The largest group of wild chimpanzees known to scientists has permanently split in two. In a study published in Science, researchers from The University of Texas at Austin and other institutions report the first clearly documented permanent fission in wild chimpanzees and the years of violence between the two groups that followed. The findings draw on three decades of ongoing field observations of the Ngogo chimpanzees of Kibale National Park, Uganda - a population featured in the Netflix documentary series "Chimp Empire" - and may offer insight into causes of conflict in our own species.
The community was cohesive for the first two decades of research. Individual chimpanzees moved between flexible subgroups, or "clusters," and maintained
social ties across the community - a fission-fusion dynamic typical of the species, in which individuals temporarily separate and reunite. In 2015, however, the team witnessed signs of polarization, with the Western and Central clusters increasingly avoiding each other. This shift coincided with a change in the male dominance hierarchy and came one year after the deaths of several adult males who may have functioned as bridges holding the larger community together.
By 2018, the fission was complete. The chimpanzees now belonged to two distinct groups with separate territories: a Western group with 83 chimps and a Central group with 107 chimps. What followed was a series of lethal attacks by the Western group on members of the Central group. Between 2018 and 2024, researchers directly observed or documented from physical evidence seven attacks on adult males and 17 on infants.
"What's especially striking is that the chimpanzees are killing former group members," said Aaron Sandel, associate professor of anthropology at UT Austin and the study's lead author. "The new group identities are overriding cooperative relationships that had existed for years."
In many primate species, large groups regularly split into smaller ones, often reducing competition for resources. But in chimpanzees, permanent fissions are extraordinarily rare. Genetic evidence suggests they occur about once every 500 years. The only previously reported case took place in the 1970s at Gombe, Tanzania, during Jane Goodall's long-term study. But that case has remained a subject of debate in part because the chimpanzees there were provided with food by researchers. At Ngogo, the chimpanzees were never given food, and the picture is more complete, thanks to nearly three decades of study by John Mitani, emeritus professor of anthropology at the University of Michigan, and a large team of researchers and Ugandan field staff members.
"I would caution against anyone calling this a civil war," Sandel said. "But the polarization and collective violence that we have observed with these chimpanzees may give us insight into our own species."
The authors describe their findings 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."
Sandel and his fellow researchers are continuing to document and analyze the ongoing conflict to gain further insight into the causes and consequences of the split.
