Male Green Treefrogs Research Articles

Adoption of satellite behavior by small male green treefrogs, Dryophytes cinereus, is not associated with age: Implications for the maintenance of alternative behavioral phenotypes

AbstractBody size is often a critical determinant of competitive ability and reproductive behavior. For example, small males characteristically lose contests with larger males and may be metabolically constrained from sustaining energetically demanding reproductive behaviors. Small males thus often utilize alternative reproductive tactics (ARTs) to maximize their reproductive success. How variation in body size among behavioral phenotypes arises can provide important insight into the maintenance of ARTs. For instance, smaller body sizes for males adopting ARTs may simply be a function of age, suggesting that the tactic is maintained by the periodic recruitment of young males into the population. Alternatively, body size differences among behavioral phenotypes that are unrelated to age suggest disparate growth rates that potentially arise as a consequence of variation in environmental conditions during development or genotypic differences between the phenotypes. Here we examine these alternative scenarios in the green treefrog, Dryophytes cinereus, using skeletochronological analysis combined with body size measures. As with many other anuran amphibians, male green treefrogs conditionally adopt a noncalling satellite mating tactic and attempt to intercept females attracted to the vocalizations of calling “host” males. We show that males adopting satellite behavior in natural choruses are smaller than calling males but do not represent a class of younger individuals, indicating that satellite males have lower growth rates than calling males. We also show that satellite males are in poorer condition than larger calling males, implicating energetic constraints in size‐related variation in mating tactic expression. Our work suggests that environmental conditions during development, genetic differences between phenotypes, or both, give rise to size discrepancies in behavioral phenotypes, neither of which have been previously explored in anurans. We discuss body size and condition differences between behavioral phenotypes in the context of energetic constraints and endocrine mediation of tactic expression in this species.

Glucocorticoid-Mediated Changes in Male Green Treefrog Vocalizations Alter Attractiveness to Females.

Adrenal glucocorticoids (GCs) are increasingly recognized as important modulators of male courtship signals, suggesting that circulating levels of these steroids can play a central role in sexual selection. However, few studies have examined whether GC-mediated effects on male sexual signals actually impact mate choice by females. Here, we examine how corticosterone (CORT)-mediated changes in the vocalizations of male green treefrogs, Dryophytes cinereus, influence attractiveness to females. In this species, agonistic acoustic signaling between rival males competing for mates increases circulating CORT levels in contest losers. Acute elevations in CORT, in turn, decrease the duration of male advertisement calls and increase the latency between successive calls, resulting in a net reduction in vocal effort (the amount of signaling per unit time) that occurs independently of changes in circulating androgens. Based on known preferences for acoustic features in D. cinereus, and other anuran species, the direction of CORT-mediated effects on temporal call characteristics is expected to compromise attractiveness to females, but whether they are of sufficient magnitude to impact female mate choice decisions is unclear. To examine whether CORT-mediated effects on male advertisement calls reduce attractiveness to females, we broadcast vocalizations in dual speaker playback experiments approximating the mean and 1 SD above and below the mean call duration and vocal effort values (the two primary vocal features impacted by elevated CORT) of males with low and high CORT levels. Results revealed strong preferences by females for the calls characteristic of males with low CORT in tests using the approximate mean and 1 SD above the mean call duration and vocal effort values, but females did not show a preference for calls of males with low CORT in trials using call values approximating 1 SD below the mean. Overall, females preferred males with signal traits predictive of low CORT, however this effect was nonlinear with attenuated preferences when signal alternatives differed only marginally indicating a possible thresholding effect. Specifically, females appeared to discriminate between males with low versus high CORT based primarily on differences in call rates associated with CORT-mediated changes in call duration and vocal effort. Our results highlight that changes in circulating CORT during male-male vocal interactions can decrease attractiveness to females, suggesting that circulating levels of CORT can play a critical role in both intra- and intersexual selection.

The potential role of reproductive interference in the decline of native green treefrogs following Cuban treefrog invasions

Reproductive interference is increasingly implicated in the decline of native taxa following alien species invasions. We examined reproductive interference in the context of acoustic modulation of hormone levels and declines in native green treefrogs (Hyla cinerea) following invasions of Cuban treefrogs (Osteopilus septentrionalis) in the southeastern United States. The motivation for our study stemmed from similarities in spectro-temporal properties of Cuban treefrog courtship calls and green treefrog aggressive calls. The aggressive calls of green treefrogs stimulate glucocorticoid production in rival conspecific males, which suppresses androgen production and reproductive behavior. We thus hypothesized that native male green treefrogs exposed to advertisement calls of this invasive species exhibit increased levels of circulating glucocorticoids. We found that broadcast aggressive calls of green treefrogs stimulated glucocorticoid production in conspecific males but that Cuban treefrog vocalizations did not increase glucocorticoid levels in green treefrogs. In natural choruses, the density of calling male green treefrogs and the proportion of non-calling males were positively correlated with glucocorticoid levels and negatively correlated with androgens, whereas body size was positively correlated with androgens. After controlling for these variables, there was no evidence that hormone levels or behavior differed in choruses of green treefrogs in the presence and absence of Cuban treefrogs. Together, our results suggest that interactions among male green treefrogs modulate hormone levels but that the presence of Cuban treefrogs, calling or not, is not altering levels of glucocorticoids or androgens in breeding male green treefrogs.

Dueling frogs: do male green tree frogs (Hyla cinerea) eavesdrop on and assess nearby calling competitors?

Individuals produce advertisement signals with intended purposes and targets. However, these signals can be received by “eavesdroppers,” who may extract information from them and alter their behavior according to the extracted information. In anuran systems, males congregate at breeding sites to produce advertisement calls to attract receptive females and fend off rival males. Both sexes directly assess these calls in dyadic encounters and make decisions based on the call’s characteristics, e.g., frequency. What is unknown is whether bystander males eavesdrop on these same calls to inform their future competitive decisions. Here, we examined whether male green tree frogs (Hyla cinerea) eavesdrop on competing males, assess their competitor’s call frequency, and respond accordingly. We exposed males to playbacks of two competing males that varied in call frequency—high, average, low—and quantified latency to call, time spent calling, and number of calling bouts. We found that males had reduced latency to call and called more when eavesdropping low-frequency competition, but not average or high-frequency competition. Focal male size also influenced how they responded, with larger males being more responsive than smaller males. Our results indicate that male green tree frogs are capable of eavesdropping on nearby male calls and produce behavioral responses accordingly. Further, it appears males are able to alternate between assessment strategies dependent upon the frequency of the eavesdropped competition. These findings indicate that males not only directly assess an opponent’s call in dyadic encounters, but also indirectly through eavesdropping. SIGNIFICANCE STATEMENT: Animals produce signals with intended purposes and targets, but which can be received by nearby eavesdroppers. Eavesdropped signals can elicit complex phenotypic changes in the eavesdropper, and lead to significant fitness consequences for the signal producer and eavesdropper. Thus, examining whether and to what extent individuals eavesdrop is an important step in understanding the evolution of signal production and response. Here, we examined whether male green tree frogs, H. cinerea, eavesdrop on nearby competing males and base their own calling behaviors on eavesdropped male’s calling characteristics. Calling behavior was mediated by focal male body size and eavesdropped male’s call frequency. Larger males reduced response latency and called more to nearby low-frequency males, while smaller males reduced response latency to nearby calls of average frequency. These results indicate that males will extract information from their competitive environment through eavesdropping and produce behavioral responses according to the eavesdropped information.

Hormonal and social correlates of courtship signal quality and behaviour in male green treefrogs

Circulating hormone levels often vary considerably among males competing for mates, but how this variation affects the signalling phenotype is poorly understood. In the green treefrog, Hyla cinerea, males competing for mates engage in vocal contests that increase circulating levels of the adrenal glucocorticoid corticosterone (CORT) and decrease androgens in contest losers, who characteristically adopt a noncalling ‘satellite’ mating tactic. We previously showed that CORT administration to calling males reduces vocal effort and increases the probability of satellite behaviour during simulated territorial intrusions, suggesting that elevated CORT compromises vocal quality and decreases the propensity to call in a context-dependent manner. However, the extent to which these effects reflect variation in vocal behaviour of males in natural choruses is unclear. Here, we examined how hormone levels, social context and other potential determinants of vocalization, including body size and condition, relate to vocal attributes and behaviour in calling males and in associated satellites after removal of calling ‘host’ males. Results revealed that satellites had higher CORT and lower androgens and were smaller and in poorer condition than host males. Host removal resulted in vocalization in 74% of satellites. These males invested less effort in vocalization than hosts, and CORT level best explained these differences. The remaining 26% of satellites did not vocalize after host removal and had lower androgens than satellites that vocalized. Results provide support for context-dependent effects (i.e. host presence/absence) of elevated CORT on the probability of vocalization and link high CORT to reduced vocal quality. In contrast, low androgens were associated with a low probability of vocalization regardless of social context, and androgen level was a poor predictor of vocal attributes. Our results link high CORT and low androgens to variation in signalling quality and behaviour that potentially reduces the probability of attracting females.

The Effects of Melatonin on Brain Arginine Vasotocin: Relationship with Sex and Seasonal Differences in Melatonin Receptor Type 1 in Green Treefrogs (Hyla cinerea).

The neuroendocrine mechanisms by which animals synchronise their physiological state with environmental cues are vital to timing life-history events appropriately. One important endocrine transducer of environmental cues in vertebrates is the pineal hormone melatonin, the secretion of which is directly sensitive to photoperiod and temperature. Melatonin modulates arginine vasotocin (AVT)-immunoreactive (-IR) cell number in the brain of green treefrogs (Hyla cinerea) during the summer breeding season, and this modulation is sexually dimorphic. In the present study, we investigated whether the influence of melatonin on vasotocin varies seasonally. We show that treatment of nonreproductive male green treefrogs with melatonin-filled silastic implants for 4weeks during the winter does not alter vasotocin-IR cell number in any brain region (i.e. nucleus accumbens, amygdala, preoptic area, suprachiasmatic nucleus or ventral hypothalamus). Taken together, these results suggest that the influence of melatonin on AVT is associated with sex and seasonal variation in melatonin receptor expression. We tested this hypothesis by using immunohistochemistry to characterise the distribution of melatonin receptor type 1 (MT1, also known as Mel1a) in the brain of reproductive and nonreproductive male and female frogs. We quantified MT1-IR cell number in regions known to contain AVT cell populations. Reproductive males had significantly more MT1-IR cells than nonreproductive males in all brain regions, including the combined nucleus accumbens, diagonal band of Broca and septum, striatum, amygdala, combined preoptic area and suprachiasmatic nucleus, as well as the ventral hypothalamus. In the accumbens region, where the effect of melatonin on AVT is known to be sexually dimorphic, males had significantly more MT1-IR cells than females during the summer breeding season. Based on these findings, we suggest that MT1 plays a role in mediating the interactions between melatonin and vasotocin that regulate seasonal and sexually dimorphic changes in sociosexual behaviour.

Close-range vocal signals elicit a stress response in male green treefrogs: resolution of an androgen-based conflict

Male courtship signals often stimulate the production of sex steroids in both female and male receivers. Such effects benefit signallers by increasing receptivity in females, but impose costs on signallers by promoting sexual behaviour and aggression in male competitors. To resolve this androgen-based conflict, males should use strategies that suppress sex steroid production in rival males. In green treefrogs, Hyla cinerea, chorus sounds (i.e. advertisement calls from aggregates of males) are known to stimulate androgen production in receiver males. Here, I examined whether males of this species counter these effects by eliciting an endocrine stress response in male conspecifics during close-range vocal interactions. I show that corticosterone (CORT) levels were higher in males that lost vocal contests in natural choruses compared to contest winners and nonaggressive males. Testosterone levels were also lower in contest losers compared to nonaggressive males, but not contest winners; dihydrotesterone levels did not differ among the three groups. Aggressive and advertisement calls were then broadcast to males in an experiment that simulated close-range vocal communication. Aggressive calls rapidly (45 min) elicited an increase in CORT and a reduction in androgens in receivers. Advertisement calls did not elicit an increase in CORT, but CORT levels were sustained relative to controls exposed to silence and were accompanied by a reduction in androgens in small males. Endocrine responses to acoustic signals in this species thus vary depending upon context, call type and size of signal receivers. Signallers benefit from eliciting CORT production in competitors because elevated CORT suppresses vocalization.

Condition-dependent signaling and adoption of mating tactics in an amphibian with energetic displays

Most alternative reproductive phenotypes are interpreted as alternative tactics within a conditional strategy: all individuals are predicted to choose among behavioral tactics based on some measure of their physiological condition or competitive status. The factors determining a male’s tactic at any particular time are generally unknown. Thus, most researchers have measured potential correlates of competitive ability (such as male body size) rather than directly measuring fighting ability, for example, in males adopting alternative mating tactics. In species in which sexual competition primarily involves production of acoustic signals, an individual’s choice of mating tactic may largely depend on physical characteristics, such as age, strength, or condition, which determine its ability to compete acoustically by producing attractive calls. Male green tree frogs (Hyla cinerea) often switch tactics (caller or satellite), but body size changes very little during the breeding season. I tested the hypothesis that mating-tactic switches are mediated via the effects of changing body condition on advertisement call characteristics. My field study shows that the switch between tactics is predicted by condition but not by size. Moreover, short-term changes in body weight result in condition-dependent changes in several properties of the advertisement call. The demonstration of condition-dependent advertisement calling and tactic adoption links a physical characteristic correlated with mating-tactic adoption with actual competitive status (calling performance). Vocal fatigue may constrain males to produce relatively unattractive advertisement calls, thus increasing the potential fitness payoffs associated with adoption of the satellite tactic.

Steroid hormone levels in calling males and males practicing alternative non-calling mating tactics in the green treefrog, Hyla cinerea

Circulating glucocorticoids and androgens often figure prominently in mating tactic expression in vertebrates. In anuran amphibians (frogs and toads), for example, recent models predict that the depletion of energy reserves during vocalization will result in increased glucocorticoid levels; high glucocorticoids are expected to negatively affect androgen level to mediate transitions from calling to non-calling behavior. Consistent with these predictions, we show that male green treefrogs (Hyla cinerea) adopting an alternative non-calling “satellite” mating tactic were in poorer condition and had higher circulating corticosterone levels and lower androgen levels than calling males. Body condition was inversely related to corticosterone level and positively related to testosterone, but not dihydrotestosterone, level. Corticosterone level was inversely related to testosterone level but not dihydrotestosterone level. Lastly, we show that calling males that were involved in aggressive bouts had higher corticosterone levels than calling males that were not involved in aggressive bouts. Our results are thus consistent with the prediction that aggressive interactions with conspecific males contribute to high corticosterone levels in satellite males that were observed to lose aggressive contests with larger calling males.

Changes in Plasma Testosterone Levels and Brain AVT Cell Number during the Breeding Season in the Green Treefrog

We exposed groups of adult male green treefrogs, Hyla cinerea, to acoustic stimuli (natural chorus or random tones) for seven consecutive nights at three time points during their natural breeding season (May, July, and September) and assessed seasonal changes in plasma androgen levels and number of arginine vasotocin (AVT) immunoreactive cells in the brain over this time period. We also tested whether social cues altered either androgens or AVT-ir cell number or size at each time point. Finally, we analyzed how these factors related to calling behavior. Data were collected over two breeding seasons. Call rate (calls/h) was assessed during the stimulus time (i.e. ‘evoked calling’) and during the remainder of the day (‘spontaneous calling’). Plasma hormone levels were measured at the end of the acoustic treatment when brains were collected for immunocytochemistry. Circulating androgen levels declined over the breeding season. Males exposed to chorus sounds, however, had higher androgen levels than males exposed to tones. AVT-ir cell number increased across the breeding season in the nucleus accumbens but not the amygdala, anterior preoptic area, or magnocellular preoptic area, and soma size decreased in the nucleus accumbens as cell number increased. Social stimulation had no significant influence on either AVT-ir cell measure. Evoked call rate was higher in males exposed to natural chorus sounds compared to those exposed to random tones, but did not change during the season. In contrast, spontaneous call rate was higher at the beginning of the breeding season compared to the end, and unlike evoked calling was correlated with circulating androgen levels across all treatments and time points. AVT-ir soma size was positively correlated with both evoked and spontaneous calling. These results suggest that social exposure can prolong the elevation of gonadal hormones in the bloodstream, thus mitigating or slowing the seasonal decline of such hormones. In contrast, social exposure does not affect the seasonal pattern of AVT-ir cell number or soma size. The reciprocal relationship between social cues and hormones and the subsequent effect on behavior may provide hidden benefits to animals engaging in social interactions. However, unlike steroid hormone levels, the seasonal change in AVT-ir cell number and size is not counteracted by social stimulation.

Dynamical interactions in a green treefrog chorus.

In southeast Texas male green treefrogs (Hyla cinerea) assemble in dense choruses and call for several hours every night to attract females. The call is a short “quank” lasting about a 100 ms, with a variable call interval of about 400 ms. Individuals call from dense vegetation surrounding the breeding area, occupying positions that range in elevation from water level to several meters above the ground. The high call-rate and duty cycle combined with their three-dimensional spatial distribution present challenging problems for signalers and receivers and for human observers. It is qualitatively apparent that within a large natural chorus, individuals dynamically adjust their calling to avoid overlap with other callers. However, the dynamical interactions and calling strategies have not been quantified. Here, a 15-microphone array was deployed within the breeding site in the vicinity of one group of callers, and the locations and call patterns of individuals were extracted by acoustic beamforming. The results show that an individual pays attention to only a few of the neighboring males and times his calls to avoid call collision. When a collision occurs one of the individuals will delay the subsequent call by approximately half a call interval to avoid further collision.

Intersexual dynamics mediate the expression of satellite mating tactics: unattractive males and parallel preferences

Male green treefrogs, Hyla cinerea, attract both potential mates and satellite males with their advertisement calls. Males using the satellite mating tactic position themselves silently near a calling male and attempt to intercept females attracted to the caller. Although use of conditional mating strategies may result from competition among males, few studies have considered the importance of female mate-choice preferences on the adoption and expression of alternative mate-acquisition behaviours. In this study, I tested two hypotheses. First, satellite males produce relatively unattractive advertisement signals and hence are at a competitive disadvantage when using the primary tactic. Second, satellite males associate with males producing the most attractive advertisement calls and hence increase the chance that they will intercept a female. Satellite males recorded in the field produced advertisement calls higher in frequency and shorter in duration than those produced by callers. Females consistently preferred the lower-frequency call alternative presented in two-speaker choice experiments, although they showed no preference in regard to call duration. In a second set of two-speaker choice experiments, noncalling males showed phonotactic behaviour similar to that of females, and their acoustic preferences roughly paralleled those of females. This phonotactic response may enable males producing unattractive calls to use the satellite mating tactic in a manner that maximizes potential reproductive fitness.

Social Context Influences Androgenic Effects on Calling in the Green Treefrog (Hyla cinerea)

Courtship behavior in frogs is an ideal model for investigating the relationships among social experience, gonadal steroids, and behavior. Reception of mating calls causes an increase in androgen levels in listening males, and calling, in turn, depends on the presence of androgens. However, previous studies found that androgen replacement does not always restore calling to intact levels, and the relationship between androgens and calling may be context dependent. We examined the influence of androgens on calling behavior in the presence and the absence of social signals in male green treefrogs (Hyla cinerea). We categorized calling during an acoustic stimulus (mating chorus or tones) as evoked and calling in the absence of a stimulus as spontaneous. Intact males received a cholesterol implant, castrated males were castrated and received a cholesterol implant, and T-implanted males were castrated and received a testosterone implant. The androgen levels (mean ± SE ng/ml of plasma) achieved by the implants were as follows: castrated males, 1.2 ± 0.2; intact males 21.9 ± 7.0; T-implanted males, 254.6 ± 39.5. As in other frogs, calling depends on the presence of androgens, as castration abolished and T replacement maintained calling. However, among intact and T-implanted males, the influence of androgens on calling differed between spontaneous and evoked calling. There was a positive effect of androgen treatment on spontaneous call rate and a positive correlation between spontaneous call rate and androgen levels. The influence of androgen levels on evoked call rate was more complex and interacted with acoustic treatment. Surprisingly, T implants suppressed the chorus-specific increase in calling that is evident in intact males. In addition, in response to the chorus, T-implanted males called less than did intact males, in spite of higher androgen levels. Furthermore, variation in androgens did not explain variation in evoked call rate. These data indicate that androgens influence the motivation to call, but that, when socially stimulated, androgens are necessary but insufficient for calling.

Temporal features mediating call recognition in the green treefrog, Hyla cinerea: amplitude modulation

Abstract. The male green treefrog, Hyla cinerea, has two signals in its vocal repertoire, the advertisement call and the aggressive call. These vocalizations are similar in frequency composition, but differ in their rate of amplitude modulation (waveform periodicity). The ability of male green treefrogs to respond to signals differing in modulation rate and depth was tested with an evoked-calling paradigm. Males were significantly more likely to vocalize in response to playbacks of stimuli modulated at the waveform periodicity of the conspecific advertisement call, and less likely to vocalize in response to stimuli modulated at the rate of the conspecific aggressive call, or to stimuli modulated at the rate of a heterospecific advertisement call. Male vocal responses showed similar changes with modulation rate in response to either modulated calls (containing both spectral and temporal cues) and modulated noise (containing temporal cues alone). Decreasing the modulation depth eliminated the effect of modulation rate on the males' vocal responses. Changes in responding with modulation rate were consistent across both field and laboratory conditions. These results suggest that the temporal feature of amplitude modulation plays an important role in call recognition in the natural environment.

Perception of complex sounds by the green treefrog, Hyla cinerea: envelope and fine-structure cues.

1. The envelope periodicity of communication signals is an important feature distinguishing advertisement and aggressive calls for the green treefrog (Hyla cinerea). Envelope periodicity, a cue for periodicity pitch perception in humans, is affected by the fine-structure of the signal, a cue for timbre perception in humans. The present study examined perception of two acoustic features affecting waveform fine-structure--harmonic structure and phase structure--in male green treefrogs. 2. We analyzed evoked vocal responses of male green treefrogs living in laboratory arenas to playbacks of digitally-generated signals resembling either conspecific advertisement or aggressive calls in their first harmonic periodicity. Systematic changes in the harmonic structure of these signals were achieved by varying the harmonic relations between frequency components in the signals, and changes in phase structure were achieved by varying the starting phases of harmonically-related components. 3. Calling was significantly influenced by the first harmonic periodicity of the signals. Males vocalized more to signals with the periodicity of the advertisement than the aggressive call. There were no differences in response to harmonic and inharmonic signals with similar spectral content. Phase structure did not significantly influence vocal responses. 4. These results suggest that the fine-structure ("timbre") of complex acoustic signals is not a significant feature guiding behavior tested using a communication response in this species.

Acoustic modulation of neural activity in the preoptic area and ventral hypothalamus of the green treefrog (Hyla cinerea).

Responses of neurons in the preoptic area and ventral hypothalamus to conspecific mating calls or white noise bursts were examined in male green treefrogs (Hyla cinerea) during different seasons. In the winter, 34.3% of preoptic neurons and 46.7% of ventral hypothalamic cells demonstrated significant changes in activity level during presentation of a conspecific mating call. In contrast, only 13.3% of preoptic units and 16.7% of ventral hypothalamic cells responded to the white noise. The percentage of preoptic and hypothalamic units responding to the advertisement call did not differ significantly during the summer breeding season. Type I units exhibited a dramatic increase in activity during acoustic stimulation followed by a rapid return to baseline activity levels after stimulus offset. Type II cells showed a robust activity increase during stimulation, but maintained an intermediate activity level after stimulus offset. In the preoptic area, a third response type exhibited suppressed activity during acoustic stimulation. Although seasonal condition did not alter the percentage of acoustically responsive units within either nucleus, the proportion of Type I units in the ventral hypothalamus was greatest during the summer.

Evolutionary and neurobiological implications of selective phonotaxis in the green treefrog, Hyla cinerea

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.

Mating strategy shifts in male green treefrogs ( Hyla cinerea): An experimental study

Four field experiments were designed to study calling and satellite mating strategies in the green treefrog, Hyla cinerea. (1) The calling male was removed from 19 satellite associations and 11 of the 19 satellite males began calling. (2) After the calling male was removed from 10 satellite associations, a speaker broadcast synthetic mating calls. All of the satellite males oriented to the speaker. (3) Synthetic mating calls were played back to 14 calling males. Eleven males stopped calling and oriented to the speaker during at least one trial. (4) Sequences of synthetic mating calls and synthetic encounter calls were broadcast to 12 calling males. Nine males became satellites when the speaker emitted mating calls; none did so when encounter calls were presented.