Reconciling conflicts in a one-male society: the case of geladas (Theropithecus gelada)

Yawn Contagion and Modality-Matching in the Female-Bonded Society of Geladas (Theropithecus gelada)

Behavioral contagion is widespread in primates, with yawn contagion (YC) being a well-known example. Often associated with ingroup dynamics and synchronization, the possible functions and evolutionary pathways of YC remain subjects of active debate. Among nonhuman animals, geladas (Theropithecus gelada) are the only species known to occasionally emit a distinct vocalization while yawning. Yet, the role of different sensory modalities in YC remains poorly understood. Due to their social and communicative complexity, geladas serve as an excellent model for investigating the effects of multimodality and social factors on behavioral contagion. Here we studied a large zoo-housed colony of geladas (103 subjects, 1422 yawns) and confirm the previous evidence for visual and auditory YC. Hearing, seeing, or hearing and seeing yawns significantly triggered contagious yawning at comparable levels. Additionally, we found no evidence of laterality influencing responses based on the side of detection. While the social bond, measured via grooming, between the trigger and receiver did not correlate with YC, a consistent sex effect emerged. Females responded more frequently to female than to male yawns and were more likely to match modality (i.e., vocalized vs. nonvocalized) and mirror morphology of other females' yawns. Effective female-female communication and affiliation are crucial for maintaining cohesion and fostering strong intra-unit relationships among geladas. Our results underscore the importance of different sensory components in the distribution of YC, particularly for species living in complex social systems. These findings raise further questions about the functional and emotional significance of yawning and potential inter-sexual differences, suggesting that the phenomenon is more complex than previously thought.

Jaw-muscle fiber architecture and skull form facilitate relatively wide jaw gapes in male cercopithecoid monkeys

In primates and other mammals, the capacity to generate a wide maximum jaw gape is an important performance variable related to both feeding and nonfeeding oral behaviors, such as canine gape display and clearing the canines for use as weapons during aggressive encounters. Across sexually dimorphic catarrhine primates, gape is significantly correlated with canine height and with musculoskeletal features that facilitate wide gapes. Given the importance of canine gape behaviors in males as part of intrasexual competition for females, functional relationships between gape, canine height, and musculoskeletal morphology can be predicted to differ between the sexes. We test this hypothesis by investigating sex-specific relationships among these variables in a maximum sample of 32 cercopithecoid species. Using phylogenetic least squares regression, we found that of 18 predicted relationships, 16 of the 18 (89%) were significant in males, whereas only six (33%) were significant in females. Moreover, 15 of the 18 correlations were higher-10 of the 18 significantly higher-in males than in females. Males, but not females, showed strong and significant positive allometry of fiber lengths, indicating that increase in male jaw length is accompanied by allometric increases in the capacity for muscle stretch. While males and females showed significant negative allometry for muscle leverage, only males showed significant negative allometry of muscle leverage relative to jaw gape and canine height. Collectively, these results provide support for the hypothesis that as selection acted to increase relative canine height in male cercopithecoids, one change was an allometric increase in relative maximum jaw gape, along with allometric increases in musculoskeletal morphologies that facilitate gape. Lastly, if gape and canine display/clearance are key targets of selection on masticatory morphology in male cercopithecoids, then cercopithecoid monkeys such as macaques, baboons, and sooty mangabeys may have diminished utility as models for drawing paleobiological inferences from musculoskeletal morphology about feeding behavior and diet in fossil hominins.

The face never lies: facial expressions and mimicry modulate playful interactions in wild geladas

Abstract

Play fighting, the most common form of social play in mammals, is a fertile field to investigate the use of visual signals in animals’ communication systems. Visual signals can be exclusively emitted during play (e.g. play faces, PF, context-dependent signals), or they can be released under several behavioural domains (e.g. lip-smacking, LS, context-independent signals). Rapid facial mimicry (RFM) is the involuntary rapid facial congruent response produced after perceiving others’ facial expressions. RFM leads to behavioural and emotional synchronisation that often translates into the most balanced and longest playful interactions. Here, we investigate the role of playful communicative signals in geladas (Theropithecus gelada). We analysed the role of PF and LS produced by wild immature geladas during play fighting. We found that PFs, but not LS, were particularly frequent during the riskiest interactions such as those including individuals from different groups. Furthermore, we found that RFM (PF→PF) was highest when playful offensive patterns were not biased towards one of the players and when the session was punctuated by LS. Under this perspective, the presence of context-independent signals such as LS may be useful in creating an affiliative mood that enhances communication and facilitates most cooperative interactions. Indeed, we found that sessions punctuated by the highest frequency of RFM and LS were also the longest ones. Whether the complementary use of PF and LS is strategically guided by the audience or is the result of the emotional arousal experienced by players remains to be investigated.

Significance Statement

Facial expressions and their rapid replication by an observer are fundamental communicative tools during social contacts in human and non-human animals. Play fighting is one of the most complex forms of social interactions that can easily lead to misunderstanding if not modulated through an accurate use of social signals. Wild immature geladas are able to manage their play sessions thus limiting the risk of aggressive escalation. While playing with unfamiliar subjects belonging to other groups, they make use of a high number of play faces. Moreover, geladas frequently replicate others’ play faces and emit facial expressions of positive intent (i.e. lip-smacking) when engaging in well-balanced long play sessions. In this perspective, this “playful facial chattering” creates an affiliative mood that enhances communication and facilitates most cooperative interactions.

Bridging Captive and Wild Studies: Behavioral Plasticity and Social Complexity in Theropithecus gelada

Cognitive ethology explores the ability of animals to flexibly adapt their behavior to rapid physical and social environment fluctuations. Although there is a historical dichotomy between field and captive studies, recently, a growing interest in questions that sit at the intersection of cognitive and adaptive perspectives has helped bridge this divide. By focusing on Theropithecus gelada, we discuss the three main reasons why this hybrid approach is extremely successful. First, captive and wild studies provide data at different social, spatial, and temporal scales that can be synthesized to give a fuller picture of the behavior. Secondly, apparently conflicting results from captive and wild settings are powerful tools to explore behavioral flexibility and latent behavioral tendencies. Third, the different settings provide ways of validating and exploring behaviors that are noticed in the other setting. Although we were able to bring together our captive and wild research to demonstrate these ideas, we could have obtained a more integrated vision on the proximate and ultimate gelada behavioral and cognitive strategies if we had considered this hybrid approach from the beginning. We hope that this manuscript stimulates scholars in designing their studies by taking into account the incredible potential of a complementary captive-wild research approach.

High-Ranking Geladas Protect and Comfort Others After Conflicts

Post-conflict affiliation is a mechanism favored by natural selection to manage conflicts in animal groups thus avoiding group disruption. Triadic affiliation towards the victim can reduce the likelihood of redirection (benefits to third-parties) and protect and provide comfort to the victim by reducing its post-conflict anxiety (benefits to victims). Here, we test specific hypotheses on the potential functions of triadic affiliation in Theropithecus gelada, a primate species living in complex multi-level societies. Our results show that higher-ranking geladas provided more spontaneous triadic affiliation than lower-ranking subjects and that these contacts significantly reduced the likelihood of further aggression on the victim. Spontaneous triadic affiliation significantly reduced the victim's anxiety (measured by scratching), although it was not biased towards kin or friends. In conclusion, triadic affiliation in geladas seems to be a strategy available to high-ranking subjects to reduce the social tension generated by a conflict. Although this interpretation is the most parsimonious one, it cannot be totally excluded that third parties could also be affected by the negative emotional state of the victim thus increasing a third party's motivation to provide comfort. Therefore, the debate on the linkage between third-party affiliation and emotional contagion in monkeys remains to be resolved.

Play, aggressive conflict and reconciliation in pre-school children: what matters?

Play provides children with the opportunity to train in fundamental social skills, including conflict management. Here, we evaluate the management of play, aggressive conflict and reconciliation in 3- to 5-year-old preschool children. 3-year-old children show the highest levels of aggressive conflicts in free play, and do not reconcile their aggressive conflicts in the first months of the preschool year because they still lack social capacities to successfully manage interactions with peers. We found no gender bias in being aggressors or victims, but gender-typed traits were reflected in the expression of aggressiveness in same-sex peers for boys, who rely more on physical contacts than girls. Gender segregation in play is seen only in boys, regardless of age. Our results emphasize the importance of considering play, aggressive conflicts, and reconciliation as a whole, in order to obtain a comprehensive overview of the development of pre- and post-conflict dynamics in humans.

Agonistic support towards victims buffers aggression in geladas (Theropithecus gelada)

Agonistic support occurs when a bystander intervenes in an ongoing conflict. The consequences of agonistic support may differ when provided to victims or aggressors. Supporting victims may not only protect them, but also limit the escalation of aggression among group members. Our results on Theropithecus gelada showed that support was preferentially directed towards victims and high-ranking individuals provided the highest levels of support. Whereas the support towards the aggressor had no effect in reducing its renewed aggression, it increased the frequency of subsequent conflicts among fellows. The support towards victims significantly reduced subsequent aggression both towards the victim and among other group members. The support was biased towards victims who were unrelated and shared weak bonds with the aggressors. In conclusion, victim support may be a social tool, which intervenes when other mechanisms are less likely to occur such as the case when the opponents are not kin or friends.

Different yawns, different functions? Testing social hypotheses on spontaneous yawning in Theropithecus gelada

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.

Female snub-nosed monkeys exchange grooming for sex and infant handling

Allogrooming in primates has acquired an important social function beyond its original hygienic function and can be exchanged either for itself or used as a currency to obtain other benefits such as copulations, access to infants or agonistic support. We explore the strategic use of grooming as a social tool in semi-wild golden snub-nosed monkeys (Rhinopithecus roxellana) in central China, a species where two desirable resources, viz. reproductive males and infants, are restricted to the mating and birth season, respectively. We predict that females expend their grooming selectively to different individuals according to their "value". Our results show that in the mating season, females devoted more grooming to the resident male than in the birth season, and this effect was particularly strong in non-mothers (females without newborn infants). Moreover, females were more likely to groom the resident male after copulation than during baseline social conditions. In the birth season, females devoted more grooming to other females than in the mating season, and mothers (females with newborn infants) were the most valuable grooming partners. The mean rate of contact by non-mothers toward infants of other females was significantly higher after grooming the mothers than in baseline social conditions. In conclusion, our findings lend credence to the notion that primate females use grooming as a strategic tool to obtain limited resources such as males and infants and vary preference for particular individuals depending on the seasonal availability of valuable resources.

Rapid facial mimicry in geladas

Rapid facial mimicry (RFM) is an automatic response, in which individuals mimic others' expressions. RFM, only demonstrated in humans and apes, is grounded in the automatic perception-action coupling of sensorimotor information occurring in the mirror neuron system. In humans, RFM seems to reflect the capacity of individuals to empathize with others. Here, we demonstrated that, during play, RFM is also present in a cercopithecoid species (Theropithecus gelada). Mother-infant play sessions were not only characterized by the highest levels of RFM, but also by the fastest responses. Our findings suggest that RFM in humans have homologous not only in apes, but also in cercopitecoids. Moreover, data point to similarities in the modality in which mother-infant synchronous behaviours are expressed among primates, suggesting a common evolutionary root in the basic elements of mother-infant affective exchanges.