Auditory brainstem responses in Cope's gray treefrog (Hyla chrysoscelis): effects of frequency, level, sex and size

Size matters: individual variation in auditory sensitivity may influence sexual selection in Pacific treefrogs (Pseudacris regilla)

The matched filter hypothesis proposes a close match between senders and receivers and is supported by several studies on variation in signal properties and sensory-processing mechanisms among species and populations. Importantly, within populations, individual variation in sensory processing may affect how receivers perceive signals. Our main goals were to characterize hearing sensitivity of Pacific treefrogs (Pseudacris regilla), assess patterns of individual variation in hearing sensitivity, and evaluate how among-individual variation in hearing sensitivity and call frequency content affect auditory processing of communication signals. Overall, males and females are most sensitive to frequencies between 2.0 and 2.5 kHz, which matches the dominant frequency of the call, and have a second region of high sensitivity between 400 and 800 Hz that does not match the fundamental frequency of the call. We found high levels of among-individual variation in hearing sensitivity, primarily driven by subject size. Importantly, patterns of among-individual variation in hearing differ between males and females. Cross-correlation analyses reveal that among-individual variation in hearing sensitivity may lead to differences on how receivers, particularly females, perceive male calls. Our results suggest that individual variation in sensory processing may affect signal perception and influence the evolution of sexually selected traits.

Peripheral hearing sensitivity is similar between the sexes in a benthic turtle species despite the larger body size of males

Sexually dimorphic hearing sensitivity has evolved in many vertebrate species, and the sex with a larger body size typically shows more sensitive hearing. However, generalizing this association is controversial. Research on sexually dimorphic hearing sensitivity contributes to an understanding of auditory sense functions, adaptations, and evolution among species. Therefore, the hypothesized association between body size and hearing needs further validation, especially in specific animal groups. In this study, we assessed hearing sensitivity by measuring auditory brainstem responses (ABRs) in both sexes of 3-year-old Chinese softshell turtles (Pelodiscus sinensis). In this species, male bodies are larger than those of female, and individuals spend most of their lives in the mud at the bottom of freshwater habitats. We found that for both sexes, the hearing sensitivity bandwidth was 0.2-0.9 kHz. Although males were significantly larger than females, no significant differences in ABR thresholds or latencies were found between males and females at the same stimulus frequency. These results indicate that P. sinensis hearing is only sensitive to low-frequency (typically <0.9 kHz) sound signals and that sexually dimorphic hearing sensitivity is not a trait that has evolved in P. sinensis. Physiological and environmental reasons may account for P. sinensis acoustic communication via low-frequency sound signals and the lack of sexually dimorphic hearing sensitivity in these benthic turtles. The results of this study refine our understanding of the adaptation and evolution of the vertebrate auditory system.

Female preferences for the spectral content of advertisement calls in Cope's gray treefrog (Hyla chrysoscelis)

Amphibians have inner ears with two sensory papillae tuned to different frequency ranges of airborne sounds. In frogs, male advertisement calls possess distinct spectral components that match the tuning of one or both sensory papillae. Female preferences for the spectral content of advertisement calls can depend on signal amplitude and can vary among closely related lineages. In this study of Cope's gray tree frog (Hyla chrysoscelis), we investigated the amplitude dependence of female preferences for the spectral content of male advertisement calls, which have a "bimodal" spectrum with separate low-frequency (1.25 kHz) and high-frequency (2.5 kHz) components. In two-alternative choice tests, females generally preferred synthetic calls with bimodal spectra over "unimodal" calls having only one of the two spectral components. They also preferred unimodal calls with a high-frequency component over one with the low-frequency component. With few exceptions, preferences were largely independent of amplitude across both a 30 dB range of overall signal amplitude and an 11 dB range in the relative amplitudes of the two spectral components. We discuss these results in the context of evolutionary lability in female preferences for the spectral content of advertisement calls in North American tree frogs in the genus Hyla.

Effect of Temperature on the Plasticity of Peripheral Hearing Sensitivity to Airborne Sound in the Male Red-Eared Slider Trachemys scripta elegans

Chelonians are considered the least vocally active group of extant reptiles and known as “low-frequency specialists” with a hearing range of &lt;1.0 kHz. As they are ectothermic organisms, most of their physiological and metabolic processes are affected by temperature, which may include the auditory system responses. To investigate the influence of temperature on turtle hearing, Trachemys scripta elegans was chosen to measure the peripheral hearing sensitivity at 10, 20, 30, and 40°C (close to the upper limit of heat resistance) using the auditory brainstem response (ABR) test. An increase in temperature (from 10 to 30°C) resulted in improved hearing sensitivity (a wider hearing sensitivity bandwidth, lower threshold, and shorter latency) in T. scripta elegans. At 40°C, the hearing sensitivity bandwidth continued to increase and the latency further shortened, but the threshold sensitivity reduced in the intermediate frequency range (0.5–0.8 kHz), increased in the high-frequency range (1.0–1.3 kHz), and did not significantly change in the low-frequency range (0.2–0.4 kHz) compared to that at 30°C. Our results suggest that although the hearing range of turtles is confined to lower frequencies than that in other animal groups, turtle hearing showed exceptional thermal regulation ability, especially when the temperature was close to the upper limit of heat resistance. Temperature increases that are sensitive to high frequencies imply that the males turtles’ auditory system adapts to a high-frequency sound environment in the context of global warming. Our study is expected to spur further research on the high-temperature plasticity of hearing sensitivity in diverse taxa or in the same group with different temperature ranges. Moreover, it facilitates forecasting the adaptive evolution of the auditory system to global warming.

Bone conduction pathways confer directional cues to salamanders

Sound and vibration are generated by mechanical disturbances within the environment, and the ability to detect and localize these acoustic cues is generally important for survival, as suggested by the early emergence of inherently directional otolithic ears in vertebrate evolutionary history. However, fossil evidence indicates that the water-adapted ear of early terrestrial tetrapods lacked specialized peripheral structures to transduce sound pressure (e.g. tympana). Therefore, early terrestrial hearing should have required nontympanic (or extratympanic) mechanisms for sound detection and localization. Here, we used atympanate salamanders to investigate the efficacy of extratympanic pathways to support directional hearing in air. We assessed peripheral encoding of directional acoustic information using directionally masked auditory brainstem response recordings. We used laser Doppler vibrometry to measure the velocity of sound pressure-induced head vibrations as a key extratympanic mechanism for aerial sound reception in atympanate species. We found that sound generates head vibrations that vary with the angle of the incident sound. This extratympanic pathway for hearing supports a figure-eight pattern of directional auditory sensitivity to airborne sound in the absence of a pressure-transducing tympanic ear.

Three auditory brainstem response (ABR) methods tested and compared in two anuran species

Hearing sensitivity has been extensively investigated, often by measuring the auditory brainstem response (ABR). ABR measurements are relatively non-invasive, easy to reproduce, and allow the assessment of sensitivity when psychophysical data are difficult to obtain. However, the experimental methods differ greatly in respect to stimulation, which may result in different audiograms. We used three different methods in the same individual frogs: stimulating with brief tone bursts (tABR), long-duration tones (ltABR) and masked ABR (mABR), where transients are masked by a long-duration sinusoid, and the sensitivity is assessed by the difference between unmasked and masked ABR. We measured sensitivity in a range from 100 to 3500 Hz, and the resulting audiograms show two sensitivity peaks at 400-600 Hz and 1500-1600 Hz (both sensitive down to 30 dB re. 20 µPa). We found similar results below 1000 Hz, but when stimulating with long-duration tones, the sensitivity decreased more rapidly above this frequency. We showed that the frequency specificity of tone bursts becomes poorly defined with shorter duration at low frequencies. Comparisons between subjectively (visual inspection by researchers) and objectively (thresholds defined by signal-to-noise ratio) defined audiograms showed very little variation. In conclusion, the mABR method gave the most sensitive audiograms. The tABR method showed a similar audiogram when using relatively long-duration tone bursts (25 ms). The ltABR method is not a good choice for studying hearing thresholds above 1000 Hz because of the bias introduced by spike rate saturation in the nerve fibers and their inability to phase lock.

Multiparametric assessment of the impact of opsin expression and anesthesia on striatal cholinergic neurons and auditory brainstem activity

Recent developments in genetic engineering have established murine models that permit the selective control of cholinergic neurons via optical stimulation. Despite copious benefits granted by these experimental advances, the sensory physiognomy of these organisms has remained poorly understood. Therefore, the present study evaluates sensory and neuronal response properties of animal models developed for the study of optically induced acetylcholine release regulation. Auditory brainstem responses, fluorescence imaging, and patch clamp recording techniques were used to assess the impact of viral infection, sex, age, and anesthetic agents across the ascending auditory pathway of ChAT-Cre and ChAT-ChR2(Ai32) mice. Data analyses revealed that neither genetic configuration nor adeno-associated viral infection alters the early stages of auditory processing or the cellular response properties of cholinergic neurons. However, anesthetic agent and dosage amount profoundly modulate the response properties of brainstem neurons. Last, analyses of age-related hearing loss in virally infected ChAT-Cre mice did not differ from those reported in wild type animals. This investigation demonstrates that ChAT-Cre and ChAT-ChR2(Ai32) mice are viable models for the study of cholinergic modulation in auditory processing, and it emphasizes the need for prudence in the selection of anesthetic procedures.

Reciprocal Matched Filtering in the Inner Ear of the African Clawed Frog (Xenopus laevis)

Anurans (frogs and toads) are the most vocal amphibians. In most species, only males produce advertisement calls for defending territories and attracting mates. Female vocalizations are the exceptions among frogs, however in the African clawed frog (Xenopus laevis) both males and females produce distinct vocalizations. The matched filter hypothesis predicts a correspondence between peripheral auditory tuning of receivers and properties of species-specific acoustic signals, but few studies have assessed this relationship between the sexes. Measuring hearing sensitivity with a binaural recording of distortion product otoacoustic emissions, we have found that the ears of the males of this species are tuned to the dominant frequency of the female's calls, whereas the ears of the females are tuned close to the dominant frequency of the male's calls. Our findings provide support for the matched filter hypothesis extended to include male-female calling. This unique example of reciprocal matched filtering ensures that males and females communicate effectively in high levels of background noise, each sex being most sensitive to the frequencies of the other sex's calls.

Behavioral and neural auditory thresholds in a frog

Vocalizations play a critical role in mate recognition and mate choice in a number of taxa, especially, but not limited to, orthopterans, frogs, and birds. But receivers can only recognize and prefer sounds that they can hear. Thus a fundamental question linking neurobiology and sexual selection asks-what is the threshold for detecting acoustic sexual displays? In this study, we use 3 methods to assess such thresholds in túngara frogs: behavioral responses, auditory brainstem responses, and multiunit electrophysiological recordings from the midbrain. We show that thresholds are lowest for multiunit recordings (ca. 45 dB SPL), and then for behavioral responses (ca. 61 dB SPL), with auditory brainstem responses exhibiting the highest thresholds (ca. 71 dB SPL). We discuss why these estimates differ and why, as with other studies, it is unlikely that they should be the same. Although all of these studies estimate thresholds they are not measuring the same thresholds; behavioral thresholds are based on signal salience whereas the 2 neural assays estimate physiological thresholds. All 3 estimates, however, make it clear that to have an appreciation for detection and salience of acoustic signals we must listen to those signals through the ears of the receivers.

The difference a day makes: Breeding remodels hearing, hormones and behavior in female Cope's gray treefrogs (Hyla chrysoscelis)

In seasonal breeders, there are behavioral, endocrine, and neural adaptations that promote the sexual receptivity of females and tune their sensory systems to detect and discriminate among advertising males and to successfully copulate. What happens immediately after this key life history event is unclear, but this transitional moment offers a window into the mechanisms that remodel sexual phenotypes. In this study of wild female Cope's gray treefrogs (Hyla chrysoscelis), we tested the hypothesis that oviposition results in a suite of coordinated changes in the sexual phenotype. Specifically, we predicted that sexual receptivity and discrimination behaviors would decline along with circulating concentrations of steroid hormones (corticosterone, estradiol, testosterone) and auditory sensitivity to the acoustic frequencies emphasized in male advertisement calls. We conducted these trait measurements before and after oviposition (ca. 24-h period). There was a 100% decrease in behavioral responsiveness after oviposition, and the concentrations of all three steroids plummeted during this brief window of time, especially testosterone. Moreover, higher concentrations of corticosterone-an important component of the endocrine stress response-were associated with longer response latencies, suggesting that adrenal hormones should be considered in future studies on the hormonal basis of mate choice. Counter to our prediction, auditory sensitivity increased following oviposition, and the amplitude of the auditory brainstem response was influenced by concentrations of estradiol. In pre-oviposition females auditory sensitivity diminished with increasing estradiol concentrations, while sensitivity increased with increasing estradiol concentrations in post-oviposition females, suggesting non-linear estrogenic modulation of peripheral auditory neural recruitment. Overall, our results indicate that there is considerable remodeling of behavioral output following oviposition that co-occurs with changes in both endocrine and sensory physiology.

Masking release in temporally fluctuating noise depends on comodulation and overall level in Cope's gray treefrog

Many animals communicate acoustically in large social aggregations. Among the best studied are frogs, in which males form large breeding choruses where they produce loud vocalizations to attract mates. Although chorus noise poses significant challenges to communication, it also possesses features, such as comodulation in amplitude fluctuations, that listeners may be evolutionarily adapted to exploit in order to achieve release from masking. This study investigated the extent to which the benefits of comodulation masking release (CMR) depend on overall noise level in Cope's gray treefrog (Hyla chrysoscelis). Masked signal recognition thresholds were measured in response to vocalizations in the presence of chorus-shaped noise presented at two levels. The noises were either unmodulated or modulated with an envelope that was correlated (comodulated) or uncorrelated (deviant) across the frequency spectrum. Signal-to-noise ratios (SNRs) were lower at the higher noise level, and this effect was driven by relatively lower SNRs in modulated conditions, especially the comodulated condition. These results, which confirm that frogs benefit from CMR in a level-dependent manner, are discussed in relation to previous studies of CMR in humans and animals and in light of implications of the unique amphibian inner ear for considerations of within-channel versus across-channel mechanisms.

Sensory Biology: How Female Treefrogs Pick Mates at a Noisy Party

A recent study has found that, despite strong acoustic masking from background noise, female treefrogs are able to select among individual males advertising for mates by taking advantage of small, periodic decreases in the overall noise structure.

Effect of the Level of Anesthesia on the Auditory Brainstem Response in the Emei Music Frog (Babina daunchina)

Anesthesia is known to affect the auditory brainstem response (ABR) in mice, rats, birds and lizards. The present study investigated how the level of anesthesia affects ABR recordings in an amphibian species, Babina daunchina. To do this, we compared ABRs evoked by tone pip stimuli recorded from 35 frogs when Tricaine methane sulphonate (MS-222) anesthetic immersion times varied from 0, 5 and 10 minutes after anesthesia induction at sound frequencies between 0.5 and 6 kHz. ABR thresholds increased significantly with immersion time across the 0.5 kHz to 2.5 kHz frequency range, which is the most sensitive frequency range for hearing and the main frequency range of male calls. There were no significant differences for anesthetic levels across the 3 kHz to 6 kHz range. ABR latency was significantly longer in the 10 min group than in the 0 and 5 min groups at frequencies of 0.5, 1.0, 1.5, 2.5 kHz, while ABR latency did not differ across the 3 kHz to 4 kHz range and at 2.0 kHz. Taken together, these results show that the level of anesthesia affects the amplitude, threshold and latency of ABRs in frogs.

An exception to the matched filter hypothesis: A mismatch of male call frequency and female best hearing frequency in a torrent frog

The matched filter hypothesis proposes that the tuning of auditory sensitivity and the spectral character of calls will match in order to maximize auditory processing efficiency during courtship. In this study, we analyzed the acoustic structure of male calls and both male and female hearing sensitivities in the little torrent frog (Amolops torrentis), an anuran species who transmits acoustic signals across streams. The results were in striking contradiction to the matched filter hypothesis. Auditory brainstem response results showed that the best hearing range was 1.6–2 kHz consistent with the best sensitive frequency of most terrestrial lentic taxa, yet completely mismatched with the dominant frequency of conspecific calls (4.3 kHz). Moreover, phonotaxis tests show that females strongly prefer high‐frequency (4.3 kHz) over low‐frequency calls (1.6 kHz) regardless of ambient noise levels, although peripheral auditory sensitivity is highest in the 1.6–2 kHz range. These results are consistent with the idea that A. torrentis evolved from nonstreamside species and that high‐frequency calls evolved under the pressure of stream noise. Our results also suggest that female preferences based on central auditory system characteristics may evolve independently of peripheral auditory system sensitivity in order to maximize communication effectiveness in noisy environments.

Sound source localization and segregation with internally coupled ears: the treefrog model

Acoustic signaling plays key roles in mediating many of the reproductive and social behaviors of anurans (frogs and toads). Moreover, acoustic signaling often occurs at night, in structurally complex habitats, such as densely vegetated ponds, and in dense breeding choruses characterized by high levels of background noise and acoustic clutter. Fundamental to anuran behavior is the ability of the auditory system to determine accurately the location from where sounds originate in space (sound source localization) and to assign specific sounds in the complex acoustic milieu of a chorus to their correct sources (sound source segregation). Here, we review anatomical, biophysical, neurophysiological, and behavioral studies aimed at identifying how the internally coupled ears of frogs contribute to sound source localization and segregation. Our review focuses on treefrogs in the genus Hyla, as they are the most thoroughly studied frogs in terms of sound source localization and segregation. They also represent promising model systems for future work aimed at understanding better how internally coupled ears contribute to sound source localization and segregation. We conclude our review by enumerating directions for future research on these animals that will require the collaborative efforts of biologists, physicists, and roboticists.

The effects of call-like masking diminish after nightly exposure to conspecific choruses in green treefrogs (Hyla cinerea)

One of the major difficulties encountered by animals that select mates using acoustic signals is discriminating individual calls from the background noise generated by other conspecifics. Reducing the effects of conspecific masking could improve discrimination of individual calls from background noise. We used auditory evoked potentials to investigate the effects of forward masking on the responses to artificial calls in male and female treefrogs (Hyla cinerea), as well as whether hearing advertisement calls over several nights, as happens in natural frog choruses, could modify the effects of masking. We found that response amplitude decreased with decreasing interstimulus interval when the masker was equal in amplitude to the stimulus. We also found evidence of a priming effect, whereby response amplitude at lower masker amplitudes was greater than when the target stimulus was not preceded by a masker. Finally, we found that the effect of masking was diminished by 10 nights of chorus exposure (i.e. responses were stronger to target stimuli), whereas there was no change in response in the control group. Our results show that hearing dynamic social stimuli, such as frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Hearing conspecific vocal signals alters peripheral auditory sensitivity

We investigated whether hearing advertisement calls over several nights, as happens in natural frog choruses, modified the responses of the peripheral auditory system in the green treefrog, Hyla cinerea. Using auditory evoked potentials (AEP), we found that exposure to 10 nights of a simulated male chorus lowered auditory thresholds in males and females, while exposure to random tones had no effect in males, but did result in lower thresholds in females. The threshold change was larger at the lower frequencies stimulating the amphibian papilla than at higher frequencies stimulating the basilar papilla. Suprathreshold responses to tonal stimuli were assessed for two peaks in the AEP recordings. For the peak P1 (assessed for 0.8-1.25 kHz), peak amplitude increased following chorus exposure. For peak P2 (assessed for 2-4 kHz), peak amplitude decreased at frequencies between 2.5 and 4.0 kHz, but remained unaltered at 2.0 kHz. Our results show for the first time, to our knowledge, that hearing dynamic social stimuli, like frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Better than fish on land? Hearing across metamorphosis in salamanders

Early tetrapods faced an auditory challenge from the impedance mismatch between air and tissue in the transition from aquatic to terrestrial lifestyles during the Early Carboniferous (350 Ma). Consequently, tetrapods may have been deaf to airborne sounds for up to 100 Myr until tympanic middle ears evolved during the Triassic. The middle ear morphology of recent urodeles is similar to that of early 'lepospondyl' microsaur tetrapods, and experimental studies on their hearing capabilities are therefore useful to understand the evolutionary and functional drivers behind the shift from aquatic to aerial hearing in early tetrapods. Here, we combine imaging techniques with neurophysiological measurements to resolve how the change from aquatic larvae to terrestrial adult affects the ear morphology and sensory capabilities of salamanders. We show that air-induced pressure detection enhances underwater hearing sensitivity of salamanders at frequencies above 120 Hz, and that both terrestrial adults and fully aquatic juvenile salamanders can detect airborne sound. Collectively, these findings suggest that early atympanic tetrapods may have been pre-equipped to aerial hearing and are able to hear airborne sound better than fish on land. When selected for, this rudimentary hearing could have led to the evolution of tympanic middle ears.

Sex differences and endocrine regulation of auditory-evoked, neural responses in African clawed frogs (Xenopus)

Mating depends on the accurate detection of signals that convey species identity and reproductive state. In African clawed frogs, Xenopus, this information is conveyed by vocal signals that differ in temporal patterns and spectral features between sexes and across species. We characterized spectral sensitivity using auditory-evoked potentials (AEPs), commonly known as the auditory brainstem response, in males and females of four Xenopus species. In female X. amieti, X. petersii, and X. laevis, peripheral auditory sensitivity to their species own dyad-two, species-specific dominant frequencies in the male advertisement call-is enhanced relative to males. Males were most sensitive to lower frequencies including those in the male-directed release calls. Frequency sensitivity was influenced by endocrine state; ovariectomized females had male-like auditory tuning while dihydrotestosterone-treated, ovariectomized females maintained female-like tuning. Thus, adult, female Xenopus demonstrate an endocrine-dependent sensitivity to the spectral features of conspecific male advertisement calls that could facilitate mating. Xenopus AEPs resemble those of other species in stimulus and level dependence, and in sensitivity to anesthetic (MS222). AEPs were correlated with body size and sex within some species. A frequency following response, probably encoded by the amphibian papilla, might facilitate dyad source localization via interaural time differences.

Stress and Auditory Responses of the Otophysan Fish, Cyprinella venusta, to Road Traffic Noise

Noise pollution from anthropogenic sources is an increasingly problematic challenge faced by many taxa, including fishes. Recent studies demonstrate that road traffic noise propagates effectively from bridge crossings into surrounding freshwater ecosystems; yet, its effect on the stress response and auditory function of freshwater stream fishes is unexamined. The blacktail shiner (Cyprinella venusta) was used as a model to investigate the degree to which traffic noise impacts stress and hearing in exposed fishes. Fish were exposed to an underwater recording of traffic noise played at approximately 140 dB re 1 μPa. Waterborne cortisol samples were collected and quantified using enzyme immunoassay (EIA). Auditory thresholds were assessed in control and traffic exposed groups by measuring auditory evoked potentials (AEPs). After acute exposure to traffic noise, fish exhibited a significant elevation in cortisol levels. Individuals exposed to 2 hours of traffic noise playback had elevated hearing thresholds at 300 and 400 Hz, corresponding to the most sensitive bandwidth for this species.

Auditory brainstem response of the Japanese house bat (Pipistrellus abramus)

Auditory brainstem responses (ABR) to high frequencies encompassing the species' vocal repertoire were recorded from the inferior colliculus of the Japanese house bat, Pipistrellus abramus. Amplitudes of tone pips were systematically decreased to obtain a threshold of response at different tone frequencies. The compiled audiogram has a broad U-shape over the frequency range from 4 to 80 kHz, with low thresholds between 20 and 50 kHz. The most sensitive frequency region of 35-50 kHz occurs at the quasi-constant-frequency terminal portion of the bat's downsweeping frequency-modulated echolocation pulses. Good sensitivity extending down to 20 kHz includes the frequency range of the first harmonic of communication sounds. The ABR audiogram does not show distinct, narrow peaks of greater sensitivity at the dominant frequencies in species vocalizations. Latencies of peaks in ABR responses lengthened as stimuli were attenuated. At 40 kHz, response latencies traded with amplitude by -7 to -9 μs/dB, a value smaller than measured in another frequency-modulated bat using lower frequencies for echolocation. These results have implications for understanding the significance of amplitude-latency trading in a comparative context.

Evolutionary adaptations for the temporal processing of natural sounds by the anuran peripheral auditory system

Sensory systems function most efficiently when processing natural stimuli, such as vocalizations, and it is thought that this reflects evolutionary adaptation. Among the best-described examples of evolutionary adaptation in the auditory system are the frequent matches between spectral tuning in both the peripheral and central auditory systems of anurans (frogs and toads) and the frequency spectra of conspecific calls. Tuning to the temporal properties of conspecific calls is less well established, and in anurans has so far been documented only in the central auditory system. Using auditory-evoked potentials, we asked whether there are species-specific or sex-specific adaptations of the auditory systems of gray treefrogs (Hyla chrysoscelis) and green treefrogs (H. cinerea) to the temporal modulations present in conspecific calls. Modulation rate transfer functions (MRTFs) constructed from auditory steady-state responses revealed that each species was more sensitive than the other to the modulation rates typical of conspecific advertisement calls. In addition, auditory brainstem responses (ABRs) to paired clicks indicated relatively better temporal resolution in green treefrogs, which could represent an adaptation to the faster modulation rates present in the calls of this species. MRTFs and recovery of ABRs to paired clicks were generally similar between the sexes, and we found no evidence that males were more sensitive than females to the temporal modulation patterns characteristic of the aggressive calls used in male-male competition. Together, our results suggest that efficient processing of the temporal properties of behaviorally relevant sounds begins at potentially very early stages of the anuran auditory system that include the periphery.

Assessing stimulus and subject influences on auditory evoked potentials and their relation to peripheral physiology in green treefrogs (Hyla cinerea)

Anurans (frogs and toads) are important models for comparative studies of communication, auditory physiology, and neuroethology, but to date, most of our knowledge comes from in-depth studies of a relatively small number of model species. Using the well-studied green treefrog (Hyla cinerea), this study sought to develop and evaluate the use of auditory evoked potentials (AEPs) as a minimally invasive tool for investigating auditory sensitivity in a larger diversity of anuran species. The goals of the study were to assess the effects of frequency, signal level, sex, and body size on auditory brainstem response (ABR) amplitudes and latencies, characterize gross ABR morphology, and generate an audiogram that could be compared to several previously published audiograms for green treefrogs. Increasing signal level resulted in larger ABR amplitudes and shorter latencies, and these effects were frequency dependent. There was little evidence for an effect of sex or size on ABRs. Analyses consistently distinguished between responses to stimuli in the frequency ranges of the three previously-described populations of afferents that innervate the two auditory end organs in anurans. The overall shape of the audiogram shared prominent features with previously published audiograms. This study highlights the utility of AEPs as a valuable tool for the study of anuran auditory sensitivity.

Spatial hearing in Cope's gray treefrog: II. Frequency-dependent directionality in the amplitude and phase of tympanum vibrations

Anuran ears function as pressure difference receivers, and the amplitude and phase of tympanum vibrations are inherently directional, varying with sound incident angle. We quantified the nature of this directionality for Cope's gray treefrog, Hyla chrysoscelis. We presented subjects with pure tones, advertisement calls, and frequency-modulated sweeps to examine the influence of frequency, signal level, lung inflation, and sex on ear directionality. Interaural differences in the amplitude of tympanum vibrations were 1-4 dB greater than sound pressure differences adjacent to the two tympana, while interaural differences in the phase of tympanum vibration were similar to or smaller than those in sound phase. Directionality in the amplitude and phase of tympanum vibration were highly dependent on sound frequency, and directionality in amplitude varied slightly with signal level. Directionality in the amplitude and phase of tone- and call-evoked responses did not differ between sexes. Lung inflation strongly affected tympanum directionality over a narrow frequency range that, in females, included call frequencies. This study provides a foundation for further work on the biomechanics and neural mechanisms of spatial hearing in H. chrysoscelis, and lends valuable perspective to behavioral studies on the use of spatial information by this species and other frogs.

Treefrogs as animal models for research on auditory scene analysis and the cocktail party problem

The perceptual analysis of acoustic scenes involves binding together sounds from the same source and separating them from other sounds in the environment. In large social groups, listeners experience increased difficulty performing these tasks due to high noise levels and interference from the concurrent signals of multiple individuals. While a substantial body of literature on these issues pertains to human hearing and speech communication, few studies have investigated how nonhuman animals may be evolutionarily adapted to solve biologically analogous communication problems. Here, I review recent and ongoing work aimed at testing hypotheses about perceptual mechanisms that enable treefrogs in the genus Hyla to communicate vocally in noisy, multi-source social environments. After briefly introducing the genus and the methods used to study hearing in frogs, I outline several functional constraints on communication posed by the acoustic environment of breeding "choruses". Then, I review studies of sound source perception aimed at uncovering how treefrog listeners may be adapted to cope with these constraints. Specifically, this review covers research on the acoustic cues used in sequential and simultaneous auditory grouping, spatial release from masking, and dip listening. Throughout the paper, I attempt to illustrate how broad-scale, comparative studies of carefully considered animal models may ultimately reveal an evolutionary diversity of underlying mechanisms for solving cocktail-party-like problems in communication.