Chimpanzee friends come together to battle out-group rivals

Chimpanzees maintain cooperative friendships within their group but are violent to outsiders. Battles with outgroups can be fatal but can also increase the success of group members. What influences chimpanzee participation in battle despite the high risk? Close friendships might be the key

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We heard a clear but far-off call from another chimpanzee group. Instantly the chimpanzees I was following reacted – this was not the call of a friend. The adult males grouped and touched, reasserting their bonds at this dangerous moment. Poseidon, one of the adult males, showed a fear response and approached to embrace another male, Richelieu. Willy, the alpha male, extended his hand to his friend, Fredy, for reassurance. The adult females Pola and Indira raced to grab their infants who were playing in vines and trees.

We just heard the neighbouring group to the west, rivals with whom recent encounters were especially violent and unpredictable, and last time ended with fatal injuries to a young female, Emma, who had become isolated and received no support from her groupmates. Almost immediately the chimpanzees started patrolling very quietly and slowly through the dense forest towards the calls, some 600m away. Tension was high. Willy was leading the movement but frequently stopped, waiting for Fredy, and then moving forward together. Similarly, close friends Pola and Indira travelled side by side. The closer we approached the calls, the more silent and vigilant each chimpanzee became.

Poseidon approaches to embrace Richelieu after hearing the calls of a neighbouring chimpanzee group. Credit: Liran Samuni/Taï Chimpanzee Project 

The attack on the neighbouring group had begun. The contrast between the silent approach and the hectic chaos and noise of the fight were striking. Deafening screams and barks surrounded me, and the previously peaceful forest instantly turned into a battlefield. Willy and Fredy were charging towards the rival group, shoulder to shoulder, scattering the out-group chimpanzees as they ran. Poseidon and Richelieu jointly chased the same chimpanzees from another direction, and Indira and Pola drummed on tree buttresses and then assisted in the chase, all while carrying their infants on their backs. Teamwork is a key component here to win the conflict, and the coordinated response of the chimpanzees meant that no injuries were suffered this time.

Interactions like these are a part of the everyday lives of chimpanzees and represent one of the riskiest forms of group actions, where success lies on a collective and cohesive response of several individuals. Although encounters between chimpanzee groups are extremely dangerous, especially to those who are left unsupported, like Emma, if successful they can also improve the reproduction rate of group members. How then does each chimpanzee decides whether to fight the rival group or not? How can chimpanzees optimize the gains of these battles and increase their own safety during these unpredictable interactions?

Grooming interactions between several group members before initiating a patrol and an attack on a neighbouring group. Credit: Liran Samuni/Taï Chimpanzee Project 

Those questions are at the heart of my research work with Drs. Roman Wittig and Catherine Crockford and the Taï Chimpanzee Project. Founded by Christophe Boesch in 1979, several groups of Taï chimpanzees are followed every day from dawn to dusk by researchers for more than 40 years. The chimpanzees are accustomed to researchers’ presence, so we have the rare opportunity to observe all the intricacies of their lives up close, whilst they live in their natural social and ecological environment. As one of our closest living relatives, chimpanzees are similar to us in many ways, and like us, their family members and close friends are those individuals they groom, support, and share food with the most. Like in our own social lives, chimpanzees rely on their family or friends when in need, as mutual support is more predictable with those individuals. If bondedness and mutual support are important during warfare in humans, can it similarly be that relationships characterized by bondedness and mutual support are important during chimpanzee battle? If so, do chimpanzee decisions to participate or not in fights with neighbouring groups depend on whether they have a predictable supporter participating alongside them?

Intimate grooming between mother and daughter who share a close social bond. Credit: Liran Samuni/Taï Chimpanzee Project 

Observing hundreds of encounters between different chimpanzee groups over 55 observation years we could reveal the patterns that influence which individuals participate in battles with rival groups and when do they decide to do so. This incredible dataset allowed us to examine whether close social relationships affect the participation decisions of males and females in encounters with out-groups. The results came in very clear: chimpanzees not only participate in risky encounters between groups when they have high numbers, but more importantly, the identity of the individuals going into battle with them matters. Despite chimpanzee males being naturally more violent and leading most conflicts, both sexes were more likely to participate when they acted together with an adult family member (such as their mother or adult offspring) or their best friend, someone with whom they share a strong, mutual, and long-lasting grooming relationship. As agonistic support during fights between groups is fundamental in reducing injury related risks, acting with those who are more likely to provide that support during dangerous moments may be an effective way of keeping as safe as possible. In that sense, strong social bonds in chimpanzees (like for us humans) are not only at the basis of everyday, one-on-one activities like food sharing or consolation, but also enable group-level activities that require multiple participants who collectively act together. Acting together in battle with someone you trust and are bonded with (the ‘buddy system’) is an effective military tactic in humans, exploited by army commanders since at least the time of the Spartans. As in humans, the bedrock of chimpanzee group-level cooperation during battle with rival groups comes from strong social bonds.

Liran Samuni

Postdoctoral Fellow, Harvard University

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