Acoustic and Temporal Variation in Gelada (Theropithecus gelada) Loud Calls Advertise Male Quality

Exploring nonlinear phenomena in animal vocalizations through oscillator theory

Animal vocalizations comprise a rich array of complex sounds that exhibit nonlinear phenomena (NLP), which have fascinated researchers for decades. From the melodic songs of birds to the clicks and whistles of dolphins, many species have been found to produce nonlinear vocalizations, offering a valuable perspective on the mechanisms underlying sound production and potential adaptive functions. By leveraging on the principles of oscillator theory and nonlinear dynamics, animal vocalizations, which are based on coupled oscillators, can be described and conveniently classified. We review the basic ingredients for self-sustained oscillations and how different NLP can emerge. We discuss important terms in the context of oscillator theory: attractor types, phase space, bifurcations and Arnold tongue diagrams. Through a comparative analysis of observed NLP and bifurcation diagrams, our study reviews how the tools of nonlinear dynamics can provide insights into the intricate complexity of animal vocalizations, as well as into the evolutionary pressures and adaptive strategies that have shaped the diverse communication systems of the animal kingdom.This article is part of the theme issue, 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.

Loud Call Production in Male Vervet Monkeys (Chlorocebus pygerythrus) Varies with Season and Signaller Rank

Adult males of many nonhuman primate species utter characteristic loud calls that are audible over long distances and serve various functions. In response to large terrestrial predators, male vervet monkeys (Chlorocebus pygerythrus) produce loud 'barks' that alert group members and are assumed to deter predators. It is less well known that male vervet monkeys also produce barks during aggressive interactions within and between groups. Here, we investigated whether barks potentially serve a dual function as alarm calls and quality signals. We used ad-libitum event sampling to record natural barking events from 45 adult males in six free-ranging groups over 24 months at Mawana Game Reserve, KwaZulu-Natal, South Africa. For each barking event in groups with multiple males, we aimed to record whether each adult male in the group barked or not. Using generalised linear mixed models, we investigated whether the number of barking events recorded per observation day and the probability that an individual male barked in each event were associated with male rank, the number of males in the group, the adult sex ratio, and the mating season. The highest-ranking males showed a very high probability of barking, and the number of daily barking events increased during the mating season. Males frequently barked in aggressive interactions, supporting the hypothesis that this signal plays a role in male-male competition. We conclude that, in addition to serving as alarm calls, barks may also advertise male competitive ability or motivation to compete.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10764-024-00475-x.

Nine Levels of Explanation : A Proposed Expansion of Tinbergen's Four-Level Framework for Understanding the Causes of Behavior

Tinbergen's classic "On Aims and Methods of Ethology" (Zeitschrift für Tierpsychologie, 20, 1963) proposed four levels of explanation of behavior, which he thought would soon apply to humans. This paper discusses the need for multilevel explanation; Huxley and Mayr's prior models, and others that followed; Tinbergen's differences with Lorenz on "the innate"; and Mayr's ultimate/proximate distinction. It synthesizes these approaches with nine levels of explanation in three categories: phylogeny, natural selection, and genomics (ultimate causes); maturation, sensitive period effects, and routine environmental effects (intermediate causes); and hormonal/metabolic processes, neural circuitry, and eliciting stimuli (proximate causes), as a respectful extension of Tinbergen's levels. The proposed classification supports and builds on Tinbergen's multilevel model and Mayr's ultimate/proximate continuum, adding intermediate causes in accord with Tinbergen's emphasis on ontogeny. It requires no modification of Standard Evolutionary Theory or The Modern Synthesis, but shows that much that critics claim was missing was in fact part of Neo-Darwinian theory (so named by J. Mark Baldwin in The American Naturalist in 1896) all along, notably reciprocal causation in ontogeny, niche construction, cultural evolution, and multilevel selection. Updates of classical examples in ethology are offered at each of the nine levels, including the neuroethological and genomic findings Tinbergen foresaw. Finally, human examples are supplied at each level, fulfilling his hope of human applications as part of the biology of behavior. This broad ethological framework empowers us to explain human behavior-eventually completely-and vindicates the idea of human nature, and of humans as a part of nature.

Vocal communication in wild chimpanzees: a call rate study

BACKGROUND

Patterns of vocal communication have implications for species conservation: a change in calling behaviour can, for instance, reflect a disturbed habitat. More importantly, call rate is a parameter that allows conservation planners to convert call density into animal density, when detecting calls with a passive acoustic monitoring system (PAM).

METHODS

We investigated chimpanzee (Pan troglodytes schweinfurthii) call rate during the late dry season in the Issa Valley, western Tanzania by conducting focal follows. We examined the socio-ecological factors that influence call production rate of savanna woodland chimpanzees.

RESULTS

We found that sex, proportion of time spent in a vegetation type, proportion of time spent travelling, time of the day, party size and swollen parous female presence had a significant effect on the call rate. Call rate differed among the different demographic classes with subadult and adult males vocalising twice as often as the subadult and adult females and three times as often as the juveniles.

APPLICATIONS

The use of PAM and recent statistical developments to estimate animal density is promising but relies on our knowing individual call rate, often not available for many species. With the improvement in automatic call detection, we anticipate that PAM will increasingly be broadly applied to primates but also across taxa, for conservation.

Tibetan Macaques with Higher Social Centrality and More Relatives Emit More Frequent Visual Communication in Collective Decision-Making

Simple Summary

It is well known that visual communication plays an important role in collective decision-making. However, there is not much research on the influencing factors of visual signals, especially kinship and social relations. In this study, we not only confirmed the function of visual communication in collective decision-making, but also found the effect of kinship and social relations on visual communication. Tibetan macaques with higher social centrality and more relatives emit more frequent visual communication, providing a reference for further research on decision-making. Understanding the link between communication and decision-making can elucidate the powers of group maintenance in animal societies.

Abstract

Animals on the move often communicate with each other through some specific postures. Previous studies have shown that social interaction plays a role in communication process. However, it is not clear whether the affinity of group members can affect visual communication. We studied a group of free-ranging Tibetan macaques (Macaca thibetana) at Huangshan Mountain, China, and answered whether and how social centrality or relatives matter in visual signals during group movement using Tobit regression modeling. All individuals emitted the signals of back-glances and pauses in collective movement. The emission of two signals decreased with the number of participants increased. The back-glance and pause signals emitted by the participating individuals were stronger as the position moved backward in the group. Sex, age, and rank had no significant influence on back-glance and pause signals. Individuals with higher social centrality would emit more pause signals, but social centrality had no effect on the back-glance signal. Individuals with more relatives in the group had more back-glance signals, but this had no effect on the pause signal. This study verifies that social centrality and the number of relatives have effects on visual signals in Tibetan macaques. We provide insights into the relationship between communication behaviors and group cooperation in social animals.

The ecological significance of time sense in animals

Time is a fundamental dimension of all biological events and it is often assumed that animals have the capacity to track the duration of experienced events (known as interval timing). Animals can potentially use temporal information as a cue during foraging, communication, predator avoidance, or navigation. Interval timing has been traditionally investigated in controlled laboratory conditions but its ecological relevance in natural environments remains unclear. While animals may time events in artificial and highly controlled conditions, they may not necessarily use temporal information in natural environments where they have access to other cues that may have more relevance than temporal information. Herein we critically evaluate the ecological contexts where interval timing has been suggested to provide adaptive value for animals. We further discuss attributes of interval timing that are rarely considered in controlled laboratory studies. Finally, we encourage consideration of ecological relevance when designing future interval-timing studies and propose future directions for such experiments.

Brevity is not a universal in animal communication: evidence for compression depends on the unit of analysis in small ape vocalizations

Evidence for compression, or minimization of code length, has been found across biological systems from genomes to human language and music. Two linguistic laws-Menzerath's Law (which states that longer sequences consist of shorter constituents) and Zipf's Law of abbreviation (a negative relationship between signal length and frequency of use)-are predictions of compression. It has been proposed that compression is a universal in animal communication, but there have been mixed results, particularly in reference to Zipf's Law of abbreviation. Like songbirds, male gibbons (Hylobates muelleri) engage in long solo bouts with unique combinations of notes which combine into phrases. We found strong support for Menzerath's Law as the longer a phrase, the shorter the notes. To identify phrase types, we used state-of-the-art affinity propagation clustering, and were able to predict phrase types using support vector machines with a mean accuracy of 74%. Based on unsupervised phrase type classification, we did not find support for Zipf's Law of abbreviation. Our results indicate that adherence to linguistic laws in male gibbon solos depends on the unit of analysis. We conclude that principles of compression are applicable outside of human language, but may act differently across levels of organization in biological systems.

Which way to the dawn of speech?: Reanalyzing half a century of debates and data in light of speech science

Recent articles on primate articulatory abilities are revolutionary regarding speech emergence, a crucial aspect of language evolution, by revealing a human-like system of proto-vowels in nonhuman primates and implicitly throughout our hominid ancestry. This article presents both a schematic history and the state of the art in primate vocalization research and its importance for speech emergence. Recent speech research advances allow more incisive comparison of phylogeny and ontogeny and also an illuminating reinterpretation of vintage primate vocalization data. This review produces three major findings. First, even among primates, laryngeal descent is not uniquely human. Second, laryngeal descent is not required to produce contrasting formant patterns in vocalizations. Third, living nonhuman primates produce vocalizations with contrasting formant patterns. Thus, evidence now overwhelmingly refutes the long-standing laryngeal descent theory, which pushes back "the dawn of speech" beyond ~200 ka ago to over ~20 Ma ago, a difference of two orders of magnitude.

Re-emergence of the leaf clip gesture during an alpha takeover affects variation in male chimpanzee loud calls

Loud calls are used by many species as long-distance signals for group defense, mate attraction, and inter- and intragroup spacing. Chimpanzee loud calls, or pant hoots, are used in a variety of contexts including group coordination and during male contests. Here, we observed an alpha male takeover in wild chimpanzees (Pan troglodytes verus) during which the leaf clipping gesture re-emerged after disappearing for almost two years in this community. Leaf clipping only occurred in males and was observed almost exclusively prior to pant hoot vocalizations, as has been observed in other chimpanzee communities of the Taï forest in Côte d'Ivoire. Consequently, we hypothesized that leaf clipping may be important for male-male competition by affecting variation in the acoustic properties of male chimpanzee loud calls. We therefore investigated whether pant hoots preceded by leaf clipping differed acoustically from those without, while also testing the influence of social context on pant hoot variation, namely male dominance rank and hierarchy instability, i.e., before, during and after the alpha takeover. We found that pant hoots preceded by leaf clipping were longer, contained more call elements and drum beats, and lower fundamental and peak frequencies. Moreover, during the alpha takeover pant hoots were shorter, contained fewer drum beats and higher fundamental frequencies. Additionally, pant hoot and aggression rates were also highest during the alpha takeover with leaf clipping more likely to occur on days when pant hooting rates were high. Overall social rank had limited effects on pant hoot variation. We suggest that elevated arousal and aggression during the alpha takeover triggered the re-emergence of leaf clipping and the associated acoustic changes in pant hoots. Further research should focus on the potential mechanisms by which leaf clipping is connected to variation in pant hoots and cross-population comparisons of the behaviour.

Evidence for mutual assessment in a wild primate

In aggressive interactions, game theory predicts that animals should assess an opponent's condition relative to their own prior to escalation or retreat. Despite the benefits of such mutual assessment, few studies have been able to reject simpler assessment strategies. Here we report evidence for mutual assessment in a wild primate. Gelada (Theropithecus gelada) males have conspicuous loud calls that may function as a signal of male quality. "Leader" males with harems putatively use loud calls to deter challenges from non-reproductive "bachelor" males. By contrast, leader males pose no threat to each other and congregate in large groups for a dilution effect against bachelors. In playback experiments and natural observations, gelada males responded to loud calls according to both their own and their opponent's attributes. Although primates routinely classify others relative to themselves using individual attributes, this represents some of the first direct evidence for mutual assessment in primate signaling contests.

Divergent acoustic properties of gelada and baboon vocalizations and their implications for the evolution of human speech

Human speech has many complex spectral and temporal features traditionally thought to be absent in the vocalizations of other primates. Recent explorations of the vocal capabilities of non-human primates are challenging this view. Here, we continue this trend by exploring the spectro-temporal properties of gelada (Theropithecus gelada) vocalizations. First, we made cross-species comparisons of geladas, chacma baboons, and human vowel space area. We found that adult male and female gelada exhaled grunts-a call type shared with baboons-have formant profiles that overlap more with human vowel space than do baboon grunts. These gelada grunts also contained more modulation of fundamental and formant frequencies than did baboon grunts. Second, we compared formant profiles and modulation of exhaled grunts to the derived call types (those not shared with baboons) produced by gelada males. These derived calls contained divergent formant profiles, and a subset of them, notably wobbles and vocalized yawns, were more modulated than grunts. Third, we investigated the rhythmic patterns of wobbles, a call type shown previously to contain cycles that match the 3-8 Hz tempo of speech. We use a larger dataset to show that the wobble rhythm overlaps more with speech rhythm than previously thought. We also found that variation in cycle duration depends on the production modality; specifically, exhaled wobbles were produced at a slower tempo than inhaled wobbles. Moreover, the variability in cycle duration within wobbles aligns with a linguistic property known as 'Menzerath's law' in that there was a negative association between cycle duration and wobble size (i.e. the number of cycles). Taken together, our results add to growing evidence that non-human primates are anatomically capable of producing modulated sounds. Our results also support and expand on current hypotheses of speech evolution, including the 'neural hypothesis' and the 'bimodal speech rhythm hypothesis'.