Spatial hearing in Cope's gray treefrog: I. Open and closed loop experiments on sound localization in the presence and absence of noise

Poison frog social behaviour under global change: potential impacts and future challenges

The current and cascading effects of global change challenges the interactions both between animal individuals (i.e. social and sexual behaviour) and the environment they inhabit. Amphibians are an ecologically diverse class with a wide range of social and sexual behaviours, making them a compelling model to understand the potential adaptations of animals faced with the effects of human-induced rapid environmental changes (HIREC). Poison frogs (Dendrobatoidea) are a particularly interesting system, as they display diverse social behaviours that are shaped by conspecific and environmental interactions, thus offering a tractable system to investigate how closely related species may respond to the impacts of HIREC. Here, we discuss the potential impacts of global change on poison frog behaviour, and the future challenges this group may face in response to such change. We pay special attention to parental care and territoriality, which are emblematic of this clade, and consider how different species may flexibly respond and adapt to increasingly frequent and diverse anthropogenic stress. More specifically, we hypothesise that some parents may increase care (i.e. clutch attendance and distance travelled for tadpole transport) in HIREC scenarios and that species with more generalist oviposition and tadpole deposition behaviours may fare more positively than their less flexible counterparts; we predict that the latter may either face increased competition for resources limited by HIREC or will be forced to adapt and expand their natural preferences. Likewise, we hypothesise that human-driven habitat alteration will disrupt the acoustic and visual communication systems due to increased noise pollution and/or changes in the surrounding light environment. We highlight the need for more empirical research combining behavioural ecology and conservation to better predict species’ vulnerability to global change and efficiently focus conservation efforts.

Woe is the loner: Female treefrogs prefer clusters of displaying males over single "hotshot" males

Communal displays such as leks and choruses are puzzling phenomena, as it is not obvious why signalers or choosers should aggregate. It has been hypothesized that signalers in leks enjoy higher per-capita reproductive success because choosers prefer to sample among dense configurations ("clusters") that are easier to compare. Although female preferences as well as the signal features of attractive males are well characterized in many chorusing species, we know little about how mate sampling is influenced by the spatial dynamics within communal displays. Here, we ask how female Eastern Gray Treefrogs (Hyla versicolor) respond to isolated and clustered call stimuli in a simple one versus three playback design. We explored (i) whether females exhibit a general preference for call clusters, (ii) whether spatial preference is robust to call-feature preference, and (iii) how this affects the relative success of attractive and unattractive males in different spatial combinations. We found generalized spatial discrimination against lone callers but did observe fine-scale assessment of call features within clusters. The prominence of the spatial preference impacts the attractiveness of males, conferring particular advantage to attractive callers within clusters, while reducing attractiveness of isolated males regardless of their acoustic features. Our findings indicate that female frogs navigate complex choruses by initially orientating toward clusters of calling males, and then assess call features within them. This study provides novel insight into the mate choice heuristics involved in animal choruses.

Lung-to-ear sound transmission does not improve directional hearing in green treefrogs (Hyla cinerea)

Amphibians are unique among extant vertebrates in having middle ear cavities that are internally coupled to each other and to the lungs. In frogs, the lung-to-ear sound transmission pathway can influence the tympanum's inherent directionality, but what role such effects might play in directional hearing remains unclear. In this study of the American green treefrog (Hyla cinerea), we tested the hypothesis that the lung-to-ear sound transmission pathway functions to improve directional hearing, particularly in the context of intraspecific sexual communication. Using laser vibrometry, we measured the tympanum's vibration amplitude in females in response to a frequency modulated sweep presented from 12 sound incidence angles in azimuth. Tympanum directionality was determined across three states of lung inflation (inflated, deflated, reinflated) both for a single tympanum in the form of the vibration amplitude difference (VAD) and for binaural comparisons in the form of the interaural vibration amplitude difference (IVAD). The state of lung inflation had negligible effects (typically less than 0.5 dB) on both VADs and IVADs at frequencies emphasized in the advertisement calls produced by conspecific males (834 and 2730 Hz). Directionality at the peak resonance frequency of the lungs (1558 Hz) was improved by ∼3 dB for a single tympanum when the lungs were inflated versus deflated, but IVADs were not impacted by the state of lung inflation. Based on these results, we reject the hypothesis that the lung-to-ear sound transmission pathway functions to improve directional hearing in frogs.

Customizable Recorder of Animal Kinesis (CRoAK): A multi-axis instrumented enclosure for measuring animal movements

Accurately quantifying animal activity and movements is of fundamental importance in a broad range of disciplines, from biomedical research to behavioral ecology. In many instances, it is desirable to measure natural movements in controlled sensory environments in which the animals are not physically or chemically restrained, but their movements are nevertheless constrained to occur within a fixed volume. Here, we describe a novel device to quantify the movements of small animals in response to sensory stimulation. The device consists of an Arduino controlled inertial measurement unit that senses angular velocity (along three axes) of a suspended mesh enclosure that temporarily houses the animal subject. We validated the device by measuring the phonotaxis behavior of gravid female frogs in response to acoustic broadcasts of male mating calls. The system, as designed, proved effective at measuring natural movements made in response to acoustic stimulation.

Social learning exploits the available auditory or visual cues

The ability to acquire a behavior can be facilitated by exposure to a conspecific demonstrator. Such social learning occurs under a range of conditions in nature. Here, we tested the idea that social learning can benefit from any available sensory cue, thereby permitting learning under different natural conditions. The ability of naïve gerbils to learn a sound discrimination task following 5 days of exposure adjacent to a demonstrator gerbil was tested in the presence or absence of visual cues. Naïve gerbils acquired the task significantly faster in either condition, as compared to controls. We also found that exposure to a demonstrator was more potent in facilitating learning, as compared to exposure to the sounds used to perform the discrimination task. Therefore, social learning was found to be flexible and equally efficient in the auditory or visual domains.

Frogs Exploit Statistical Regularities in Noisy Acoustic Scenes to Solve Cocktail-Party-like Problems

Noise is a ubiquitous source of errors in all forms of communication [1]. Noise-induced errors in speech communication, for example, make it difficult for humans to converse in noisy social settings, a challenge aptly named the "cocktail party problem" [2]. Many nonhuman animals also communicate acoustically in noisy social groups and thus face biologically analogous problems [3]. However, we know little about how the perceptual systems of receivers are evolutionarily adapted to avoid the costs of noise-induced errors in communication. In this study of Cope's gray treefrog (Hyla chrysoscelis; Hylidae), we investigated whether receivers exploit a potential statistical regularity present in noisy acoustic scenes to reduce errors in signal recognition and discrimination. We developed an anatomical/physiological model of the peripheral auditory system to show that temporal correlation in amplitude fluctuations across the frequency spectrum ("comodulation") [4-6] is a feature of the noise generated by large breeding choruses of sexually advertising males. In four psychophysical experiments, we investigated whether females exploit comodulation in background noise to mitigate noise-induced errors in evolutionarily critical mate-choice decisions. Subjects experienced fewer errors in recognizing conspecific calls and in selecting the calls of high-quality mates in the presence of simulated chorus noise that was comodulated. These data show unequivocally, and for the first time, that exploiting statistical regularities present in noisy acoustic scenes is an important biological strategy for solving cocktail-party-like problems in nonhuman animal communication.

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.

Inherent Directionality Determines Spatial Release from Masking at the Tympanum in a Vertebrate with Internally Coupled Ears

In contrast to humans and other mammals, many animals have internally coupled ears that function as inherently directional pressure-gradient receivers. Two important but unanswered questions are to what extent and how do animals with such ears exploit spatial cues in the perceptual analysis of noisy and complex acoustic scenes? This study of Cope's gray treefrog (Hyla chrysoscelis) investigated how the inherent directionality of internally coupled ears contributes to spatial release from masking. We used laser vibrometry and signal detection theory to determine the threshold signal-to-noise ratio at which the tympanum's response to vocalizations could be reliably detected in noise. Thresholds were determined as a function of signal location, noise location, and signal-noise separation. Vocalizations were broadcast from one of three azimuthal locations: frontal (0 °), to the right (+90 °), and to the left (-90 °). Masking noise was broadcast from each of 12 azimuthal angles around the frog (0 to 330 °, 30 ° separation). Variation in the position of the noise source resulted in, on average, 4 dB of spatial release from masking relative to co-located conditions. However, detection thresholds could be up to 9 dB lower in the "best ear for listening" compared to the other ear. The pattern and magnitude of spatial release from masking were well predicted by the tympanum's inherent directionality. We discuss how the magnitude of masking release observed in the tympanum's response to spatially separated signals and noise relates to that observed in previous behavioral and neurophysiological studies of frog hearing and communication.

The biological significance of acoustic stimuli determines ear preference in the music frog

Behavioral and neurophysiological studies support the idea that right ear advantage (REA) exists for perception of conspecific vocal signals in birds and mammals. Nevertheless, few studies have focused on anuran species that typically communicate through vocalization. The present study examined the direction and latencies of orientation behaviors in Emei music frogs (Babina daunchina) produced in response to six auditory stimuli emitted by a speaker placed directly behind the subjects. The stimuli included male advertisement calls produced from within burrow nests, which have been shown to be highly sexually attractive (HSA), calls produced from outside burrows, which are of low sexual attractiveness (LSA), screech calls produced when frogs are attacked by snakes, white noise, thunder and silence. For all sound stimuli except the screech, the frogs preferentially turned to the right. Right ear preference was strongest for HSA calls. For the screech and thunder stimuli, there was an increased tendency for subjects to move further from the speaker rather than turning. These results support the idea that in anurans, right ear preference is associated with perception of positive or neutral signals such as the conspecific advertisement call and white noise, while a left ear preference is associated with perception of negative signals such as predatory attack.

From uni- to multimodality: towards an integrative view on anuran communication

Undeniably, acoustic signals are the predominant mode of communication in frogs and toads. Acoustically active species are found throughout the vast diversity of anuran families. However, additional or alternative signal modalities have gained increasing attention. In several anurans, seismic, visual and chemical communications have convergently evolved due to ecological constraints such as noisy environments. The production of a visual cue, like the inevitably moving vocal sac of acoustically advertising males, is emphasized by conspicuously coloured throats. Limb movements accompanied by dynamic displays of bright colours are additional examples of striking visual signals independent of vocalizations. In some multimodal anuran communication systems, the acoustic component acts as an alert signal, which alters the receiver attention to the following visual display. Recent findings of colourful glands on vocal sacs, producing volatile species-specific scent bouquets suggest the possibility of integration of acoustic, visual and chemical cues in species recognition and mate choice. The combination of signal components facilitates a broadened display repertoire in challenging environmental conditions. Thus, the complexity of the communication systems of frogs and toads may have been underestimated.

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.