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

James Brooks,Ena Onishi,Shinya Yamamoto,Manuel Bohn,Isabelle R Clark,Katie E Slocombe

doi:10.1371/journal.pone.0246869

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.

Highlights

The evolution of intergroup relations is of great importance for theories of both human evolution and animal behavioral ecology
There was a significant interaction between condition and trial in social grooming in the playback phase (β = -1.44, SE = 0.43, χ2 = 9.71, p = 0.0018; odds ratio (OR) = 0.24; Fig 2), where in the first trial individuals engaged in more social grooming in the outgroup condition than control condition but this effect decreased across trials
In the playback phase there was a significant interaction between condition and trial on posture (β = -0.32, SE = 0.14, χ2 = 4.42, p = 0.036; OR = 0.73; Fig 2) where individuals in the first trials spent less time lying down in the outgroup compared to control condition but the effect decreased across trials

Summary

Introduction

The evolution of intergroup relations is of great importance for theories of both human evolution and animal behavioral ecology. Outgroup threat has long been proposed as a key driver of ingroup cohesion and cooperation and has been emphasized as a potentially significant factor in humans’ great capacity for both cooperation and aggression [1,2,3,4], but to date evidence from our closest relatives in controlled experiments is lacking. Competition between groups over resources may incur a high cost during dangerous aggressive encounters but may result in territorial expansion enhancing group fitness [2, 5]. In particular in situations involving limited resources, has been proposed as a context that may simultaneously select for both greater aggression towards outgroup members and greater.

Methods

Results

Conclusion