Chimpanzee friends fight together to battle rivals

Strong social bonds increases the willingness to join others in battle

Humans cooperate with each other in large groups to defend territories or wage war. But what underlies the evolution of this kind of cooperation? Researchers at the Max PIanck Institute for Evolutionary Anthropology and Harvard University show that there may be a link between social bonds and participation in large-scale cooperation: Chimpanzees join their close bond partners when fighting rivals. In humans, too, social bonds may have been essential to the evolution of cooperative abilities.

Chimpanzees join their close bond partners – related group members and friends – to battle rivals.
© Liran Samuni, Taï Chimpanzee Project

Chimpanzees, one of the closest relatives of humans, cooperate on a group level – in combative disputes, they even cooperate with group members to whom they are not related. Those involved in fights with neighbouring groups put themselves at risk of serious injury or even death.

Within the context of the Taï Chimpanzee Project researchers observed three chimpanzee communities in Taï National Park in Côte d’Ivoire documenting social relationships, territory range and intergroup encounters amongst others. “We have been able to analyze almost 500 vocal and physical battles from the last 25 years with participation of at least one of the three habituated communities, some of which have caused severe injury or death”, says Liran Samuni, the first author of the study.

The study showed that males, as well as females participate in the battles and that three factors increased the likelihood of participation in the intergroup encounter when there were many individuals participating, when maternal kin joined and when non-kin social bond partners were present. “It seems chimpanzees not only consider the sheer number in their sub-group when moving into battle, but they consider the presence of a trusted group member, who will support them in case of an attack”, adds Catherine Crockford, senior author of the study. “These results suggest”, Liran Samuni continues, “that the link between strong, enduring social relationships and costly collective acts is not uniquely human, but is present in one of our closest living relatives, the chimpanzee.”

“This study is part of a series of several investigations linking in-group cooperation with out-group competition”, explains Roman Wittig, director of the Taï Chimpanzee Project and senior author of the studies. “We were able to show that out-group competition reduces chimpanzees’ reproduction and their territory size. On the other hand, out-group competition increases in-group cohesion and, likely facilitated by the neurohormone oxytocin, reduces the likelihood of defection in battle.” Data from the Taï Chimpanzee Project, with four neighboruing communities observed on a daily basis, will be a key source for scientific investigations into the ultimate and proximate causes of group-level cooperation. “The Taï chimpanzees can teach us”, Roman Wittig points out, “what social tools enable human’s unique capacity for large-scale cooperation with non-kin”.

[RW/SJ]

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