Uniting against a common enemy: Perceived outgroup threat elicits ingroup cohesion in chimpanzees

Abstract

Outgroup threat has been identified as an important driver of ingroup cohesion in humans, but the evolutionary origin of such a relationship is unclear. Chimpanzees (Pan troglodytes) in the wild are notably aggressive towards outgroup members but coordinate complex behaviors with many individuals in group hunting and border patrols. One hypothesis claims that these behaviors evolve alongside one another, where outgroup threat selects for ingroup cohesion and group coordination. To test this hypothesis, 5 groups of chimpanzees (N = 29 individuals) were observed after hearing either pant-hoots of unfamiliar wild chimpanzees or control crow vocalizations both in their typical daily environment and in a context of induced feeding competition. We observed a behavioral pattern that was consistent both with increased stress and vigilance (self-directed behaviors increased, play decreased, rest decreased) and increased ingroup cohesion (interindividual proximity decreased, aggression over food decreased, and play during feeding competition increased). These results support the hypothesis that outgroup threat elicits ingroup tolerance in chimpanzees. This suggests that in chimpanzees, like humans, competition between groups fosters group cohesion.

The founder sociality hypothesis

“…The decrease in bond strength among individuals of nondispersing sex cannot easily be explained by reduced reactive aggression alone. Additionally, as early as Darwin as well as ongoing research today suggests coevolution of ingroup cooperation with outgroup aggression (Bowles, 2009;Choi & Bowles, 2007;Darwin, 1871;Yamamoto, 2020) and preliminary tests in chimpanzees have found evidence for a correlation in both wild (Samuni et al, 2019) and captive (Brooks, Onishi, et al, 2021) contexts. Whether the three predictions presented in this paper are independent or stem from a common evolutionary pressure will be an important piece of evidence in testing the validity of the founder sociality hypothesis.…”

The dynamics of social cohesion in response to simulated intergroup conflict in banded mongooses

“…In other systems, the effect of intergroup conflict on post‐conflict aggression is mixed. In some species, within‐group aggression is elevated in the post‐conflict period, for example, in capuchin monkeys (Polizzi di Sorrentino et al, 2012 ) and bonnet macaques (Cooper et al, 2004 ), and in others, it is lower, for example, chimpanzees (Brooks et al, 2021 ). By contrast, intergroup encounters resulted in no change in within‐group aggression in the cichlid (Bruintjes et al, 2015 ), dwarf mongooses (Morris‐Drake et al, 2019 ), or ring‐tailed lemurs (Nunn & Deaner, 2004 ).…”

Modelling Personality Change During Extreme Exogenous Conditions

“…In one of these studies, the mere presence of danger made people search for more physical proximity and communicate more. Such effects occurred in other hominidae, as well, and seemed to serve as survival behaviour (Brooks et al, 2021).…”