–90 dB), females of H. avivoca and of two of three mtDNA-defined lineages of H. versicolor preferred unimodal signals with a high-frequency peak to those with a low-frequency peak. Females of H. chrysoscelis, H. arenicolor and the third lineage of H. versicolor did not show a preference, indicating that receiver mechanisms may be at least as evolutionarily labile as call structure. Spectral-peak preferences of gray treefrogs from Missouri, USA were intensity-dependent. Whereas females chose low-frequency calls at 65 dB spl, there was either no preference (H. chrysoscelis) or a preference for high-frequency calls (H. versicolor) at 85 and 90 dB spl. These non-linear effects indicate that there is an increasing influence of high-frequency energy on preferences as females approach calling males, and these results serve to emphasize that playback experiments conducted at a single level may have limited generality.

Two closely related wood-cricket species, Gryllus fultoni (Orthoptera: Gryllidae) and G. vernalis, produce similar calling songs, consisting of 3-pulse chirps. Analysis of field and laboratory recordings of calling songs showed that, after correction to a common temperature, there was a divergence in chirp and pulse rates between far allopatric populations of G. fultoni and populations sympatric with G. vernalis. To determine whether the divergence in calling songs potentially provides reproductive isolation between G. fultoni and G. vernalis throughout the temperature range over which these insects sing, we recorded calling songs of lab-reared G. fultoni and G. vernalis populations between 18 and 28°C. Mean chirp rate significantly differed between sympatric and far allopatric G. fultoni populations as well as between sympatric G. fultoni and G. vernalis populations. Although there was a significant difference in mean pulse rate between sympatric G. fultoni and G. vernalis populations, pulse rate did not differ between sympatric and far allopatric G. fultoni populations in the laboratory study. Considering the magnitudes of differences in calling song characters discriminated by females of G. fultoni and the mean differences and the variability in calling song characters between the two species, the joint difference in chirp and pulse rates may be adequate for species discrimination over most of the range at which these crickets breed.

Two species of closely related wood cricket, Gryllus fultoni (Orthoptera: Gryllidae) and Gryllus vernalis, occur together in some parts of the eastern United States and have a similar calling song structure, consisting of three-pulse chirps. A previous study revealed that chirp rate in G. fultoni was highest (greatest difference vis-a-vis chirp rate in G. vernalis) in sympatric populations, intermediate in near allopatric populations that were located close to the sympatric zone, and lowest in allopatric populations. A similar trend was observed in pulse rate, but the mean values of this trait showed much more convergence than chirp rate at the low end of the range of calling temperatures. In this study, we investigated the song discrimination of females from sympatric and allopatric populations of G. fultoni at about 23°C, which is near the middle of the normal range of calling temperatures. We used both single-stimulus and two-stimulus playback experiments to learn if geographical differences in song preferences paralleled those in calling songs. Stimuli presented were representative of calling songs in three classes of G. fultoni populations (sympatric, near allopatric, and far allopatric), a calling song of G. vernalis, and three calling songs with parameter values that were intermediate with respect to those of the songs of far allopatric G. fultoni and G. vernalis. In the single-stimulus playbacks, females of all G. fultoni populations responded poorly if at all to the heterospecific stimulus. Females of sympatric and near allopatric populations responded poorly to all intermediate stimuli, but females of far allopatric populations frequently responded to these sounds. In the two-stimulus playbacks, females of sympatric and near allopatric populations generally discriminated against intermediate and heterospecific stimuli. However, females of far allopatric populations often did not discriminate against intermediate stimuli, whose characteristics resembled the calling songs of G. vernalis. The divergent pattern of female phonotactic discrimination between sympatric and far allopatric populations was thus generally congruent with the pattern of divergence in chirp and pulse rates and would be expected to significantly reduce heterospecific mating in sympatry. These geographical patterns of female song discrimination and male calling songs conform to a commonly used definition of reproductive character displacement.

Females are expected to use spectral properties of acoustic signals for mate choice because dominant frequency often provides reliable information about male size. The advertisement calls of grey treefrogs ( Hyla chrysoscelis and H. versicolor ) have two main frequency bands: the amplitude of the dominant, high-frequency band averages 7–10 dB greater than that of the low-frequency band. The frequencies of these peaks were negatively correlated with male body size in both species. I used two-speaker playbacks of synthetic calls to generate preference functions for both species based on simultaneous variation in the frequencies of the two bands and, for H. versicolor , on variation in single-component calls; frequencies of most of these stimuli fell within the range of the dominant, high-frequency band. When two-component sounds were used, preference functions were bimodal and broad, with peaks close to ( H. versicolor ) or slightly higher than ( H. chrysoscelis ) mean frequencies in conspecific calls. Because preferences were not strong, I predict that females exert only weak stabilizing or directional selection on male call frequencies. When single-component calls were used in tests of H. versicolor , the preference-function peak was shifted to a lower-than-average frequency within the high-frequency range. I hypothesize that such a frequency may better stimulate both of the inner ear organs, whose tuning roughly matches frequencies in the two bands in conspecific calls, than a single component of average or much lower-than-average frequency. Females of both species preferred calls with both spectral peaks to calls with a single, high-frequency peak, even when the low-frequency peak was attenuated by 24–30 dB. This result serves to emphasize that typically subdominant frequency components can have seemingly disproportionate effects on signal attractiveness.

Signals used for mate choice and receiver preferences are often assumed to coevolve in a lock-step fashion. However, sender-receiver coevolution can also be nonparallel: even if species differences in signals are mainly quantitative, females of some closely related species have qualitatively different preferences and underlying mechanisms. Two-alternative playback experiments using synthetic calls that differed in fine-scale temporal properties identified the receiver criteria in females of the treefrog Hyla chrysoscelis for comparison with female criteria in a cryptic tetraploid species (H. versicolor); detailed preference functions were also generated for both species based on natural patterns of variation in temporal properties. The species were similar in three respects: (1) pulses of constant frequency were as attractive as the frequency-modulated pulses typical of conspecific calls; (2) changes in preferences with temperature paralleled temperature-dependent changes in male calls; and (3) preference functions were unimodal, with weakly defined peaks estimated at values slightly higher than the estimated means in conspecific calls. There were also species differences: (1) preference function slopes were steeper in H. chrysoscelis than in H. versicolor; (2) preferences were more intensity independent in H. chrysoscelis than in H. versicolor; (3) a synergistic effect of differences in pulse rate and shape on preference strength occurred in H. versicolor but not in H. chrysoscelis; and (4) a preference for the pulse shape typical of conspecific calls was expressed at the species-typical pulse duration in H. versicolor but not in H. chrysoscelis. However, females of H. chrysoscelis did express a preference based on pulse shape when tested with longer-than-average pulses, suggesting a hypothesis that could account for some examples of nonparallel coevolution. Namely, preferences can be hidden or revealed depending on the direction of quantitative change in a signal property relative to the threshold for resolving differences in that property. The results of the experiments reported here also predict patterns of mate choice within and between contemporary populations. First, intraspecific mate choice in both species is expected to be strongly influenced by variation in temperature among calling males. Second, simultaneous differences in pulse rate and pulse shape are required for effective species discrimination by females of H. versicolor but not by females of H. chrysoscelis. Third, there is greater potential for sexual selection within populations and for discrimination against calls produced by males in other geographically remote populations in H. chrysoscelis than in H. versicolor.

Interactions between species can affect the evolution of their sexual signals, receiver selectivity, or both. One commonly expected outcome is reproductive character displacement, whereby adverse consequences of mismating select for greater differentiation of communication systems in areas of sympatry than in areas of allopatry. We found evidence of reproductive character displacement in the acoustic communication system of green tree frogs (Hyla cinerea). The strength of female preferences for the spectral properties of calls that distinguish conspecific calls from those of a closely related congener, H. gratiosa, was greater in areas of sympatry with H. gratiosa than in areas of allopatry. We also found subtle differences in advertisement calls and in the heights of male calling perches when we restricted our comparisons to localities in which H. gratiosa was also breeding (syntopy) with localities where this species was absent. Hyla cinerea and H. gratiosa show only weak genetic incompatibility, but the calls representative of interspecific hybrids were unattractive to females of both parental species. Hybrids might also be at an ecological disadvantages because of different habitat preferences of the two taxa. Thus, selection against production of less fit or less attractive hybrid or backcross offspring are probably the main causes responsible for the differences documented in this paper.

Evolutionary change in signals may occur within and between populations. A pre-requisite of signal evolution is that heritable traits vary sufficiently between individuals for selection to operate....

Despite intense interest in mate choice, relatively little is known about how individuals sample prospective mates. Indeed, a key issue is whether females sample males or simply mate with the first male encountered. We investigated mate sampling by female barking treefrogs (Hyla gratiosa). Females choosing mates in natural choruses did not move between males but instead mated with the first male they approached closely. Most females mated with the male closest to them at the start of their mate-choice process, and females were more likely to mate with the closest male when the distance to other males was large. These observations are consistent with the hypothesis that females do not sample potential mates but instead mate with the first male they distinguish from the rest of the chorus. To test this initial detection hypothesis, we conducted a playback experiment in which we offered females a choice between two calls, one of which was detectable above the background chorus sound at the female’s release point, and one of which became detectable only as females moved toward the initially detectable call. Females did not prefer the initially detectable call, thus ruling out the initial detection hypothesis and implicating sampling of potential mates by females. Based on the behavior of females in natural choruses, we hypothesize that females approach the chorus, move to locations where they are able to detect the calls of several males simultaneously, and choose a mate from among these males at some distance from the males. Such simultaneous sampling may be common in lekking and chorusing species, which have been the subjects of many studies of sexual selection. Key words: Hyla gratiosa, mate choice, mate sampling, simultaneous sampling, treefrogs. [Behav Ecol 13:472–480 (2002)]

In his book Descent of Man and Selection in Relation to Sex, Darwin (1) made a compelling argument that evolution could not be explained solely on the basis of differential survival (natural selection). In particular, traits that enhance an individual’s reproductive success (sexual selection) often decrease its chances of survival. For example, the risks of injury and death can be high for males that fight for direct access to females or for territories that contain resources needed by females. Courtship ornaments and displays, which often determine territorial ownership or mating success, are not only energetically costly but frequently result in increased chances of predation. Some of the most striking and well-documented examples of the latter phenomenon involve reductions in sexual ornaments and modifications of behavior that reduce the conspicuousness of courting individuals in areas where predation is high compared with areas where predators are absent or uncommon (2, 3). These changes almost certainly increase survivorship at the expense of less effective territorial defense and lower mating success. In PNAS, Steinberg et al. (4) found that males of a semiarboreal lizard, the brown anole (Anolis sagrei), on islands in the Bahamas where a predator was introduced show a significant reduction in the amplitude of head bobs compared with lizards from predator-free islands. This behavioral change reduces the communication range of these visual signals, thus lowering the chances of being detected by a predator. By the same token, such signals are likely to be less effective in repelling more distant territorial rivals and attracting potential mates. Whereas a common response to predation is to reduce the duration or frequency of their displays (5), the main change in A. sagrei on islands with predators was head-bob amplitude, a display component used in territorial defense and mate attraction.

We studied female mate choice by Hyla versicolor in three venues to examine how acoustic and spatial complexity, background noise, and the calling behavior of males might influence preferences manifest in previous laboratory two-stimulus choice tests. Our laboratory-based two-stimulus choice tests with and without broadcasts of chorus noise demonstrated that females prefer long calls relative to short calls when calling efforts of alternatives are equivalent. Background noise impaired the ability of females to discriminate in favor of longer over shorter calls, but the magnitude of the effect was small. Captures of females at eight speakers broadcasting 6- to 27-pulse calls at the edge of a pond revealed strong discrimination against only the shortest call variant. In natural choruses, females may only rarely encounter males using such unattractive vocalizations. Female phonotaxis at an artificial pond with caged and electronically monitored calling males also indicated that consequences of female preferences for temporal aspects of calling observed in two-stimulus choice tests are much attenuated in choruses and explain only small portions (

In nearly all female choice experiments with anurans, the values of acoustic properties of experimental stimuli have been fixed. In nature, however, a female usually experiences an array of calls in which acoustic properties, especially dynamic ones, differ between males in variability as well as in mean values. High variability per se does not make a synthetic call more or less attractive to female grey treefrogs, Hyla versicolor, than a non-variable or low-variability call with about the same mean values of pulse number per call, call period, or both. Females did prefer a low-variability alternative with a slightly higher number of sound pulses per unit time than the variable stimulus, suggesting that females can resolve small differences in mean values between the calls of different males despite considerable within-male variability.

SYNOPSIS. Insects and anurans show various patterns of selective responsiveness to playbacks of natural and synthetic sounds. Preferences and tuning of the auditory system most often match one or more strongly emphasized frequency components in long-range, mate-attracting signals typical of conspecific individuals, but exceptions, in which frequencies lower or higher than the mean are preferred, occur in some species. In some insects the most preferred frequencies may correspond to regions that maximize localizability of signals rather than regions of maximum sensitivity. Patterns of preference for fine-temporal properties are typically stabilizing within a population; females prefer values near the mean and few males produce calls with values that deviate sufficiently to make them less attractive than calls with average values. Preferences for gross temporal properties {e.g., the rate and duration of signaling) are usually highly directional, with much higher than mean values preferred. In anurans, call rate is a better predictor of male mating success than dominant frequency. Nevertheless, a variety of factors, especially close-range assessments that are common in insects, may modify or negate the advantages of producing long-range signals of high relative attractiveness. The evolutionary consequences of selective responsiveness in anurans and insects are discussed

Abstract. Because male treefrogs indiscriminately try to mate with other frogs of comparable size that move close to them, a major cause of heterospecific matings is thought to be inadvertent contact between a female of one species and a vocalizing male of another species. It follows that the probability of such mismatings would be reduced if females recognized the calls of sympatric, heterospecific males and avoided moving near those males during phonotactic approaches to vocalizing conspecific males. Gravid female treefrogs of three species were tested in situations in which they could avoid a source of heterospecific calls as they responded to conspecific calls. The results of these experiments provide no evidence that females avoid sources of heterospecific calls. Because females of all three species sometimes show positive phonotactic responses in 'no-choice' situations to the same kinds of heterospecific calls used in this study, these results suggest that the mechanisms responsible for homospecific pairing are qualitatively no different than those mediating intraspecific mate choice. The implications of these results for current debates about processes underlying mate choice are discussed.

Abstract. Females of the grey treefrog, Hyla chrysoscelis, from three areas where a genetically incompatible, sibling species, H. versicolor, also occurred, were much more likely to prefer synthetic sounds with fine-temporal properties typical of conspecific males than were females from two areas of remote allopatry. Specifically, more than 90% of the females from sympatric populations weighted a species-specific cue (pulse-repetition rate = pulse rate) more strongly than they did a cue (call duration) that mediates intraspecific mate choice, whereas only about 50% of the females from remote allopatric populations did so. These results suggest that selection for the avoidance of mating mistakes has acted on females in sympatric populations. If this is a general phenomenon, then the apparent rarity of reproductive character displacement may be explained at least in part by the fact that nearly all previous studies have focused solely on geographical patterns of variation in male courtship signals.

Patterns of variation in the acoustic properties of advertisement calls of several species of treefrogs (family Hylidae) are related to patterns of female preferences for these same properties. Properties were unequivocally classified as static or dynamic based on within-male variability during bouts of calling. Static properties are highly stereotyped within males and between males in natural populations. Dynamic properties often change by as much as 100% during single bouts of calling; dynamic properties are usually much more variable among males in natural populations than are static properties. An analysis of the calling performance of individual males over the course of a breeding season indicated that at least one dynamic property, pulse number (or call duration) in Hyla versicolor, was sufficiently repeatable to differentiate between males. Playback experiments that employed synthetic calls showed that: (1) females exhibit patterns of preferences that should result in selection that is stabilizing or weakly directional on static properties, and highly directional on dynamic properties; and (2) the overall attractiveness of a signal is affected by variation in both static and dynamic properties. Similar results were found in laboratory and field studies of other taxa, primarily anurans and insects. Patterns of variability in call properties of males and in female preferences are discussed in terms of proximate mechanisms and theoretical models of the evolution of female choice.

A fundamental assumption of the Hamilton and Zuk (1982) model of sexual selection is that parasite-free males should produce mate attracting displays that are substantially more vigorous and effective than those of their parasitized counterparts. Further, the model predicts that parasite-free males should be preferred as mating partners by females and, as a consequence, should have higher reproductive success than their parasitized competitors. Gray treefrogs ( Hyla versicolor ) constitute an ideal species for testing both assumptions and predictions of the Hamilton-Zuk model in that: (i) mate attraction by males requires the production of energetically costly calls and (ii) males of this species have been shown to harbor a wide range of helminth parasites. The present paper summarizes results of analyses of calling and mating behavior by male gray treefrogs in relation to their parasite infection levels. In general, these analyses revealed no consistent relationship between the number and kinds of parasites harbored by males and either the duration or duty-cycle (a direct measure of energetic cost) of their calls. Furthermore, males found in amplexus (i.e., males actually selected as mating partners by females) showed a slightly higher prevalence of parasite infection (but a slightly lower intensity of infection) than did a comparable sample of unmated males. In sum, the present study failed to support the major within-species prediction of the Hamilton-Zuk model of sexual selection and also calls into question one of the fundamental assumptions of the model.

The “good genes” hypothesis predicts that mating preferences enable females to select mates of superior genetic quality. The genetic consequences of the preference shown by female gray tree frogs for long-duration calls were evaluated by comparing the performance of maternal half-siblings sired by males with different call durations. Offspring of male gray tree frogs that produced long calls showed better performance during larval and juvenile stages than did offspring of males that produced short calls. These data suggest that call duration can function as a reliable indicator of heritable genetic quality.

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.

A mating preference function describes the relationship between variation in a trait in potential mates and the strength of the preference for that trait. Few studies have measured mating preference functions either at a population level or for individuals. We used two-choice playback experiments to determine the mating preference functions of individual female barking treefrogs for two call characteristics: call-repetition rate and fundamental frequency. We tested each female four times with each pair of stimuli and with three to six pairs of stimuli. Individual females exhibited directional preferences for higher call rates and stabilizing preferences for intermediate fundamental fre- quency. These individual preference functions were similar to population-level preferences documented in previous studies. Within a stimulus type (call rate or fundamental frequency), nearly all females exhibited the same general preference function. However, females varied in the minimum difference between stimuli necessary to elicit a unan- imous choice for the higher call rate, and they differed in both the intermediate fundamental frequency they preferred most and the minimum difference in fundamental frequency that elicited a unanimous choice for one of the two alternatives. The variation we observed among females was not repeatable; repeatabilities were in general low and statistically nonsignificant. The observed variation in the preferences of females, even if unrepeatable, should weaken selection on male traits relative to selection in the absence of such variation.

The first cytologically confirmed natural hybrids between the diploid-tetraploid gray treefrogs, Hyla chrysoscelis and H. versicolor, were two triploid males initially detected by their distinctive advertisement calls. Analyses of the pulserepetition rate and shape of the advertisement calls of these triploid hybrids suggest that one of the two previously reported putative hybrids was a hybrid and the other male was a H. chrysoscelis. The calls of the field-recorded hybrids were intermediate in pulse rate in comparison to the parental species and significantly different in structure than were the calls of laboratory-reared hybrids described by Mable and Bogart (1991). Mated pairs were collected in six syntopic populations in south-central Missouri. Flow cytometric identifications revealed that five (about 6.7%) of the 75 pairs in two of these ponds were mismated; these five mismatings represented about 3% of all 181 pairs examined from all Missouri populations. The rarity of sexually mature hybrids, despite a fairly high frequency of mismatings in at least two localities, indicates severe selection against hybrids. In four of the five mismated pairs, the female was H. versicolor. Analyses of mtDNA sequences showed that the female parent of the two cytologically confirmed natural hybrids was also H. versicolor.

1. Synthetic calls with a waveform periodicity of 300/sec and a bimodal spectrum attracted female green treefrogs as effectively as natural calls. 2. Effectiveness was markedly reduced if the relative amplitude of the two spectral peaks differed by as little as 10 dB. 3. Optimal frequency bands were 900-1100 Hz in the low range and 2700-3300 Hz in the high. 4. Addition of components at 1800 and 2100 Hz rendered the call less attractive. 5. In the absence of a bimodal stimulus, most females responded to a call containing a single spectral peak in the high or low range. 6. Mating call recognition in the green treefrog is compared with that in the bullfrog.

Green treefrogs, Hyla cinerea , at a study site in eastern Georgia behaved as prolonged breeders: there were many more males present each night than females, the arrival of which was unpredictable. Individual males were usually consistent in their behaviour during a single night, and were most often observed calling. There was a weak, negative correlation between male size (snout-vent length, unless otherwise stated) and the frequency of satellite behaviour, defined as a non-calling male's being situated near a calling male. In 2 of 3 years, the mean size of males observed as satellites was less than that of the calling males with which they were associated on a given night; however, many satellites were the same size or larger than calling males. The distributions of matings per male during three seasons were not significantly different than Poisson distributions with the same means. Individual mating success was best predicted by attendance at the breeding site; mating success was negatively related to the frequency of satellite behaviour in some analyses, but very little of the variance was explained. An exhaustive series of analyses failed to show any pattern of size-dependent mating, including weight-assortative mating, even when males that were frequently satellites were excluded. The relative sizes of males and females in amplexus had little effect on fertilization success. On a seasonal basis, males were clumped in their spatial distribution, and matings tended to occur where male density was highest. Individual males were highly variable in their patterns of movement, which were not correlated with size or mating success. Mating success was also unrelated to the frequency (Hz) of the low-frequency spectral peak in the advertisement call of the male; male size and weight were negatively correlated with the frequency of the low-frequency peak.

Sound pressure levels of the vocalizations of 21 species of North American anurans were measured in the field. In most species the peak levels at 50 cm in front of the male exceeded 100 dBre 2×10−4 μbar (Table 2). The sound pressure levels of the calls of most individuals varied by less than 2.5 dB, but intraspecific variation was significantly higher, averaging about 8.5 dB (Tables 1, 2). Sound pressure levels of mating calls were positively correlated with body size in the toad,Bufo americanus (Fig. 2); however, interspecific differences in sound pressure levels were not clearly related to interspecific differences in body size. Measurements of directivity patterns indicated that sound fields around three males ofHyla crucifer were uniform, whereas two males ofH. chrysoscelis represented directional sound sources (Table 3).

Two hundred and eighty-eight female green treefrogs responded in two-choice experiments using synthetic calls. The spectral properties of the sounds were similar ; the fine-temporal properties were systematically varied. Females preferred sounds in which the waveform periodicity was about 300/s, a rate typical of natural mating calls. They discriminated strongly against calls with periodicities of 100/s, 60/s and 50/s. Aperiodic calls and calls with periodicities of 225/s and 900/s were intermediate in attractiveness. The results of four experiments indicated that periodicity preferences depended on corresponding changes in the amplitude-time envelopes of the signals. Neurophysiological correlations and the implications of the behavioural results are discussed. The biological significance in species recognition and in the differentiation of the mating call from other signals in the repertoire of the green treefrog is discussed.

SYNOPSIS. Playback experiments using synthetic sounds indicate that gravid females of the gray treefrogs ( Hyla versicolor and H. chrysoscelis ) and the green treefrog ( H. cinerea ) selectively respond to sounds on the basis of physical properties that show inter-individual variation in the calls of conspecific males. Although the specificity is adequate to differentiate among the signals of many males, this potential for mate choice may not be fully realized in acoustically complex situations such as natural breeding aggregations. Female selectivity in H. cinerea was less precise in four-speaker experiments than in two-speaker experiments. An analysis of female preferences with respect to a call property correlated with body size was only partially successful in predicting the size distribution of successful males in a natural population. Finally, the existence of other synchronously breeding species of frogs with similar calls may impose significant constraints on intraspecific mate choice, and female specificity with respect to some acoustic properties of mating calls is difficult to explain otherwise.

An extensive series of two-speaker and four-speaker playback experiments using synthetic advertisement calls identified acoustic criteria that may be used by females of the green treefrog, Hyla cinerea, in choosing a mate. At a sound pressure level (SPL) of 75 dB (decibels re 20 μPa) females preferred signals which had a low-frequency spectral peak in the range 0·8–1·0 kHz and a high-frequency spectral peak in the range 2·4–3·6 kHz. Some frequency preferences were reduced or abolished by changing the absolute or relative SPLs of the experimental stimuli. Frequency selectivity in the low-frequency range decreased with playback level; selectivity in the high-frequency range increased with playback level. Females were less selective with respect to variation of the low-frequency peak in four-speaker experiments than in two-speaker experiments. Frequency preferences were independent of female body size. Females responded selectively to signals that differed in call duration and relative SPL; some preferences depended on absolute SPL. Females preferred signals with higher (15–100%) than average repetition rates to those with average rates, but did not respond at all to calls repeated at about four times the normal rate. These results are discussed in terms of current knowledge on auditory physiology and recent data on patterns of call variation and mating success among male green treefrogs.

The vocalizations of natural hybrids between various species of treefrogs (Hylidae) are compared with the vocalizations of the parental species : 1. In most respects, the calls of two males of H. avivoca X H. chrysoscelis and of six males of H. cinerea X H. gratiosa are intermediate. 2. The vocalizations of three males of H. femoralis X H. clarysoscelis tend to be similar to those of males of H. chrysoscelis. 3. The calls of each of the hybrids are composed, however, of some features which are intermediate, some which are similar to one of the parental species, and others which are unique to hybrid calls. Differentiation of the calls of males of H. avivoca, H. chrysoscelis, H. femoralis, and hybrids in this group is mainly in the temporal domain. Females of H. chrysoscelis and H. femoralis responded only to the calls of males of their own species in discrimination experiments where hybrid calls were alternative stimuli. The calls of males of H. cinerea, H. gratiosa and hybrids between these two differ primarily in their spectral composition : the two spectral peaks have different locations in each kind of call. In discrimination experiments, females of H. gratiosa responded only to conspecific calls when the calls of hybrids were equally accessible. Females of H. cinerea showed partial discrimination against the calls of hybrids. Females of both kinds often responded to the calls of hybrids when conspecific signals were unavailable. In general, the closer the spectrum of a natural call resembled that of conspecific calls, the more attractive was the acoustic signal.

1. The frequency preferences of gravid females ofHyla cinerea for the low-frequency peak of bimodal synthetic mating calls shifted by about 400 Hz when the temperature of the animals was changed by about 7 °C. Changes in preference involving the high-frequency peak were much less dramatic (Table 1). 2. The low-frequency peaks of the mating calls of four individual males ofH. cinerea changed only slightly (≦70 Hz) when they were recorded at high and low temperatures (Table 2, Fig. 1). The same acclimation times (around one hour) were used in testing female preferences and in recording male calls at different temperatures. 3. Since shifts in female preference are not paralleled by concomitant changes in the spectral properties of the vocal signals, temperature coupling based on spectral information appears to be absent in the acoustic communication system of this species. 4. Spectral properties are important for species recognition and at low temperatures females prefer calls with low-frequency peaks that are more similar to those of another, sympatric species (Hyla gratiosa) than to those of conspecific males. This implies that species isolation in nature could be jeopardized at low temperatures if females become responsive to mating calls under these conditions. 5. Temperature effects on frequency preferences were predicted by a neurophysiological study of the auditory thalamic area inH. cinerea. The present results support the conclusion that this area is involved in processing biologically significant sounds. 6. The results indicate, however, that resonance properties of the buccal cavity alone are not sufficient to match the mating call frequencies with the frequency sensitivity of the auditory system.

Male hybrids of reciprocal crosses between gray and pinewoods tree frogs (Hyla chtysoscelis x H. femoralis) that were raised to sexual maturity in the laboratory produced distinctive vocalizations. Hybrid females preferred the calls of hybrids to calls of gray tree frogs and also chose synthetic calls with a pulse repetition rate typical of the hybrids in preference to calls with a rate typical of pinewoods tree frogs.

The evolutionary significance of female choice is controversial1. Central to the issue are the mechanisms and criteria used by females to choose among potential sexual partners. For example, females merely responding to conspicuous displays that are uncorrelated with male condition or quality would neither incur costs of assessment nor gain benefits over and above random mating2. Here we show that selective phonotaxis by female gray tree frogs depends on non-arbitrary acoustic cues that indicate the male's species and energetic costs of signal production. Furthermore, some female preferences cannot be explained by differences in signal conspicuousness (acoustic energy) alone. Females obviously benefit by rejecting genetically incompatible, heterospecific males, which can be identified solely by pulse rate. Females also select among conspecific males on the basis of call duration, which is correlated with a male's energetic investment in courtship and probably indicates his physical condition and fitness.

Noncalling adult male tree frogs were found in close association with about 16 percent of the calling males in a pond in Georgia. In 13 of 30 field experiments a noncalling satellite male intercepted and achieved amplexus with a gravid female moving toward the calling male. This mating strategy, which conserves energy required for calling, resembles the strategy employed by other vertebrates.

The gray tree frog mates over a temperature range of at least 9 degrees C. Gravid females, tested at two different temperatures, preferred synthetic mating calls with temperature-dependent temporal properties similar to those produced by a male at about the same temperature as their own. Thus, the vocalization system and the temporal pattern recognition system are affected by temperature in a qualitatively similar fashion.

Mating behavior of Hyla meridionalis was observed at an artificial reservoir in Punta del Hidalgo, Tenerife. Playback experiments were conducted in the field near the reservoir. The density of calling males was extremely high, and individuals sometimes called within 5 cm of each other. Males often changed calling positions and successfully attempted to clasp one another. Gravid females were common and most often found in amplexus. In three of four observations, the male initiated amplexus from a distance of 5 to 10 cm. Only 11 of 107 gravid females responded in two-choice playback experiments. Ten chose the calls of a conspecific male recorded at Punta del Hidalgo, and one female chose the calls of a male of H. a. arborea recorded in Germany. Our observations and experimental results are compared with those of similar studies of North American hylids.