Flexible decision-making in grooming partner choice in sooty mangabeys and chimpanzees

Abstract

Living in permanent social groups forces animals to make decisions about when, how and with whom to interact, requiring decisions to be made that integrate multiple sources of information. Changing social environments can influence this decision-making process by constraining choice or altering the likelihood of a positive outcome. Here, we conceptualized grooming as a choice situation where an individual chooses one of a number of potential partners. Studying two wild populations of sympatric primate species, sooty mangabeys (Cercocebus atys atys) and western chimpanzees (Pan troglodytes verus), we tested what properties of potential partners influenced grooming decisions, including their relative value based on available alternatives and the social relationships of potential partners with bystanders who could observe the outcome of the decision. Across 1529 decision events, multiple partner attributes (e.g. dominance ranks, social relationship quality, reproductive state, partner sex) influenced choice. Individuals preferred to initiate grooming with partners of similar global rank, but this effect was driven by a bias towards partners with a high rank compared to other locally available options. Individuals also avoided grooming partners who had strong social relationships with at least one bystander. Results indicated flexible decision-making in grooming interactions in both species, based on a partner's value given the local social environment. Viewing partner choice as a value-based decision-making process allows researchers to compare how different species solve similar social problems.

Urinary Cortisol, Aggression, Dominance and Competition in Wild, West African Male Chimpanzees

“…We increased the density of our pant grunt data by pooling the pant grunt dataset collected by the Taï Chimpanzee Project's researchers and field assistants. Inter-observer reliability between researchers were >90% and data from field assistants were only included once they had more than 80% overlap with a trained reference observer when collecting data simultaneously (Mielke et al, 2017(Mielke et al, , 2018. Ordinal ranks were standardized daily between 0 and 1, with 1 being the highest rank.…”

Long-term repeatability in social behaviours suggests stable social phenotypes in wild chimpanzees

“…While grooming forms an important component of social bond formation (84), chimpanzees also make contingent grooming choices based on a range of parameters, such as audience, partner rank or context (e.g. reconciliation after an aggression) (68). Therefore, our results highlight this contrast in social decision making between levels of association (constrained preferences) and grooming (flexible choices).…”

“…Our study shows that consistent individual differences in social behaviour extends to patterns of aggression and affiliation, both of which should influence fitness more than association alone (43,44,85–88,100). Indeed, both aggression and grooming involve direct, typically physical interactions with other group members, meaning that variation in these phenotypes will be important for factors such as rank acquisition (51,64,68,101,102), disease transmission (103–105) and group cohesion (45,106,107). Given the potential significance of these social tendencies, understanding how certain individuals come to be more aggressive or affiliative, as well as gregarious, than others, requires further empirical exploration.…”

“…Chimpanzee societies also feature various cooperative behaviours, such as alliance formation, food sharing, group hunting, and territorial patrols, which likely impact on fitness and for which strong social relationships are required (42–45). Therefore, these individuals face diverse social environments and important choices regarding their social behaviour (46), which may predict a degree of flexibility in social tendencies.…”

“…Therefore, we expected a high degree of within-individual variation in aggressive tendencies in males in relation to lifetime fluctuations in dominance rank, whereas within-individual variation in aggression would be lower in females due to relatively stable hierarchy structure. Individual chimpanzees are highly strategic and flexible in grooming partner choice (46), while time devoted to grooming in this species can vary depending on demographic factors such as group size (54). Therefore, we expected low repeatability in grooming behaviour across long time periods.…”