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Identifying universal principles underpinning diverse natural systems is a key goal of the life sciences. A powerful approach in addressing this goal has been to test whether patterns consistent with linguistic laws are found in nonhuman animals. Menzerath’s law is a linguistic law that states that, the larger the construct, the smaller the size of its constituents. Here, to our knowledge, we present the first evidence that Menzerath’s law holds in the vocal communication of a nonhuman species. We show that, in vocal sequences of wild male geladas (Theropithecus gelada), construct size (sequence size in number of calls) is negatively correlated with constituent size (duration of calls). Call duration does not vary significantly with position in the sequence, but call sequence composition does change with sequence size and most call types are abbreviated in larger sequences. We also find that intercall intervals follow the same relationship with sequence size as do calls. Finally, we provide formal mathematical support for the idea that Menzerath’s law reflects compression—the principle of minimizing the expected length of a code. Our findings suggest that a common principle underpins human and gelada vocal communication, highlighting the value of exploring the applicability of linguistic laws in vocal systems outside the realm of language.

Recognizing vocal behaviours intended to benefit others is a crucial yet understudied social skill. Primates with rich vocal repertoires and complex societies are excellent models to track the evolution of such capacity. Here, we exposed wild geladas ( Theropithecus gelada ) to vocal exchanges between unfamiliar female victim screams and male affiliative calls. The stimuli were arranged in sequences either simulating vocal affiliation towards victims (scream-affiliative call) or violating such order (affiliative call-scream), with varying emotional arousal conveyed by the affiliative call type. Measuring gazing activity towards the loudspeaker and the interruptions of feeding, we show that monkeys were sensitive to the sequential order in vocal exchanges as well as to the emotional arousal conveyed by affiliative calls. Our field study suggests a prosocial use of vocalizations in wild monkeys and reveals that foundational cognitive elements for processing vocal exchanges as meaningful third-party interactions may have existed in our common ancestors with monkeys.

Yawn contagion in humans has been proposed to be related to our capacity for empathy. It is presently unclear whether this capacity is uniquely human or shared with other primates, especially monkeys. Here, we show that in gelada baboons (Theropithecus gelada) yawning is contagious between individuals, especially those that are socially close, i.e., the contagiousness of yawning correlated with the level of grooming contact between individuals. This correlation persisted after controlling for the effect of spatial association. Thus, emotional proximity rather than spatial proximity best predicts yawn contagion. Adult females showed precise matching of different yawning types, which suggests a mirroring mechanism that activates shared representations. The present study also suggests that females have an enhanced sensitivity and emotional tuning toward companions. These findings are consistent with the view that contagious yawning reveals an emotional connection between individuals. This phenomenon, here demonstrated in monkeys, could be a building block for full-blown empathy.

Primates are intensely social and exhibit extreme variation in social structure, making them particularly well suited for uncovering evolutionary connections between sociality and vocal complexity. Although comparative studies find a correlation between social and vocal complexity, the function of large vocal repertoires in more complex societies remains unclear. We compared the vocal complexity found in primates to both mammals in general and human language in particular and found that non-human primates are not unusual in the complexity of their vocal repertoires. To better understand the function of vocal complexity within primates, we compared two closely related primates (chacma baboons and geladas) that differ in their ecology and social structures. A key difference is that gelada males form long-term bonds with the 2–12 females in their harem-like reproductive unit, while chacma males primarily form temporary consortships with females. We identified homologous and non-homologous calls and related the use of the derived non-homologous calls to specific social situations. We found that the socially complex (but ecologically simple) geladas have larger vocal repertoires. Derived vocalizations of geladas were primarily used by leader males in affiliative interactions with ‘their’ females. The derived calls were frequently used following fights within the unit suggesting that maintaining cross-sex bonds within a reproductive unit contributed to this instance of evolved vocal complexity. Thus, our comparison highlights the utility of using closely related species to better understand the function of vocal complexity.

By living in social groups with potential competitors, animals forgo monopolizing access to resources. Consequently, debate continues over how selection might favour sociality among competitors. For example, several models exist to account for the evolution of shared reproduction in groups. The ‘concession model’ hypothesizes that dominant reproducers benefit from the presence of subordinates, and hence tolerate some reproduction by subordinates. This mutual benefit to both dominants and subordinates may provide a foundation for the formation of social groups in which multiple members reproduce—a necessary step in the evolution of cooperation. To date, however, the concession model has received virtually no support in vertebrates. Instead, the vast majority of vertebrate data support ‘limited control models’, which posit that dominant reproducers are simply unable to prevent subordinates from reproducing. Here we present the most comprehensive evidence to date in support of the concession model in a vertebrate. We examined natural variation in the number of adult males in gelada (Theropithecus gelada) reproductive units to assess the extent of reproductive skew in multi-male units. Dominant (‘leader’) males in units that also had subordinate (‘follower’) males had a 30 per cent longer tenure than leaders in units that did not have followers, mainly because followers actively defended the group against potential immigrants. Follower males also obtained a small amount of reproduction in the unit, which may have functioned as a concession in return for defending the unit. These results suggest that dominants and subordinates may engage in mutually beneficial reproductive transactions, thus favouring male–male tolerance and cooperation.

Female rodents are known to terminate pregnancies after exposure to unfamiliar males (\"Bruce effect\"). Although laboratory support abounds, direct evidence for a Bruce effect under natural conditions is lacking. Here, we report a strong Bruce effect in a wild primate, the gelada (Theropithecus gelada). Female geladas terminate 80% of pregnancies in the weeks after a dominant male is replaced. Further, data on interbirth intervals suggest that pregnancy termination offers fitness benefits for females whose offspring would otherwise be susceptible to infanticide. Taken together, data support the hypothesis that the Bruce effect can be an adaptive strategy for females.

Although monitoring social information is a key aspect of the social complexity hypothesis, surprisingly little work has compared social knowledge across different species of wild animals. In the present study, I use playback experiments to test for individual recognition in wild male geladas (Theropithecus gelada) to compare with published accounts of social knowledge in chacma baboons (Papio ursinus). Geladas and baboons are closely related primates living in socially complex groups that differ dramatically in group size—geladas routinely associate with more than 10 times the number of conspecifics than do baboons. Using grunts from non-rival males to simulate approaches, I examined the strength of a subject male's response when the ‘approach’ was from the direction of (i) non-rival males (control), or (ii) rival males (a more salient stimulus if playback grunts are not recognized by the subject). I compared responses separately based on the degree of social overlap between the caller and the subject. Responses indicate that male geladas, unlike baboons, do not use vocalizations to recognize all of the individuals they regularly encounter. This represents, to my knowledge, the first documented evidence of ‘missing’ social knowledge in a natural primate population. The sharp distinction between baboons and geladas suggests that geladas are either unable or unmotivated to keep track of the individual identity of other males in their multi-level society—even males with whom they have a large degree of social overlap. Thus, these results are consistent with the central assumption of the social complexity hypothesis that social cognition is costly.

Primates have adapted to numerous environments and lifestyles but very few species are native to high elevations. Here we investigated high-altitude adaptations in the gelada ( Theropithecus gelada ), a monkey endemic to the Ethiopian Plateau. We examined genome-wide variation in conjunction with measurements of haematological and morphological traits. Our new gelada reference genome is highly intact and assembled at chromosome-length levels. Unexpectedly, we identified a chromosomal polymorphism in geladas that could potentially contribute to reproductive barriers between populations. Compared with baboons at low altitude, we found that high-altitude geladas exhibit significantly expanded chest circumferences, potentially allowing for greater lung surface area for increased oxygen diffusion. We identified gelada-specific amino acid substitutions in the alpha-chain subunit of adult haemoglobin but found that gelada haemoglobin does not exhibit markedly altered oxygenation properties compared with lowland primates. We also found that geladas at high altitude do not exhibit elevated blood haemoglobin concentrations, in contrast to the normal acclimatization response to hypoxia in lowland primates. The absence of altitude-related polycythaemia suggests that geladas are able to sustain adequate tissue-oxygen delivery despite environmental hypoxia. Finally, we identified numerous genes and genomic regions exhibiting accelerated rates of evolution, as well as gene families exhibiting expansions in the gelada lineage, potentially reflecting altitude-related selection. Our findings lend insight into putative mechanisms of high-altitude adaptation while suggesting promising avenues for functional hypoxia research. Analysing a newly assembled genome of the gelada, an endemic Ethiopian monkey that lives in high altitudes, the authors identify a novel karyotype and genomic elements associated with high-altitude adaptation.

Here, we tested hypotheses about the potential functions of yawning based on its intensity and social contexts. Due to their spectrum intensity of yawns (covered teeth/YW1; uncovered teeth/YW2; uncovered gums/YW3), geladas are a good model species for this purpose. We suggest that yawns of different intensity can bear different information according to the performer, the context and the behavioural pattern temporally associated to the yawn event. YW3, mainly performed by high ranking males during periods of high social tension, was frequently associated with an auditory component and often accompanied by scratching (a measure of anxiety). YW1 and YW2, preferentially performed by females, were frequently associated to lip smacking, an affiliative display. In conclusion, even though a clear-cut functional distinction of geladas' yawn intensity is difficult, YW1 and YW2 seem to be more linked to affiliative social interactions; whereas, YW3 seems to be more linked to agonistic and tension situations.

Theory predicts that cheating individuals should alter their behaviour to avoid detection, yet empirical data for such ‘deceptive’ behaviour (and its putative consequence—punishment) is almost entirely absent from the literature. This dearth of evidence, particularly among primates, limits our understanding of the evolution of deception and punishment. Here, we quantify deception and punishment in a reproductive context in wild geladas ( Theropithecus gelada ). Individuals involved in extra-pair copulations (9% of observed copulations) exhibited behaviour consistent with tactical deception: they were less likely to vocalize and more likely to copulate when the cuckolded male was a sizable distance away (>20 m). Further, many extra-pair copulations (∼20%) elicited post-copulatory aggression—likely, punishment—from cuckolded males. This rare empirical evidence of both tactical concealment and retaliatory aggression offers a potential model system for examining the co-occurrence of deception and punishment in natural settings. Although theory indicates that deception and punishment are successful social strategies, there is little evidence for either in the wild. This study presents the first systematic evidence of tactical deception and punishment of reproductive cheating in a wild primate, the gelada ( Theropithecus gelada ).