The largest group of wild chimpanzees known to scientists has permanently split into warring factions. In a study published in Science, an international team of researchers, including a University of Arizona primatologist, report this rare event, building on three decades of observations on the Ngogo chimpanzees in Kibale National Park in Uganda.

The chimpanzee group was cohesive for the first two decades of research, but a rise in polarization in 2015 led to a permanent fission into two distinct groups by 2018. After the social division, one group waged multiple lethal attacks on the other. At least 24 chimpanzees have been killed, including seven adult males.

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Jacob Negrey, an assistant professor of anthropology in the College of Social and Behavioral Sciences and a study co-author, said that the findings suggest that, for chimpanzees, there is a profound link between shared space and group identity. As social relationships fray and group members no longer gather to eat, rest, and socialize, "friends" can become killers.

"What's shocking to me is that these chimps were able to kill individuals with whom they had long maintained friendly social relationships," added Negrey, who has studied the Ngogo chimps for about 13 years. "Decades of affiliative interactions in some cases did not preclude lethal violence."

In other primate species, large groups splitting into smaller groups in not uncommon and can benefit social systems by reducing competition for resources. But such splits are extremely rare in chimpanzees, estimated through genetic evidence to occur only about once every 500 years. 

The Ngogo chimpanzees' social lives were carefully documented every summer by John Mitani, emeritus professor of anthropology at the University of Michigan, a large team of other scientists, students and Ugandan field staff. The evidence on the fission and lethal aggression at Ngogo suggests that such events are not an anomaly or due to human influence. 

The authors describe the Ngogo chimpanzees as a large group with subgroups or "clusters." These clusters remained flexible – with individual chimpanzees often switching between clusters and maintaining social ties across the two – and exhibited a fission-fusion dynamic common to the species, where members temporarily split into smaller social units that subsequently reunite.

In 2015, however, the team witnessed signs of polarization, with the western and central clusters increasingly avoiding each other. This coincided with a change in the male dominance hierarchy and occurred one year after several adult male chimpanzees died that may have served as bridges among the clusters. 

By 2018, the fission was complete, with chimpanzees belonging to distinct western and central groups with separate territories. This split was followed by a series of attacks from the western group on the central group. Between 2018 and 2024 the authors observed or inferred with high confidence seven such attacks on adult males and 17 on infant chimpanzees.

"I would caution against anyone calling this a civil war," said Aaron Sandel, associate professor of anthropology at UT Austin and lead author of the study. "But the social processes and collective violence that we have observed with these chimpanzees may give us insight into our own species."

The authors describe their results as a challenge to the hypothesis that human warfare, including civil war, is driven primarily by cultural markers of group identity, such as ethnic or religious differences. 

"If relational dynamics alone can drive polarization and lethal conflict in chimps without language, ethnicity, or ideology, then in humans, those cultural markers might be secondary to something more basic," Sandel said. "If that's true, then we may have the potential to reduce societal conflicts in our personal lives, and that gives me hope. As our paper concludes, it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace."