Directional speakers as a tool for animal vocal communication studies.

Audio playbacks are a common experimental tool in vocal communication research. However, low directionality of sound makes it hard to control the audience exposed to the stimuli. Parametric speakers offer a solution for transmitting directional audible signals by using ultrasonic carrier waves. The targeted transmission of vocal signals offers exciting opportunities for testing the diffusion of information in animal groups and mechanisms for resolving informational ambiguities. We have field tested the quality and directionality of a commercial parametric speaker, Soundlazer SL-01. Additionally, we assessed its usability for performing playback experiments by comparing behavioural responses of free-ranging meerkats (Suricata suricatta) with calls transmitted from conventional and parametric speakers. Our results show that the tested parametric speaker is highly directional. However, the acoustic structure of meerkat calls was strongly affected and low frequencies were not reliably reproduced by the parametric speaker. The playback trials elicited weakened behavioural responses probably due to the partial distortion of the signal but also indicating the potential importance of social facilitation for initiating mobbing events in meerkats. We conclude that parametric speakers can be useful tools for directed transmission of animals calls but after a careful assessment of signal fidelity.

Male rock hyraxes that maintain an isochronous song rhythm achieve higher reproductive success.

Rhythmic stability (nonrandom temporal structure) is required for many neural and physiological functions, whereas rhythmic irregularities can indicate genetic or developmental deficiencies. Therefore, rhythmic courtship or contest signals are widespread in nature as honest advertisement displays. Examination of bird songs revealed the pervasiveness of categorical rhythmic patterns that can be described as small integer ratios between sequential inter-call intervals. As similar rhythmic profiles are prevalent in human music, it was suggested that a shared functionality could drive both animal songs and human musical rhythms, facilitating synchrony between signallers and enabling easy identification of performance errors. Here we examined whether the rhythmic structure and the rhythmic stability of vocal displays are related to reproductive success in male rock hyraxes (Procavia capensis), which presents an unusual case of a terrestrial singing mammal. We combined long-term parentage analysis of 13 male hyraxes (22 male/years) with an analysis of an audio library of 105 hyrax songs. Male annual reproductive success was determined by the number of offspring that survived to the age of 1 year. The frequency of singing events was used to determine the seasonal singing effort for each male. Songs were analysed for rhythmic structure, focusing on the presence of categorical rhythms and the contribution of rhythmic stability to annual reproductive success. We found that male hyraxes that sing more frequently tend to have more surviving offspring and that the rhythmic profile of hyrax songs is predominantly isochronous with sequential vocal element pairs nearly equally spaced. The ratio of isochronous vocal element transitions (on-integer) to element transitions that deviate from an isochronous pattern (off-integer) in hyrax songs is positively correlated with male reproductive success. Our findings support the notion that isochronous rhythmic stability can serve as an indication of quality in sexually selected signals and is not necessarily driven by the need for multiple caller synchronization. The relative scarcity of nonisochronous rhythmic categories in individually performed hyrax songs raises the question of whether such rhythmic categories could be a product of collective, coordinated signalling, while being selected against in individual performance.

Disentangling influence over group speed and direction reveals multiple patterns of influence in moving meerkat groups.

Animals that travel together in groups must constantly come to consensus about both the direction and speed of movement, often simultaneously. Contributions to collective decisions may vary among group members, yet inferring who has influence over group decisions is challenging, largely due to the multifaceted nature of influence. Here we collected high-resolution GPS data from five habituated meerkat groups in their natural habitat during foraging and developed a method to quantify individual influence over both group direction and speed. We find that individual influence over direction and speed are correlated, but also exhibit substantial variation. Comparing patterns across social statuses reveals that dominant females have higher influence than other individuals over both group direction and speed. Individuals with high influence also tend to spend more time in the front of the group. We discuss our results in light of meerkat life-history and current literature on influence during group movement. Our method provides a general approach which can be applied to disentangle individual influence over group direction and speed in a wide range of species with cohesive movement, emphasizing the importance of integrating multiple lines of inquiry when inferring influence in moving animal groups.

A practical guide for generating unsupervised, spectrogram-based latent space representations of animal vocalizations.

BACKGROUND: The manual detection, analysis and classification of animal vocalizations in acoustic recordings is laborious and requires expert knowledge. Hence, there is a need for objective, generalizable methods that detect underlying patterns in these data, categorize sounds into distinct groups and quantify similarities between them. Among all computational methods that have been proposed to accomplish this, neighbourhood-based dimensionality reduction of spectrograms to produce a latent space representation of calls stands out for its conceptual simplicity and effectiveness. Goal of the study/what was done: Using a dataset of manually annotated meerkat Suricata suricatta vocalizations, we demonstrate how this method can be used to obtain meaningful latent space representations that reflect the established taxonomy of call types. We analyse strengths and weaknesses of the proposed approach, give recommendations for its usage and show application examples, such as the classification of ambiguous calls and the detection of mislabelled calls. What this means: All analyses are accompanied by example code to help researchers realize the potential of this method for the study of animal vocalizations.

Vocalization-associated respiration patterns: thermography-based monitoring and detection of preparation for calling.

Vocal emission requires coordination with the respiratory system. Monitoring the increase in laryngeal pressure, which is needed for vocal production, allows detection of transitions from quiet respiration to vocalization-supporting respiration. Characterization of these transitions could be used to identify preparation for vocal emission and to examine the probability of it manifesting into an actual vocal production event. Specifically, overlaying the subject's respiration with conspecific calls can highlight events of call initiation and suppression, as a means of signalling coordination and avoiding jamming. Here, we present a thermal imaging-based methodology for synchronized respiration and vocalization monitoring of free-ranging meerkats. The sensitivity of this methodology is sufficient for detecting transient changes in the subject's respiration associated with the exertion of vocal production. The differences in respiration are apparent not only during the vocal output, but also prior to it, marking the potential time frame of the respiratory preparation for calling. A correlation between conspecific calls with elongation of the focal subject's respiration cycles could be related to fluctuations in attention levels or in the motivation to reply. This framework can be used for examining the capability for enhanced respiration control in animals during modulated and complex vocal sequences, detecting 'failed' vocalization attempts and investigating the role of respiration cues in the regulation of vocal interactions.

The "Law of Brevity" in animal communication: Sex-specific signaling optimization is determined by call amplitude rather than duration.

The efficiency of informational transfer is one of the key aspects of any communication system. The informational coding economy of human languages is often demonstrated by their almost universal fit to Zipf's "Law of Brevity," expressing negative relationship between word length and its usage frequency. Animal vocal systems, however, provided mixed results in their adherence to this relationship, potentially due to conflicting evolutionary pressures related to differences in signaling range and communicational needs. To examine this potential parallel between human and animal vocal communication, and also to explore how divergent, sex-specific, communicational settings affect signaling efficiency within a species, we examined the complete vocal repertoire of rock hyraxes (Procavia capensis). As male and female hyraxes differ in their sociality levels and male hyraxes vocal repertoire is dominated by sexual advertisement songs, we hypothesized that sex-specific vocal repertoires could be subjected to different signaling optimization pressures. Our results show that the sexes differ in repertoire size, call usage, and adherence to coding efficiency principles. Interestingly, the classic call length/call usage relationship is not consistently found in rock hyraxes. Rather, a negative relationship between call amplitude and call usage is found, suggesting that the efficiency of the vocal repertoire is driven by call amplitude rather than duration. We hypothesize that, in contrast to human speech that is mainly intended for short distance, the need for frequent long-range signaling shapes an animal's vocal repertoire efficiency according to the cost of call amplitude rather than call length. However, call duration may be a secondary factor affecting signaling efficiency, in cases where amplitude is under specific selection pressures, such as sexual selection.

Social context mediates testosterone's effect on snort acoustics in male hyrax songs.

Testosterone affects physical and motivational states, both of which may strongly influence vocalization structure and acoustics. The loud complex calls (i.e., songs) of male rock hyraxes (Procavia capensis) are used as honest signals for advertising physical and social states. The snort, a low frequency, noisy element of the song, encodes information on the singer's age and social rank via harshness, as measured by jitter (i.e., acoustic frequency stability) and duration; suggesting that the snort concomitantly advertises both vocal stability and aggression. Our past findings revealed that testosterone levels are related to both vocal elements and social status of male hyraxes, suggesting that hormonal mechanisms mediate the motivation for aggressive and courtship behaviors. Here we examined whether long-term androgen levels are related to snort acoustics and song structure by comparing levels of testosterone in hair with acoustic and structural parameters. We found that songs performed by individuals with higher testosterone levels include more singing bouts and longer, smoother snorts, but only in those songs induced by external triggers. It is possible that hyraxes with higher levels of testosterone possess the ability to perform higher-quality singing, but only invest in situations of high social arousal and potential benefit. Surprisingly, in spontaneous songs, hyraxes with high testosterone were found to snort more harshly than low-testosterone males. The context dependent effects of high testosterone on snort acoustics suggest that the aggressive emotional arousal associated with testosterone is naturally reflected in the jittery hyrax snort, but that it can be masked by high-quality performance.

Vocal Turn-Taking in Meerkat Group Calling Sessions.

Coordination is a fundamental aspect of social living, underlying processes ranging from the maintenance of group cohesion to the avoidance of competition. Coordination can manifest as synchronization, where individuals perform the same action at the same time but can also take the form of anti-synchronization or turn-taking. Turn-taking has mainly been studied in the context of the development of language [1] due to the fact that it is a universal feature in all languages and has been found to appear early in infancy [2, 3]. Recently, turn-taking has received attention in animal communication research [4-7] as a potential foundation on which social communication was formed [1, 3]. In this study, we describe turn-taking in group-wide vocal interactions of meerkats (Suricata suricatta) during low-conflict sunning behavior, which is accompanied by the production of specific "sunning calls." We show that sunning-call production is socially stimulated and that at the group level, meerkats avoid overlap, thus fulfilling a key principle of turn-taking [8]. Through observational data and playback experiments, we show that these group-level patterns arise from two individual-level rules: call inhibition over short timescales, which prevents mutual interference, and call excitation over longer timescales, which stimulates further group calling. These simple rules suggest that hierarchy formation and turn allocation are not required for achieving group-wide coordination of communication. We also suggest that the potential bonding function of turn-taking shown in humans might have similar effects in animal interactions. VIDEO ABSTRACT.

The progression pattern of male hyrax songs and the role of climactic ending.

The study of animal vocal signals can either focus on the properties of distinct vocal elements or address the signal as a whole. Although some attention has been given to the continuous progression patterns of bird songs, such patterns in mammalian vocalisations have been largely overlooked. We examined temporal changes in structural and acoustic parameters in male rock hyrax songs. We found a gradual increase in call frequency and amplitude towards the song ending, as well as an abrupt increase in bout syntactic complexity, peaking in the last quintile of a song. In musical terms, such a pattern can be described as a crescendo (amplitude increase) with a terminal climax. In Western music, crescendos are used to maintain attention and direct the listeners towards a memorable highpoint of the musical piece. This structure may have an analogous function in animal communication, recruiting audience attention towards the climactic and potentially most informative part of the signal. Our playback experiments revealed that hyrax males tend to reply more to songs with a climactic ending, indicating that this progression pattern is important for hyrax communication. We suggest that animal vocal communication research can benefit from adding musical concepts to the analysis toolbox.