Animal Acoustic Signals Research Articles

Geographic variation in the acoustic signals of lesser treefrogs Dendropsophus minutus (Anura, Hylidae)

Biological sound archives are a major source to investigate geographic variation in animal acoustic signals and their evolutionary drivers. The acoustic signals of anuran species with wide distribution ranges often vary geographically as a result of isolation by distance and climate amongst other factors. We examined whether the acoustic structure of call notes would vary geographically in lesser treefrogs Dendropsophus minutus using recordings from bioacoustics repositories. We also tested whether climate (mean annual temperature and annual precipitation) drive geographic variation in those signals. The acoustic distance was unrelated to geographic distance, suggesting that isolation by distance solely cannot explain geographic variation in call structure. Overall, lesser treefrogs uttered call notes with lower frequencies and bandwidths in the west of their range. In addition, frogs produced shorter call notes in hotter, wetter sites and narrow-bandwidth notes in hotter environments. We suggest that frogs produce more calls (not measured here) of shorter durations to maximize transmission and minimize the metabolic costs of calling at high air temperatures. We also suggest that hotter environments favor the propagation of lower-bandwidth calls. This study reinforces the feasibility and power of citizen science and acoustic data repositories for understanding the evolution of anuran acoustic signals.

Higher-order dialectic variation and syntactic convergence in the complex warble song of budgerigars.

Dialectic signatures in animal acoustic signals are key in the identification of and association with group members. Complex vocal sequences may also convey information about behavioral state, and may thus vary according to social environment. Some bird species, such as psittaciforms, learn and modify their complex acoustic signals throughout their lives. However, the structure and function of vocal sequences in open-ended vocal learners remains understudied. Here, we examined vocal sequence variation in the warble song of budgerigars, and how these change upon contact between social groups. Budgerigars are open-ended vocal learners which exhibit fission-fusion flock dynamics in the wild. We found that two captive colonies of budgerigars exhibited colony-specific differences in the syntactic structure of their vocal sequences. Individuals from the two colonies differed in the propensity to repeat certain note types, forming repetitive motifs which served as higher-order signatures of colony identity. When the two groups were brought into contact, their vocal sequences converged, and these colony-specific repetitive patterns disappeared, with males from both erstwhile colonies now producing similar sequences with similar syntactic structure. We present data suggesting that the higher-order temporal arrangement of notes/vocal units is modified throughout life by social learning as groups of birds continually associate and dissociate. Our study sheds light on the importance of examining signal structure at multiple levels of organization, and the potential for psittaciform birds as model systems to examine the influence of learning and social environment on acoustic signals.

Acoustic modulation of reproductive hormones in the blacktail shiner, Cyprinella venusta, a soniferous cyprinid

Animal communication networks consist of multisensory signals and often include an acoustic element. Many freshwater fishes, including the blacktail shiner, Cyprinella venusta , produce specific vocalizations during courtship and agonistic behaviours. Similar to other vocal taxa, fish communication networks are vulnerable to noise pollution. The dominant frequencies of anthropogenic noise often overlap spectrally with animal acoustic signals. To investigate the role of acoustic signalling in breeding soniferous fishes in a physiological context, we conducted a series of acoustic playback trials. We measured waterborne levels of oestradiol (E2) and prostaglandin F2 α (PGF2 α ) in gravid females exposed to courtship calls, and levels of 11-ketotestosterone (11KT) in males exposed to agonistic calls. In a separate series of trials, we exposed individuals to calls masked with bridge traffic noise. Hormone levels were quantified using enzyme-linked immunosorbent assays (ELISA) and analysed using repeated measures ANOVAs. Gravid females exhibited a significant drop in E2 ( P = 0.023) and an increase in PGF2 α during courtship calls ( P = 0.01). Our research suggests that growl signals contribute to the onset of spawning behaviours in female C. venusta and this response may be dampened by anthropogenic noise pollution. • We examined effects of anthropogenic noise on acoustic signalling in C. venusta. • Males produce growls (courtship/spawning) and knocks (male–male aggression). • Unmasked growls modulated levels of reproductive hormones in gravid females. • Males did not respond hormonally to unimodal stimuli. • Fish exposed to masked vocalizations did not exhibit hormonal changes.

Community Bioacoustics: Studying Acoustic Community Structure for Ecological and Conservation Insights

The diversity of animal acoustic signals has evolved due to multiple ecological processes, both biotic and abiotic. At the level of communities of signaling animals, these processes may lead to diverse outcomes, including partitioning of acoustic signals along multiple axes (divergent signal parameters, signaling locations, and timing). Acoustic data provides information on the organization, diversity and dynamics of an acoustic community, and thus enables study of ecological change and turnover in a non-intrusive way. In this review, we lay out how community bioacoustics (the study of acoustic community structure and dynamics), has value in ecological monitoring and conservation of diverse landscapes and taxa. First, we review the concepts of signal space, signal partitioning and their effects on the structure of acoustic communities. Next, we highlight how spatiotemporal ecological change is reflected in acoustic community structure, and the potential this presents in monitoring and conservation. As passive acoustic monitoring gains popularity worldwide, we propose that the analytical framework of community bioacoustics has promise in studying the response of entire suites of species (from insects to large whales) to rapid anthropogenic change.

Treefrogs adjust their acoustic signals in response to harmonics structure of intruder calls

AbstractSpectral properties of animal acoustic signals may help individuals to assess the characteristics of rivals and to adjust their competitive strategies in territorial disputes. Thus, we hypothesized that the distribution of energy across frequency bands in anuran calls determines behavioral responses in male–male competition. Using playback experiments, we investigated the relevance of the harmonic calls in the acoustic communication of the treefrog Dendropsophus minutus. We exposed territorial males to three synthetic acoustic stimuli composed of aggressive notes: 1) standard call (all harmonics and peak frequency corresponding to the second band); 2) inverted-energy call (all harmonics and peak frequency corresponding to the first band); and 3) concentrated-energy call (all energy contained in the second harmonic). Males responded aggressively to all stimuli, mainly by increasing the rate and duration of their aggressive notes. However, when exposed to stimuli with different harmonic configurations, males changed the harmonic structure of their own calls, emitting more A- and B-notes with peak power in the fundamental frequency, particularly when exposed to the concentrated-energy call. Our results suggest that male frogs may use the harmonic structure of calls to assess opponents and modulate territorial and aggressive behavior, triggering complex acoustic adjustments. This study contributes to our knowledge about the functions of acoustic traits in amphibian social interactions, and particularly of the presence of harmonics that has received less attention compared to other acoustic properties in the study of animal acoustic communication.

Can you hear me now? A review of signal transmission and experimental evidence for the acoustic adaptation hypothesis

ABSTRACT The Acoustic Adaptation Hypothesis (AAH) posits that animal acoustic signals used in long-range communication should be adapted to transmit well within the habitats in which they evolved. However, comparative studies of signal form indicate mixed support for predictions of the AAH. Several studies have employed experimental playback approaches to testing signal transmission which can complement comparative studies. Here, we summarise these experimental playback tests of the AAH in birds, mammals, insects, and anurans, we describe the methodologies used in these tests, and we assess the evidence for habitat-specific signal degradation and species-specific acoustic fidelity (i.e. whether signals propagate best in native versus foreign habitats). Experimental evidence, like comparative evidence, varies across habitats and taxa. Although transmission properties consistently differed by habitat, with closed habitats degrading signals more than open habitats, animal signals were not always adapted to propagate best within their native habitats. Researchers felt they had convincing evidence for species-specific acoustic fidelity in less than half of the 67 reviewed studies, with the most support found for birds and the least for anurans. We discuss potential explanations for differences within and between habitats and taxa and conclude with suggestions for standardised methodology and areas of future research.

How ambient noise may shape peripheral auditory sensitivity: a theoretical model on the trade-off between signal detection and recognition

Ambient noise affects hearing in natural environments and may therefore affect the evolution of animal acoustic signals and auditory sensitivity. An earlier fitness model examining variable ambient noise conditions predicted higher sensitivity as the best strategy for species living in quiet habitats as opposed to lower sensitivity for the ones in noisy habitats. The trade-off between detection and recognition of acoustic signals appeared to be a key factor determining hearing sensitivity for acoustic communication in the presence of noise. The original model focused on the best auditory range of two U-shaped audiograms differing in sensitivity only (i.e., low and high). Here the model is extended by employing additional sensitivity levels and investigating the full range of hearing in order to examine (a) conditions favoring auditory sensitivity in a model characterized by the presence of multiple sensitivity and sound levels, and (b) the importance of other audiogram features, such as bandwidth and shape, which are assessed by analyzing the fitness consequences associated with bandwidth variation in the auditory system displaying best sensitivity. The model also allows theoretical insights into the importance of the detection-recognition trade-off for the evolution of the auditory critical bandwidths and ratios. The model predicted that a successful receiver should evolve an auditory system of either low or high sensitivity, but not intermediate. When low sensitivity pays, the audiogram shape should follow the profile of maximum noise levels encountered in the receiver’s environment. When high sensitivity pays, the audiogram should maximize bandwidth. A high sensitivity system with larger critical bandwidths may be successful only if the ensuing cost due to lower detection performance is outweighed by the benefit of improved sound recognition.

Vocal responses of austral forest frogs to amplitude and degradation patterns of advertisement calls

Degradation phenomena affecting animal acoustic signals may provide cues to assess the distance of emitters. Recognition of degraded signals has been extensively demonstrated in birds, and recently studies have also reported detection of degraded patterns in anurans that call at or above ground level. In the current study we explore the vocal responses of the syntopic burrowing male frogs Eupsophus emiliopugini and E. calcaratus from the South American temperate forest to synthetic conspecific calls differing in amplitude and emulating degraded and non-degraded signal patterns. The results show a strong dependence of vocal responses on signal amplitude, and a general lack of differential responses to signals with different pulse amplitude modulation depths in E. emiliopugini and no effect of relative amplitude of harmonics in E. calcaratus. Such limited discrimination of signal degradation patterns from non-degraded signals is likely related to the burrowing habits of these species. Shelters amplify outgoing and incoming conspecific vocalizations, but do not counteract signal degradation to an extent comparable to calling strategies used by other frogs. The limited detection abilities and resultant response permissiveness to degraded calls in these syntopic burrowing species would be advantageous for animals communicating in circumstances in which signal alteration prevails.

WarbleR: an r package to streamline analysis of animal acoustic signals

Summary Animal acoustic communication is one of the most fruitful research areas in behavioural and evolutionary biology. Work in this area depends largely on quantifying the structure of acoustic signals, which has often depended upon closed‐source or graphical user interface (GUI)‐based software. Here, we describe the r package warbleR, a new package for the analysis of animal acoustic signal structure. The package offers functions for downloading avian vocalizations from the open‐access online repository Xeno‐Canto, displaying the geographic extent of the recordings, manipulating sound files, detecting acoustic signals or importing detected signals from other software, assessing performance of methods that measure acoustic similarity, conducting cross‐correlations, measuring acoustic parameters and analysing interactive vocal signals, among others. Functions working iteratively allow parallelization to improve computational efficiency. We present a case study showing how warbleR functions can be used in a workflow to evaluate the structure of acoustic signals. We analyse geographic variation in long‐billed hermit hummingbirds (Phaethornis longirostris) songs obtained from Xeno‐Canto. The code in warbleR can be executed by less experienced r users, but has also been thoroughly commented, which will facilitate further customization by advanced users. The combination of the tools described here with other acoustic analysis packages in r should significantly expand the range of analytical approaches available.

The Lombard effect in male ultrasonic frogs: Regulating antiphonal signal frequency and amplitude in noise.

Acoustic communication in noisy environments presents a significant challenge for vocal animals because noise can interfere with animal acoustic signals by decreasing signal-to-noise ratios and masking signals. Birds and mammals increase call intensity or frequency as noise levels increase, but it is unclear to what extend this behavior is shared by frogs. Concave-eared torrent frogs (Odorrana tormota) have evolved the capacity to produce various calls containing ultrasonic harmonics and to communicate beside noisy streams. However, it is largely unclear how frogs regulate vocalization in response to increasing noise levels. We exposed male frogs to various levels of noise with playback of conspecific female courtship calls and recorded antiphonal signals and spontaneous short calls. Males were capable of rapidly adjusting fundamental frequency and amplitude of antiphonal signals as noise levels increased. The increment in fundamental frequency and amplitude was approximately 0.5 kHz and 3 dB with every 10 dB increase in noise level, indicating the presence of noise-dependent signal characteristics. Males showed the noise-tolerant adaption in response to female calls in noise level from 40 to 90 dB SPL. The results suggest that the noise-dependent signal characteristics in O. tormota have evolved as a strategy to cope with varying torrent noise.

Propagation of natural toad calls in a Mediterranean terrestrial environment

Propagation patterns of animal acoustic signals provide insights into the evolution of signal design to convey signaler's information to potential recipients. However, propagation properties of vertebrate calls have been rarely studied using natural calls from individuals; instead playback calls broadcast through loudspeakers have been used extensively, a procedure that may involve acoustical and physical features differing from natural sounds. Measurements of the transmission characteristics of natural advertisement calls, which are simple tonal sounds, of the Iberian midwife toad, Alytes cisternasii, were carried out, and the results were compared with previously published results broadcasting recorded calls of the same species. Measurements of sound pressure level (SPL) of calls from individual male A. cisternasii revealed that the call amplitude decreases at distances of 1-8 m from the source at rates averaging 1-5 dB above spherical transmission loss in an omni-directional pattern. A comparison between SPLs of natural calls in the current study and of playback calls from a previous study showed that patterns of propagation did not differ in average values, but variance was significantly higher for natural calls. Results suggest that using broadcast signals for transmission experiments may result in a simplification of the conditions in which actual animals communicate in nature.

Quantifying noise in animal acoustic signals.

Variability in noisy calls or call segments is often difficult to quantify but critical in order to understand animal acoustic communication. Such components represent a large proportion of animal acoustic signals and they are behaviorally meaningful. Such elements have also collectively been labeled as nonlinear phenomena. Three approaches are reviewed. The harmonics‐to‐noise ratio (HNR) can be related to perceptual characteristics of an acoustic signal as well as to the mechanism of its production. The HNR seems to be a useful numeric parameter to quantify vocal variability. Two problems with the procedure will be outlined. The nonlinear measure (NLM) is designed to provide an overall estimate of the strength of nonlinearity in a signal. The rationale is that if signal noise reflects low‐dimensional chaos as produced through nonlinear processes, deterministic‐nonlinear modeling will produce a relatively small error component. A stochastic linear model, on the other hand, will produce a relatively large ...

A Brief Review of Anthropogenic Sound in the Oceans

Sound in the oceans is generated by a variety of natural sources, such as breaking waves, rain, and marine animals, as well as a variety of human-produced sources, such as ships, sonars and seismic signals. This overview will begin with a quick review of some basic properties of sound waves with particular reference to differences between the behaviours of these waves underwater versus in air. A basic understanding of the physics of underwater sound is critical to understanding how marine animal acoustic signals have evolved relative to their different functions and how changes in the marine acoustic environment due to increasing anthropogenic sound in the oceans may impact these species. We will then review common sources of anthropogenic sound in the oceans. The frequency contributions of three major sources of underwater anthropogenic sound and their relative intensities will be discussed: naval exercises, seismic surveys and commercial shipping. Finally, a case study examining relative inputs to a regional noise budget, that of the Gerry E. Studds Stellwagen Bank National Marine Sanctuary, will be presented to introduce the audience to methodologies for characterizing and managing sound on an ecosystem level.

Bimodal signal requisite for agonistic behavior in a dart-poison frog, Epipedobates femoralis.

Animal acoustic signals play seminal roles in mate attraction and regulation of male spacing, maintenance of pairbonds, localization of hosts by parasites, and feeding behavior. Among vertebrate signals, it is becoming clear that no single stereotyped signal feature reliably elicits species-specific behavior, but rather, that a suite of characters is involved. Within the largely nocturnal clade of anuran amphibians, the dart-poison frog, Epipedobates femoralis, is a diurnal species that physically and vigorously defends its calling territory against conspecific intruders. Here we report that physical attacks by a territorial male are provoked only in response to dynamic bimodal stimuli in which the acoustic playback of vocalizations is coupled with vocal sac pulsations, but not by either unimodal cues presented in isolation or static bimodal stimuli. These results suggest that integration of dynamic bimodal cues is necessary to elicit aggression in this species.

Chicken food calls are functionally referential

Male chickens, Gallus gallus domesticus, usually produce characteristic ‘food’ calls upon discovering edible objects, and are more likely to do so in the presence of a hen. Food calling is thus dependent upon food and modulated by social context, which is consistent with the idea that hens respond because they anticipate a feeding opportunity. An alternative model suggests that female behaviour is not mediated by the predicted presence of food but rather by social information, such as a low probability of male aggression. We conducted two playback experiments to explore the type of information encoded in food-associated vocal signals. Isolated hens were played recorded food calls and we compared their responses with those evoked by ground alarm calls (which have similar acoustic characteristics) and by contact calls (which are produced under similar social circumstances). Hens responded to food call playbacks by fixating downwards with the frontal binocular field. This anticipatory feeding movement was specific to food calls and did not occur in either of the control conditions. Food calls also affected looking downwards selectively. There were no differences between the call types in their effects on social behaviour, such as approach and contact calling, nor were there differences in the nonspecific effects of sound playback, such as orienting towards the loudspeaker or increased locomotor activity. Chicken food calls appear to provide conspecifics with information about the presence of food. This property has not hitherto been demonstrated in any natural system of animal acoustic signals.

AN ELECTRONIC TECHNIQUE FOR MONITORING THE TEMPORAL ASPECTS OF ACOUSTIC SIGNALS OF CAPTIVE ORGANISMS

ABSTRACT We introduce an inexpensive electronic technique for monitoring the temporal aspects of any captive animal's acoustic signals. The electronic apparatus, attached to a data acquisition unit and personal computer, compares microphone output to a pre-set level and stores calling/non-calling data to disk. Total time calling and temporal signaling patterns of up to 256 individuals can be monitored for indefinite lengths of time. Sampling rate is adjustable, with a maximum rate of 6 samples/microphone/second. The capabilities of the system are illustrated with the field cricket Gryllus integer. Temporal aspects of acoustic signaling are discussed in terms of monitoring time scale and recognition of individual variation, energetics research, and hypothesis testing of the costs and benefits associated with mating success and predation.