Chimpanzees unite against a common enemy

In the face of threats from other groups, people become more cohesive and cooperative with their own group, an association that Charles Darwin says could be an evolved ability. Now, a research group at Kyoto University has experimentally shown for the first time that this tendency is shared with chimpanzees, one of our closest relatives.

“Despite the importance of understanding how humans can interact with their in-group and still carry out extreme out-group aggression, little research has so far been conducted into whether the association between these behaviors is true in non-human primates,” says first author James Brooks.

Building on field research suggesting that chimpanzees had more cohesion in days and months that they had outgroup encounters, the team tested the direct relationship between outgroup threat and internal cohesion by simulating an outgroup encounter and observing the subjects’ behavior.

Five groups of chimpanzees listened to sounds from unknown individuals, along with a mastery of the sounds of crows. The team found that subjects who heard the outgroups became more vigilant and stressed, but instead of translating this into tension in the group, the chimpanzees got closer together, showed more affiliative behavior, and were less aggressive when given limited food. . compared to the control group.

This suggests that in chimpanzees, as in humans, intergroup competition promotes cohesion, and further that intergroup competition in human evolution may have allowed us to cooperate in large groups in the presence of a common enemy and maintain tolerant relationships.

“This is the first experimental evidence that humans share this tendency with chimpanzees,” explains tutor Shinya Yamamoto, “but it remains to be tested whether this is due to the strong evolutionary history of intergroup competition of both species or a more common feature that is shared with other great apes. “

The team is currently investigating whether the same pattern is observed in bonobos – other close relatives of humans – that are not known to commit lethal aggression outside of the group.

The newspaper “Unite Against a Common Enemy: Threats to the Group Promotes Group Cohesion in Chimpanzees” appears in the magazine February 24, 2021 PLOS ONE