Chorus Noise Research Articles

Sensory-motor tuning allows generic features of conspecific acoustic scenes to guide rapid, adaptive, call-timing responses in túngara frogs.

Male frogs court females from within crowded choruses, selecting for mechanisms allowing them to call at favourable times relative to the calls of rivals and background chorus noise. To accomplish this, males must continuously evaluate the fluctuating acoustic scene generated by their competitors for opportune times to call. Túngara frogs produce highly frequency- and amplitude-modulated calls from within dense choruses. We used similarly frequency- and amplitude-modulated playback tones to investigate the sensory basis of their call-timing decisions. Results revealed that different frequencies present throughout this species' call differed in their degree of call inhibition, and that lower-amplitude tones were less inhibitory. Call-timing decisions were then driven by fluctuations in inhibition arising from underlying frequency- and amplitude-modulation patterns, with tone transitions that produced steeper decreases in inhibition having higher probabilities of triggering calls. Interactions between the varied behavioural sensitivities to different conspecific call frequencies revealed here, and the stereotyped amplitude- and frequency-modulation patterns present in this species' calls, can explain previously surprising patterns observed in túngara frog choruses. This highlights the importance of understanding the specific sensory drivers underpinning conspecific signalling interactions, and reveals how sensory systems can mediate the interplay between signal perception and production to facilitate adaptive communication strategies.

Túngara frog call-timing decisions arise as internal rhythms interact with fluctuating chorus noise

Abstract For chorusing males, optimally timing their calls relative to nearby rivals’ calls and fluctuations in background chorus noise is crucial for reproductive success. A caller’s acoustic environment will vary by chorus density and the properties of his chorus-mates’ calls and will fluctuate unpredictably due to chorusing dynamics emerging among his chorus-mates. Thus, callers must continuously monitor moment-to-moment fluctuations in the acoustic scene they perceive at the chorus for advantageous times to call. In live experimental choruses, we investigated the factors influencing túngara frog call-timing responses to chorus-mates’ calls on an interaction-by-interaction basis, revealing that intrinsic and extrinsic factors influenced call-timing decisions. Callers were more likely to overlap calls from smaller chorus-mates and chorus-mates at intermediate distances, as well as calls containing lower frequencies and exhibiting lower final amplitude minima. Consequently, variation among males in call properties led to variation in levels of call-interference received when calling in the same social environment. Additionally, callers were more likely to overlap chorus-mates’ calls after experiencing extended periods of inhibition and were less likely to overlap synchronized chorus-mates’ calls relative to single calls. In chorusing species, female choice is influenced by inter-caller dynamics, selecting for male call-timing strategies which, in turn, constitute the selective environment further refining these same strategies. Thus, understanding the specific factors driving call-timing decisions is essential for understanding how sexual selection operates in chorusing taxa.

Can Extensive Training Transform a Mouse into a Guinea Pig? An Evaluation Based on the Discriminative Abilities of Inferior Colliculus Neurons.

Humans and animals maintain accurate discrimination between communication sounds in the presence of loud sources of background noise. In previous studies performed in anesthetized guinea pigs, we showed that, in the auditory pathway, the highest discriminative abilities between conspecific vocalizations were found in the inferior colliculus. Here, we trained CBA/J mice in a Go/No-Go task to discriminate between two similar guinea pig whistles, first in quiet conditions, then in two types of noise, a stationary noise and a chorus noise at three SNRs. Control mice were passively exposed to the same number of whistles as trained mice. After three months of extensive training, inferior colliculus (IC) neurons were recorded under anesthesia and the responses were quantified as in our previous studies. In quiet, the mean values of the firing rate, the temporal reliability and mutual information obtained from trained mice were higher than from the exposed mice and the guinea pigs. In stationary and chorus noise, there were only a few differences between the trained mice and the guinea pigs; and the lowest mean values of the parameters were found in the exposed mice. These results suggest that behavioral training can trigger plasticity in IC that allows mice neurons to reach guinea pig-like discrimination abilities.

Intraspecific variation in advertisement call characteristics and acoustic strategies among male forest green tree frogs

AbstractFrogs are a representative taxon that use advertisement calls to aid in reproduction. In most frog species, calls vary with body size, and allometric constraints between body size and call frequency have been widely reported among anuran species. Although this variation is an important driver of sexual selection in frogs, male advertisement call strategies may also vary according to body size. In this study, we conducted playback experiments on the male forest green tree frog (Zhangixalus arboreus) to determine whether male advertisement call characteristics and strategies vary according to body size and the amplitude of intraspecific chorus noise. The results indicated that the calls of larger individuals are louder and lower than those of smaller ones, who call more frequently; moreover, the calls become lower, and the number of calls decreases, as noise levels increase. These findings suggest that forest green tree frog emits lower calls or refrains from calling when chorus noise increases, and that intraspecific variation in advertisement call characteristics can induce different strategies in response to chorus noise. Because advertisement call variation with body size is common among frog species, intraspecific variation in male advertisement call strategies may also be a common phenomenon.

Can you hear/see me? Multisensory integration of signals does not always facilitate mate choice

Abstract Females of many species choose mates using multiple sensory modalities. Multimodal noise may arise, however, in dense aggregations of animals communicating via multiple sensory modalities. Some evidence suggests multimodal signals may not always improve receiver decision-making performance. When sensory systems process input from multimodal signal sources, multimodal noise may arise and potentially complicate decision-making due to the demands on cognitive integration tasks. We tested female túngara frog, Physalaemus (=Engystomops) pustulosus, responses to male mating signals in noise from multiple sensory modalities (acoustic and visual). Noise treatments were partitioned into three categories: acoustic, visual, and multimodal. We used natural calls from conspecifics and heterospecifics for acoustic noise. Robotic frogs were employed as either visual signal components (synchronous vocal sac inflation with call) or visual noise (asynchronous vocal sac inflation with call). Females expressed a preference for the typically more attractive call in the presence of unimodal noise. However, during multimodal signal and noise treatments (robofrogs employed with background noise), females failed to express a preference for the typically attractive call in the presence of conspecific chorus noise. We found that social context and temporal synchrony of multimodal signaling components are important for multimodal communication. Our results demonstrate that multimodal signals have the potential to increase the complexity of the sensory scene and reduce the efficacy of female decision making.

Fish chorus and vessel noise in a marine protected coastal reef vary with lunar phase

Fish chorus and vessel noise in a marine protected coastal reef vary with lunar phase

Spoiled for choice: number of signalers constrains mate choice based on acoustic signals

AbstractAnimal communication mediates social interactions with important fitness consequences for individuals. Receivers use signals to detect and discriminate among potential mates. Extensive research effort has focused on how receiver behavior imposes selection on signalers and signals. However, animals communicate in socially and physically complex environments with important biotic and abiotic features that are often excluded from controlled laboratory experiments, including noise. “Noise” is any factor that prevents signal detection and discrimination. The noise caused by aggregates of acoustic signalers is a well-known impediment to receivers, but how many individual signalers are required to produce the emergent effects of chorus noise on receiver behavior? In Teleogryllus oceanicus, the Australian field cricket, we assayed female preferences for a temporal property of male advertisement signals, the number of long chirp pulses, using two-, four-, six-, and eight-choice phonotaxis experiments. We found that, as the number of individual signalers increased, receivers became less likely to respond phonotactically and less likely to express their well-documented preference for more long chirp pulses. We found that very few individual signalers can create a sufficiently noisy environment, due either to acoustic interference or choice overload, to substantially impair female preference expression. Our results suggest that receivers may not always be able to express their well-documented mating preferences in nature.

Robustness to Noise in the Auditory System: A Distributed and Predictable Property.

Background noise strongly penalizes auditory perception of speech in humans or vocalizations in animals. Despite this, auditory neurons are still able to detect communications sounds against considerable levels of background noise. We collected neuronal recordings in cochlear nucleus (CN), inferior colliculus (IC), auditory thalamus, and primary and secondary auditory cortex in response to vocalizations presented either against a stationary or a chorus noise in anesthetized guinea pigs at three signal-to-noise ratios (SNRs; −10, 0, and 10 dB). We provide evidence that, at each level of the auditory system, five behaviors in noise exist within a continuum, from neurons with high-fidelity representations of the signal, mostly found in IC and thalamus, to neurons with high-fidelity representations of the noise, mostly found in CN for the stationary noise and in similar proportions in each structure for the chorus noise. The two cortical areas displayed fewer robust responses than the IC and thalamus. Furthermore, between 21% and 72% of the neurons (depending on the structure) switch categories from one background noise to another, even if the initial assignment of these neurons to a category was confirmed by a severe bootstrap procedure. Importantly, supervised learning pointed out that assigning a recording to one of the five categories can be predicted up to a maximum of 70% based on both the response to signal alone and noise alone.

Silence is sexy: soundscape complexity alters mate choice in túngara frogs

Abstract Many animals acoustically communicate in large aggregations, producing biotic soundscapes. In turn, these natural soundscapes can influence the efficacy of animal communication, yet little is known about how variation in soundscape interferes with animals that communicate acoustically. We quantified this variation by analyzing natural soundscapes with the mid-frequency cover index and by measuring the frequency ranges and call rates of the most common acoustically communicating species. We then tested female mate choice in the túngara frog (Physalaemus pustulosus) in varying types of background chorus noise. We broadcast two natural túngara frog calls as a stimulus and altered the densities (duty cycles) of natural calls from conspecifics and heterospecifics to form the different types of chorus noise. During both conspecific and heterospecific chorus noise treatments, females demonstrated similar preferences for advertisement calls at low and mid noise densities but failed to express a preference in the presence of high noise density. Our data also suggest that nights with high densities of chorus noise from conspecifics and heterospecifics are common in some breeding ponds, and on nights with high noise density, the soundscape plays an important role diminishing the accuracy of female decision-making.

Species Recognition Is Constrained by Chorus Noise, but Not Inconsistency in Signal Production, in Cope’s Gray Treefrog (Hyla chrysoscelis)

Optimal mate choice based on the assessment of communication signals can be constrained by multiple sources of noise. One well-known impediment to acoustically guided mating decisions is the ambient noise created by multiple signaling individuals in large social groups, in which ambient noise can mask signals by impairing signal recognition and discrimination by receivers. Although studied far less often, another potential source of noise in communication systems stems from variability or inconsistency in how signalers produce their signals. Consistency is especially important in the context of mate choice because sexual advertisement signals are frequently produced repeatedly through time and are composed of constituent parts (e.g., notes, pulses) that are repeated within signals. Inconsistent signal production within individuals has the potential to mask between-individual differences that are often the target of receiver decision-making. In this study of Cope’s gray treefrog, Hyla chrysoscelis, we tested the hypothesis that ambient noise and inconsistent signaling, both independently and synergistically, impair discrimination of species identity. We assayed female discrimination based on pulse rate, a signal of species identity, in quiet and at three levels of ambient noise designed to simulate a breeding chorus. We used synthetic advertisement calls that were invariant or generated with one of three experimental levels of inconsistency in pulse rate, chosen based on levels of within-individual variation observed in natural calls. Pulse rate discrimination was impaired by average and above-average levels of chorus noise, but not by inconsistency in signal production. Receivers spent slightly more time making decisions at the highest level of chorus noise, but response latencies were unaffected by inconsistency. There was no evidence of synergism between ambient noise and inconsistency. Our results suggest that ambient noise, but not inconsistency in signal production, may limit sexual selection on a signal of species identity in natural settings.

The paradox of hearing at the lek: auditory sensitivity increases after breeding in female gray treefrogs (Hyla chrysoscelis).

Both behavioral receptivity and neural sensitivity to acoustic mate attraction signals vary across the reproductive cycle, particularly in seasonally breeding animals. Across a variety of taxa receptivity to signals increases, as does peripheral auditory sensitivity, as females transition from a non-breeding to breeding condition. We recently documented decreases in receptivity to acoustic mate attraction signals and circulating hormone levels, but an increase in peripheral auditory sensitivity to call-like stimuli following oviposition in Cope's gray treefrogs (Hyla chrysoscelis). However, it is not known if changes in auditory sensitivity are confined to the frequency range of calls, or if they result from more generalized changes in the auditory periphery. Here, we used auditory brainstem responses (ABRs) to evaluate peripheral frequency sensitivity in female Cope's gray treefrogs before and after oviposition. We found lower ABR thresholds, greater ABR amplitudes, and shorter ABR latencies following oviposition. Changes were most pronounced and consistent at lower frequencies associated with the amphibian papilla, but were also detectable at higher frequencies corresponding to the tuning of the basilar papilla. Furthermore, only ABR latencies were correlated with circulating steroid hormones (testosterone). Changes in peripheral processing may result from changes in metabolic function or sensorineural adaptation to chorus noise.

Within-individual variation in sexual displays: signal or noise?

Male sexual displays are variable within individuals, but we know very little about how this variation in signal production affects animal communication. Within-individual variation does not appear to signal male quality as a potential mate and may simply obscure mean male trait values. Female treefrogs nevertheless prefer males who produce consistent signals of male quality but may not be able to express this preference in dense breeding aggregations, which produce high levels of chorus 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 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.

Within-individual variation in signal production limits female choice in noisy treefrog choruses

In acoustically communicating animals such as anurans and orthopterans, individual signalers can display remarkable variation in signal traits, even within a single bout of calling. This within-individual variation has the potential to “mask” the between-individual variation that is the target of sexual selection. Thus, receivers may not always be able to discriminate among males based on properties of their acoustic signals, even when female choice is well documented in the lab. The effect of within-individual variation may be compounded by noise generated by breeding choruses of conspecifics, which can prevent signal detection or discrimination. We manipulated within-individual variation in two signal traits (call rate and pulse rate) that are critically important for mate choice in Hyla chrysoscelis, Cope’s gray treefrog. We replicated our design at three realistic levels of chorus noise. Within-individual variation in signal traits reduced the strength of sexual selection on one of two traits, and receivers were less likely to discriminate against lower-quality signals under conditions of realistic chorus noise. Our results suggest that preference functions generated using stimuli that lack within-individual variation, under ideal listening conditions, may chronically over-estimate the strength of sexual selection on signal traits in natural populations.

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.

Raising a racket: invasive species compete acoustically with native treefrogs

Environmental noise is increasing worldwide, limiting the space available for species to send and receive important acoustic information. Many invasive species produce acoustic signals that alter the spectrotemporal characteristics of available signalling space. This provides an opportunity to test ideas about competitive exclusion by quantifying whether species with shared requirements for acoustic resources will become excluded or partition resource use to permit coexistence. We conducted a field playback experiment to test whether native treefrogs (green treefrogs, Hyla cinerea; pine woods treefrogs, Hyla femoralis) modify their acoustic behaviour to minimize acoustic competition from chorus noise of the invasive Cuban treefrog, Osteopilus septentrionalis. We demonstrate that noise from an invasive species differentially affects the vocal behaviour of native species. Those with similar calls (H. cinerea) shortened calls, called louder and persisted calling in response to masking stimuli while those with different calls (H. femoralis) did not modify behaviour. This evidence suggests that acoustic competition by invasive O. septentrionalis has altered the acoustic community structure, identifying acoustic competition as a mechanism by which invasive species can impact communities. Furthermore, these results broaden the concept of noise pollution, demonstrating fitness-relevant consequences of noise produced by invasive species.

Humans, Fish, and Whales: How Right Whales Modify Calling Behavior in Response to Shifting Background Noise Conditions.

This study investigates the role of behavioral plasticity in the variation of sound production of southern right whales (Eubalaena australis) in response to changes in the ambient background noise conditions. Data were collected from southern right whales in Brazilian waters in October and November 2011. The goal of this study was to quantify differences in right whale vocalizations recorded in low background noise as a control, fish chorus noise, and vessel noise. Variation in call parameters were detected among the three background noise conditions and have implications for future studies of noise effects on whale sound production.

Effects of anthropogenic noise on endocrine and reproductive function in White's treefrog, Litoria caerulea.

Urbanization is a major driver of ecological change and comes with a suite of habitat modifications, including alterations to the local temperature, precipitation, light and noise regimes. Although many recent studies have investigated the behavioural and ecological ramifications of urbanization, physiological work in this area has lagged. We tested the hypothesis that anthropogenic noise is a stressor for amphibians and that chronic exposure to such noise leads to reproductive suppression. In the laboratory, we exposed male White's treefrogs, Litoria caerulea, to conspecific chorus noise either alone or coupled with pre-recorded traffic noise nightly for 1 week. Frogs presented with anthropogenic noise had significantly higher circulating concentrations of corticosterone and significantly decreased sperm count and sperm viability than did control frogs. These results suggest that in addition to having behavioural and ecological effects, anthropogenic change might alter physiology and Darwinian fitness. Future work should integrate disparate fields such as behaviour, ecology and physiology to elucidate fully organisms' responses to habitat change.

Allometric Convergence, Acoustic Character Displacement, and Species Recognition in the Syntopic Cricket Frogs Acris crepitans and A. gryllus

Evidence for reproductive character displacement (RCD) has accumulated more slowly than for ecological character displacement, perhaps because sampling scales and environmental covariates can obscure the role of RCD in speciation. We examined an early example of RCD in an anuran species group, the vocalizations of the sympatric cricket frogs Acris crepitans and A. gryllus. With a relatively fine spatial scale, we compared mixed-species choruses (syntopy), nearby locations where A. gryllus is recently extirpated (historic sympatry), and surrounding areas without secondary contact (allopatry). In each of these areas, we evaluated variation in dominant frequency, click rate, and mass of males. In addition, we determined the acoustic preferences of syntopic females. Temperature influenced dominant frequency of vocalizations in A. crepitans, but not in A. gryllus. Body size varied more and had a stronger influence on dominant frequency in A. crepitans than in A. gryllus. Consequently, the decrease in mass of A. crepitans in syntopy resulted in convergence of body size and divergence of dominant frequencies of the two species. In contrast, dominant frequency of A. crepitans did not differ between historic sympatry and allopatry. Females of both species used fine temporal structure to discriminate between conspecific and heterospecific vocalizations and showed no preferences for dominant frequency. Chorus noise limited the ability of A. gryllus females to detect and discriminate vocalizations, so convergence in mass might have resulted from RCD in dominant frequency to reduce heterospecific acoustic interference. However, influences other than RCD might have caused syntopic convergence in body size.

Multitasking males and multiplicative females: dynamic signalling and receiver preferences in Cope's grey treefrog

The ‘multitasking hypothesis’ for complex signal function predicts performance trade-offs between signal components that negatively covary (e.g. due to energetic or mechanical constraints) and receiver preferences for more extreme values of the negatively covarying components that are difficult to produce simultaneously. We tested these two predictions in Cope's grey treefrogs, Hyla chrysoscelis. In a field study, we recorded and analysed 1000 advertisement calls from males calling in breeding choruses to test the prediction that two signal components important in female mating decisions (call rate and call duration) negatively covary under natural conditions. In a laboratory study, we conducted phonotaxis tests with female subjects to test the prediction that females prefer calls with higher overall ‘call efforts’ (the product of call rate × call duration). Consistent with predictions of the multitasking hypothesis, call rate and call duration were significantly negatively related and females preferred calls produced with higher call efforts, manifested through preferences for greater values of both call rate and call duration. We conducted an additional playback experiment to test the hypothesis that males increase their call effort in competitive situations to maximize their attractiveness to females. Compared to quiet conditions, male subjects increased their call duration and decreased their call rate, but did not alter call effort, in response to a simulated calling neighbour or broadcasts of chorus noise. Together our data have implications for understanding the function of multicomponent signals when signallers must balance performance trade-offs in mate attraction with dynamic signal modifications in other social contexts.