Alejandro Vélez

Sound transmission and the recognition of temporally degraded sexual advertisement signals in Cope's gray treefrog (Hyla chrysoscelis)

SUMMARY Acoustic communication signals degrade as they propagate between signalers and receivers. While we generally understand the degrading effects of sound propagation on the structure of acoustic signals, we know considerably less about how receivers make behavioral decisions based on the perception of degraded signals in sonically and structurally complex habitats where communication occurs. In this study of acoustic mate recognition in Copes gray treefrog, Hyla chrysoscelis (Cope 1880), we investigated how the temporal structure of male advertisement calls was compromised by propagation in a natural habitat and how females responded to stimuli mimicking various levels of temporal degradation. In a sound transmission experiment, we quantified changes in the pulsed structure of signals by broadcasting synthetic calls during active choruses from positions where we typically encountered signalers, and re-recording the signals from positions where we typically encountered potential receivers. Our main finding was that the silent gaps between pulses become increasingly ‘filled in’ by background noise and reverberations as a function of increasing propagation distance. We also conducted female phonotaxis experiments to determine the threshold modulation depth required to elicit recognition of the pulsatile structure of the call. Females were surprisingly tolerant of degraded temporal structure, and there was a tendency for greater permissiveness at lower playback levels. We discuss these results in terms of presumed mechanisms of call recognition in complex environments and the acoustic adaptation hypothesis.

Signal Recognition by Green Treefrogs (Hyla cinerea) and Cope's Gray Treefrogs (Hyla chrysoscelis) in Naturally Fluctuating Noise

This study tested three hypotheses about the ability of female frogs to exploit temporal fluctuations in the level of background noise to overcome the problem of recognizing male advertisement calls in noisy breeding choruses. Phonotaxis tests with green treefrogs (Hyla cinerea) and Copes gray treefrogs (Hyla chrysoscelis) were used to measure thresholds for recognizing calls in the presence of noise maskers with (a) no level fluctuations, (b) random fluctuations, or level fluctuations characteristic of (c) conspecific choruses and (d) heterospecific choruses. The dip-listening hypothesis predicted lower signal recognition thresholds in the presence of fluctuating maskers compared with nonfluctuating maskers. Support for the dip-listening hypothesis was weak; only Copes gray treefrogs experienced dip listening and only in the presence of randomly fluctuating maskers. The natural soundscapes advantage hypothesis predicted lower recognition thresholds when level fluctuations resembled those of natural soundscapes compared with artificial fluctuations. This hypothesis was rejected. In noise backgrounds with natural fluctuations, the species-specific advantage hypothesis predicted lower recognition thresholds when fluctuations resembled species-specific patterns of conspecific soundscapes. No evidence was found to support this hypothesis. These results corroborate previous findings showing that Copes gray treefrogs, but not green treefrogs, experience dip listening under some noise conditions. Together, the results suggest level fluctuations in the soundscape of natural breeding choruses may present few dip-listening opportunities. The findings of this study provide little support for the hypothesis that receivers are adapted to exploit level fluctuations of natural soundscapes in recognizing communication signals.

Song structure, not high‐frequency song content, determines high‐frequency auditory sensitivity in nine species of New World sparrows (Passeriformes: Emberizidae)

Summary The evolution of vocal signals can be constrained by a host of factors including habitat effects on sound propagation, morphology of sound-producing structures and phylogenetic relationships among species. Here, we asked whether auditory sensitivity over a broad range of frequencies correlates with the spectral content of conspecific vocalizations, or whether it is constrained by the overall structure of vocalizations, habitat effects on sound propagation or relatedness among species. We studied nine New World sparrows (Passeriformes: Emberizidae) including three open-habitat species, three scrub-like habitat species and three forest species. For each habitat, one species had pure-trilled songs, another had tonal songs and another had complex songs with tones, trills and amplitude-modulated buzzes. As predicted by the acoustic adaptation hypothesis, song spectral properties (specifically frequency and entropy) had the highest values in open-habitat species and the lowest values in forest species. Based on our results from song analyses, and the sender–receiver matching hypothesis, we predicted that open-habitat species would be more sensitive to high-frequency sounds compared to forest species. Contrary to this prediction, habitat and high-frequency song content had little effect on audiogram shape. Song type, however, had a strong effect, with species that produce complex songs showing higher sensitivity to high-frequency sounds than all other species. Our results suggest that the use of song frequency by receivers depends on song structure and not necessarily on song spectral content. Therefore, our current understanding of how signal-processing mechanisms should match signal properties appears to be too simple. When thinking about the evolution of signal-processing mechanisms, the multidimensionality of signals, and how the different dimensions can interact, should be considered.

Habitat-related differences in auditory processing of complex tones and vocal signal properties in four songbirds

We examined temporal processing of harmonic tone complexes in two woodland species (tufted titmice and white-breasted nuthatches) and two open-habitat species (house sparrows and white-crowned sparrows). Envelope and fine-structure processing were quantified using the envelope following response (EFR) and frequency following response (FFR). We predicted stronger EFRs in the open-habitat species based on broader auditory filters and greater amplitude modulation of vocal signals in this group. We predicted stronger FFRs in woodland species based on narrower auditory filters. As predicted, EFR amplitude was generally greatest in the open habitat species. FFR amplitude, in contrast, was greatest in white-crowned sparrows with no clear difference between habitats. This result cannot be fully explained by species differences in audiogram shape and might instead reflect greater acoustic complexity of songs in the white-crowned sparrow. Finally, we observed stronger FFRs in woodland species when tones were broadcast with the next higher harmonic in the complex. Thus, species such as nuthatches that have songs with strong harmonics may process these sounds using enhanced spectral processing instead of enhanced amplitude-envelope processing. The results suggest coevolution between signal design and temporal processing of complex signals and underscore the need to study auditory processing with a diversity of signals.

Pulse-number discrimination by Cope's gray treefrog (Hyla chrysoscelis) in modulated and unmodulated noise

In Copes gray treefrog (Hyla chrysoscelis), thresholds for recognizing conspecific calls are lower in temporally modulated noise backgrounds compared with unmodulated noise. The effect of modulated noise on discrimination among different conspecific calls is unknown. In quiet, females prefer calls with relatively more pulses. This study tested the hypotheses that noise impairs selectivity for longer calls and that processes akin to dip listening in modulated noise can ameliorate this impairment. In two-stimulus choice tests, female subjects were allowed to choose between an average-length call and a shorter or longer alternative. Tests were replicated at two signal levels in quiet and in the presence of chorus-shaped noise that was unmodulated, modulated by a sinusoid, or modulated by envelopes resembling natural choruses. When subjects showed a preference, it was always for the relatively longer call. Noise reduced preferences for longer calls, but the magnitude of this reduction was unrelated to whether the noise envelope was modulated or unmodulated. Together, the results are inconsistent with the hypothesis that dip listening improves a female gray treefrogs ability to select longer calls in modulated compared with unmodulated noise.

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 Copes 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.

The signal in noise: Acoustic information for soundscape orientation in two North American tree frogs

Lay Summary Information conveyed by the timing and temporal structure of calls embedded in sounds of breeding choruses is necessary for eliciting orientation in treefrogs. In contrast, information based on emergent acoustic properties arising from the collective mixture of calls constituting the chorus is not enough. Orientation based on the timing and temporal properties of the actual calls composing choruses could limit the distances over which female frogs could use sound to orient toward and localize breeding aggregations.

Avian Auditory Processing at Four Different Scales: Variation Among Species, Seasons, Sexes, and Individuals

Previous research on songbirds has typically focused on variation in production of vocal communication signals. These studies have addressed the mechanisms and functional significance of variation in vocal production across species and, within species, across seasons and among individuals (e.g., males of varying resource-holding capacity). However, far less is known about parallel variation in sensory processing, particularly in non-model species. A relationship between vocal signals and auditory processing is expected based on the sender–receiver matching hypothesis. Here, we review our recent comparative studies of auditory processing in songbirds conducted using auditory evoked potentials (AEPs) in a variety of field-caught songbird species. AEPs are voltage waveforms recorded from the scalp surface that originate from synchronous neural activity and provide insight into the sensitivity, frequency resolution, and temporal resolution of the subcortical auditory system. These studies uncovered variation in auditory processing at a number of different scales that was generally consistent with the sender–receiver matching hypothesis. For example, differences in auditory processing were uncovered among species and across seasons that may enhance perception of communication signals in species-specific habitats and during periods of mate selection, respectively. Sex differences were also revealed, often in season-specific patterns, and surprising individual differences were observed in auditory processing of mate attraction signals but not of calls used in interspecific contexts. While much remains to be learned, these studies highlight a previously unrecognized degree of parallel variation in songbirds, existing at diverse hierarchical scales, between production of vocal communication signals and subcortical auditory processing.