Mapping vocal interactions in space and time differentiates signal broadcast versus signal exchange in meerkat groups

Animal vocal communication research traditionally focuses on acoustic and contextual features of calls, yet substantial information is also contained in response selectivity and timing during vocalization events. By examining the spatiotemporal structure of vocal interactions, we can distinguish between 'broadcast' and 'exchange' signalling modes, with the former potentially serving to transmit signallers' general state and the latter reflecting more interactive signalling behaviour. Here, we tracked the movements and vocalizations of wild meerkat (Suricata suricatta) groups simultaneously using collars to explore this distinction. We found evidence that close calls (used for maintaining group cohesion) are given as signal exchanges. They are typically given in temporally structured call-response sequences and are also strongly affected by the social environment, with individuals calling more when they have more neighbours and juveniles responding more to adults than the reverse. In contrast, short note calls appear mainly in sequences produced by single individuals and show little dependence on social surroundings, suggesting a broadcast signalling mode. Despite these differences, both call categories show similar clustering in space and time at a group level. Our results highlight how the fine-scale structure of vocal interactions can give important insights into the usage and function of signals in social groups. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamics.'

A multifaceted framework to establish the presence of meaning in non-human communication

Does non-human communication, like language, involve meaning? This question guides our focus through an interdisciplinary review of the theories and terminology used to study meaning across disciplines and species. Until now, it has been difficult to apply the concept of meaning to communication in non-humans. This is partly because of the varied approaches to the study of meaning. Additionally, while there is a scholarly acknowledgement of potential meaning in non-human cognition, there is also scepticism when the topic of communication arises. We organise some of the key literature into a coherent framework that can bridge disciplines and species, to ensure that aspects of meaning are accurately and fairly compared. We clarify the growing view in the literature that, rather than requiring multiple definitions or being split into different types, meaning is a multifaceted yet still unified concept. In so doing, we propose that meaning is an umbrella term. Meaning cannot be summed up with a short definition or list of features, but involves multiple complexities that are outlined in our framework. Specifically, three global facets are needed to describe meaning: a Signal Meaning Facet, an Interactant Meaning Facet, and a Resultant Meaning Facet. Most importantly, we show that such analyses are possible to apply as much to non-humans as to humans. We also emphasise that meaning nuances differ among non-human species, making a dichotomous approach to meaning questionable. Instead, we show that a multifaceted approach to meaning establishes how meaning appears within highly diverse examples of non-human communication, in ways consistent with the phenomenon's presence in human non-verbal communication and language(s). Therefore, without further recourse to 'functional' approaches that circumvent the critical question of whether any non-human meaning exists, we show that the concept of meaning is suitable for evolutionary biologists, behavioural ecologists, and others to study, to establish exactly which species exhibit meaning in their communication and in what ways.

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.

Habitat preferences, estimated abundance and behavior of tree hyrax (Dendrohyrax sp.) in fragmented montane forests of Taita Hills, Kenya

We studied a previously almost unknown nocturnal mammal, an apparently undescribed species of tree hyrax (Dendrohyrax sp.) in the moist montane forests of Taita Hills, Kenya. We used thermal imaging to locate tree hyraxes, observe their behavior, and to identify woody plants most frequently visited by the selective browsers. We also documented acoustic behavior in forest fragments of different sizes. Data on calling type and frequency were analyzed together with lidar data to estimate population densities and to identify forest stand characteristics associated with large populations. Viable populations were found only in the largest forest fragments (> 90 ha), where tree hyraxes preferred most pristine forest stands with high, multilayered canopies. The estimated population sizes in smaller forest fragments were very limited, and hyraxes were heard to call only during late night and early morning hours, presumably in order to avoid detection. While we frequently recorded tree hyrax songs in the largest forest fragments, we almost never heard songs in the small ones. All remaining subpopulations of the Taita tree hyrax are under threat of human disturbance and further habitat deterioration. Conservation efforts should include protection of all remaining habitat patches, but also reforestation of former habitat is urgently needed.

Sex-associated and context-dependent leadership in the rock hyrax

In many mammalian species, both sexes may take leadership role, but different traits may play a role in determining variation within species. Here we examine the effect of sex on leadership. We present three complementary datasets derived from a well-studied population of wild rock hyrax (Procavia capensis). The findings demonstrated that male and female rock hyraxes take on different leadership positions, depending on the context. When risk is moderate, more likely to lead are younger resident males, which experience high cortisol and lower testosterone levels. However, during acute predation scenarios, more likely to lead are males with lower centrality status. We suggest that hyrax males exhibit risky behaviors that may reflect their need for self-advertisement. In contrast, leadership among group females is more equally distributed. Females have little to gain from risky actions due to the lack of competition among them, but nonetheless take leadership positions.

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Different traits play a role in hyrax male and female leaders, in different contexts

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On moderate risk, younger resident males with high cortisol and low testosterone lead

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During predation scenario, lower centrality status males lead

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Among group females, leadership is more equally distributed

Review of sensory modalities of sirenians and the other extant Paenungulata clade

Extant members of Paenungulata (sirenians, proboscideans, and hyracoideans) form a monophyletic clade which originated in Africa. While paenungulates are all herbivorous, they differ greatly in size, life history, and habitat. Therefore, we would expect both phylogenetically related similarities and ecologically driven differences in their use and specializations of sensory systems, especially in adaptations in sirenians related to their fully aquatic habitat. Here we review what is known about the sensory modalities of this clade in an attempt to better elucidate their sensory adaptations. Manatees have a higher frequency range for hearing than elephants, who have the best low-frequency hearing range known to mammals, while the hearing range of hyraxes is unknown. All paenungulates have vibrissae assisting in tactile abilities such as feeding and navigating the environment and share relatively small eyes and dichromatic vision. Taste buds are present in varying quantities in all three orders. While the olfactory abilities of manatees and hyraxes are unknown, elephants have an excellent sense of smell which is reflected by having the relatively largest cranial nerve related to olfaction among the three lineages. Manatees have the relatively largest trigeminal nerve-the nerve responsible for, among other things, mystacial vibrissae-while hyraxes have the relatively largest optic nerve (and therefore, presumably, the best vision) among the Paenungulata. All three orders have diverged significantly; however, they still retain some anatomical and physiological adaptations in common with regard to sensory abilities.

Vocalization Analyses of Nocturnal Arboreal Mammals of the Taita Hills, Kenya

Three poorly known nocturnal mammal species from the montane forests of the Taita Hills in Kenya, were studied via vocalization analysis. Here, their acoustic behaviour is described. The studied animals were the tree hyrax (Dendrohyrax sp.), the small-eared greater galago (Otolemur garnettii), and the dwarf galago (Paragalago sp.). High-quality loud calls were analysed using RAVEN PRO, and compared to calls of presumed closest relatives. Our findings include the first detailed descriptions of tree hyrax songs. Moreover, our results suggest that the tree hyrax of Taita Hills may be a taxon new to science, as it produces a characteristic call, the ‘strangled thwack’, not previously known from other Dendrohyrax populations. Our data confirms that the small-eared greater galago subspecies living in the Taita Hills is Otolemur garnettii lasiotis. The loud calls of the elusive Taita Hills dwarf galago closely resemble those of the Kenya coast dwarf galago (Paragalago cocos). Thus, the population in the Taita Hills probably belongs to this species. The Taita Hills dwarf galagos are geographically isolated from other dwarf galago populations, and live in montane cloud forest, which is an unusual habitat for P. cocos. Intriguingly, two dwarf galago subpopulations living in separate forest patches in the Taita Hills, Ngangao and Mbololo, have clearly different contact calls. The Paragalagos in Mbololo Forest may represent a population of P. cocos with a derived call repertoire, or, alternatively, they may actually be mountain dwarf galagos (P. orinus). Hence, differences in habitat, behaviour, and contact call structure suggest that there may be two different Paragalago species in the montane forests of the Taita Hills.

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.

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.