Grunt Usage and Social Monitoring: Investigation of the Signaller and the Receiver Perspectives in a Tolerant Species of Baboons

Understanding society, communication and cognition in non-human primates sheds light on the evolution of our own species. While the acoustic structure in primates is mainly innate, their vocal usage and comprehension/response are more flexible and fluctuate based on social experiences. Because relationships can vary between individuals within the same group and change over time, the contexts of call usage (signaller perspective) and the patterns of response to vocalisations (receiver perspective) is supposed to differ even among closely related species. My thesis project is in line with the current framework investigating to which degree social system shapes communication and social cognition in primates. More specifically, I studied the usage of an affiliative vocalisation – the ‘grunt’ – and the allocation of social attention of a wild population of Guinea baboons (Papio papio) living in the Niokolo Koba National Park, Senegal. By combining behavioural observations and experimental designs, I investigated the perspectives of the signaller and the receiver focusing on the grunt, the most common affiliative vocalisation in baboons. Savannah baboons (P. ursinus, P. cynocephalus, P. kinda, P. anubis) live in polygynandrous groups characterized by high male-male competition and stable female-bonded societies. In contrast, the social organization of Guinea and hamadryas baboons (P. hamadryas) is defined by a nested multi-level society, female-biased dispersal and a polygyny-monandrous mating system. In Guinea baboons, the ‘unit’ — i.e., a ‘primary’ male, 1-6 females, their youngster, and frequently ‘secondary’ males — represents at the core of the society, and the agglomeration of several units is called a ‘party’. Males are mainly philopatric within a party and share high spatial tolerance, facilitating the formation of strong and enduring social bonds. To investigate if the social system of Guinea baboons influences their grunt usage, I collected behavioural observations of sexually mature males and females. First, I examined if the nature of post-approach interactions was modulated by the grunt production of the approaching animal (i.e., the subject). Following approaches with grunts in both sexes, the dyad was more likely to interact in an affiliative way. Moreover, grunts increased the probability of manipulation of the partner’s infant and reduced the risk of displacement. Subsequently, I tested how infants in close-proximity of the receiver and the quality of relationships between the partners potentially influence the subject’s decision to grunt or to remain silent. Both sexes were more likely to grunt to female than to male partners. Between females, the probability of vocal production was lower when the relationship strength was high, but only when an infant was present with her partner. The unit membership also impacted the grunt utterance as females were more likely to vocalise while approaching a female from a different unit. Dominance status and dyadic rank distance did not affect the probability to grunt between females, even if a linear dominance hierarchy was detected. In correspondence with the high tolerance between males of this species, relationship strength had no impact on the likelihood to grunt during male-male approaches. Finally, males were more likely to grunt when an infant was in close-proximity of the female partner. Overall, these results indicate that Guinea baboons use grunts strategically to advertise their benign intent during specific unpredictable situations (e.g., while approaching non-preferential partners). Although genetic constraints limit the structure and extent of vocal repertoire within taxa, baboons can adapt their vocal usage flexibly depending on their social environment. In a second study, I explored the receiver perspective in adult male Guinea baboons by testing the attention to third-party social interactions. I carried out playback experiments to assess if males keep track of the current pattern of male-female associations within their own party (but outside their own unit). First, I established that grunts occur more frequently between females and primary males of the same unit than from different units. Then, I created consistent and inconsistent acoustic sequences simulating grunt exchanges between a female and a primary male from the same or from different units respectively. I tested those grunt sequences on primary and non-primary males, as male status (association with a female) could influence the value of the social information. Surprisingly, male Guinea baboons looked longer toward the speaker when exposed to the consistent compared to the inconsistent condition, a result opposite to comparable playback experiments on chacma baboon males. Moreover, primary males reacted more than non-primary males regardless of the experimental condition. Thus, this result reveal how differences of the mating system and the degree of male competition affect the value allocated to social information even between closely related species. By comparing my results with the literature on geladas and other baboon species, the variations observed in call usage and the motivation to keep track of third-party interactions in Guinea baboons are likely to be related to different aspects of their social systems. Although Guinea baboons use grunts strategically to signal benign intent, like in many other primate species, the lack of a significant effect of male- male and male-female bond strength, as well as female dominance hierarchy on grunt utterance may be related to the low competition level observed both within and between sexes in this species. Considering the low interest toward recordings of simulated incongruent male-female associations, my playback experiments support that the allocation of social attention — and potentially the extent of social knowledge — is highly dependent on the mating system and the level of competition/tolerance among conspecifics. Future research should consider a set of measures to reveal how the competition, cohesion and/or cooperation potentially impact the evolution of communication and cognition. Furthermore, extensive comparative investigations across species and populations fluctuating in some dimensions of their social systems are necessary, as the social environment seems to account for more variation among species than genetic relatedness