In the 1970s, the late Jane Goodall observed a community of chimpanzees in Gombe, Tanzania, breaking into two factions; the males in one group ended up killing all the males in the rival group over the next four years, along with one female chimp. But the case was considered an anomaly, although there is genetic evidence suggesting this kind of split is a rare event occurring every 500 years or so. Now researchers have observed the largest known community of Ngogo chimpanzees in Uganda also permanently splitting into two rival groups with a similar outbreak of violence, according to a new paper published in the journal Science.
“What’s especially striking is that the chimpanzees are killing former group members,” said co-author Aaron Sandel, an anthropologist at the University of Texas, Austin. “The new group identities are overriding cooperative relationships that had existed for years. I would caution against anyone calling this a civil war. But the polarization and collective violence that we have observed with these chimpanzees may give us insight into our own species.”
The authors analyzed 24 years’ worth of data from social networks, 10 years of GPS tracking, and 30 years of demographic data on the Ngogo chimps in Uganda’s Kibale National Park. They identified three distinct phases to the split. First there was an abrupt shift as chimp relationships became polarized into two distinct clusters: Western and Central. The chimps then spent the next two years increasingly avoiding those in their rival cluster; there were very few interactions across clusters, and Western male chimps started patrolling their territory, showing increased aggression toward Central males. By 2018 the fissure had become permanent.
Finally, the authors observed the outbreak of lethal violence resulting in the deaths of six adult males in 2018; the violence extended to the killing of 14 infant chimps in 2021. There were 14 additional chimp deaths between 2021 and 2024 that were not directly observed, and the authors suggest they may also have been killed since there had been no signs of illness in any of those animals. All the observed violent attacks were committed by the Western group chimps, despite being smaller in number. This contradicts the usual intergroup conflict models for imbalance of power, in which larger groups are thought to have the advantage.
Fracturing social networks
In a related perspective, James Brooks—a primatologist at the German Primate Center in Gottingen, Germany who was not involved with the study—noted that 50 years ago, a community of wild bonobos split permanently into two groups without the same kind of intersectional violence; the groups coexist peacefully to this today. Various hypotheses have been proposed for this very different outcome, most notably that there was a relative abundance of food in the bonobos’ environment. Sandal et al.’s findings challenge that hypothesis, however, since the chimps also seemed to have an abundance of food and yet were not able to peacefully coexist once they had split into two factions.
Sandel et al. offer several possibilities for why the Ngogo chimps split into two camps. The original group had grown to nearly 200 chimps, much larger than other chimp communities, and there were more than 30 adult males, pushing the limits of maintaining inter-community relationships. There may have been some feeding competition as well; although the Ngogo territory has abundant resources, there can be local variations in availability. And once there was reproductive isolation between the two groups, there would be a corresponding increase in competition among males.
Even those factors might not have been sufficient, but the authors also identified three possible catalysts. In 2014, five adult males and one adult female died of unknown causes, although several had shown signs of illness. Those losses likely disrupted the social network by weakening social ties across clusters. There was also a new alpha male from the Western cluster the following year, coincidentally the same time when the first sustained separation occurred. The two prior alphas had been from the Central cluster, so that change in the dominance hierarchy may have exacerbated inter-group tensions.
Finally, there was a respiratory outbreak January 2027 that killed 25 chimps, which could have sped up the final separation. “Taken together, these events suggest how networks may fracture in the face of multiple demographic and social changes,” the authors concluded.
“A hostile split among wild chimpanzees is a reminder of the danger that group divisions can present to human societies,” Brooks wrote in his perspective. “However, humans also engage with, bond, and cooperate at multiple levels across intersecting groups. The group relationships of humans are nuanced, diverse, and flexible. This flexibility enables deep cooperation but also underlies acts of violence. Humans must learn from studying the group-based behavior of other species, both in war and at peace, while remembering that their evolutionary past does not determine their future.
“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,” said Sandel. “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.”
DOI: Science, 2026. 10.1126/science.adz4944 (About DOIs).
