Social bonds facilitate cooperative resource sharing in wild chimpanzees

Abstract

Why share when access to benefits is uncertain is crucial to our understanding of the evolution of humans' extensive cooperation. Here, we investigated some of the different human sharing hypotheses and potential neuroendocrine mechanisms, in one of our closest living relatives, chimpanzees. The strongest predictor of sharing across food types was the presence of enduring and mutually preferred grooming partners, more than harassment, direct signalling, or trade. Moreover, urinary oxytocin levels were higher after the sharing of both individually and jointly acquired resources compared with controls. We conclude that the emotional connection inherent in social bonds was a key factor determining sharing patterns, with the oxytocinergic system potentially facilitating long-term cooperative exchanges. Testing for the role of social bonds in increasing predictability of sharing behaviour, a feature frequently overlooked, may help us to identify the evolutionary drivers of resource sharing and mechanisms that sustain delayed reciprocity between non-kin.

Maternal influence on the development of nut‐cracking skills in the chimpanzees of the Taï forest, Côte d'Ivoire (Pan troglodytes verus)

“…Infants’ intrusive behavior has been suggested as the major driver for food transfer among primates (Thornton & Raihani, ). However, recent evidence clearly argues against the hypothesis that harassment mediates food sharing among chimpanzees in Taï, suggesting instead, that the social bond between individuals supports this behavior (Samuni et al, ). Therefore, in the context of nut‐cracking learning, the delayed food reward resulting from mothers refusing nut sharing could lead, as a byproduct, to keeping immatures’ attention focused on mothers’ actions.…”

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

“…Much research to date on social phenotypes has focused solely on association patterns (19,25), or on single forms of dyadic interaction (20, 34), with a minority of studies examining consistency in multiple forms of social behaviours (89,98,99). 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).…”

“…The three behaviours of aggression, grooming (affiliation) and association represent three important components of sociality for chimpanzees, facilitating social goals such as dominance rank attainment (82,83) and the formation of social bonds (42,84), both of which influence fitness (43,44,85–88). Our study expands on former research in this field in terms of its scale, both temporally and in the range of behaviours examined, in a wild population, demonstrating that social phenotypes are a phenomenon in long-lived species occupying complex social environments.…”

Maternal cannibalism in two populations of wild chimpanzees

“…Despite differences in maternal experience (WAN: experienced mother with other offspring in the community; MON: nulliparous, recent immigrant), both mothers took part in the meat sharing with the other community members. Meat sharing is associated with oxytocin secretion, both in Taï (Samuni et al 2018b) and Budongo (Wittig et al 2014), potentially acting as a social buffer against stress by down-regulating cortisol secretion [reviewed in McQuaid et al (2016)]. Sharing the carcass of her own infant, in other words, may temporarily buffer a mother's stress associated with the death of her infant.…”