KIBALE NATIONAL PARK — The largest known community of wild chimpanzees, the Ngogo group in Uganda's Kibale National Park, has split into two hostile factions that have engaged in lethal violence since 2015, according to a study published in the journal Science. The study, drawing on more than 30 years of observations, described the intergroup violence as a primate equivalent of a civil war.

At its peak, nearly 200 individual chimpanzees were members of the Ngogo group and lived in smaller subgroups called clusters. Males and females from different clusters intermingled, mated, hunted together, and worked together to fight off outside groups. Researchers took videos of males from different clusters holding hands.

The natural deaths of five adult males and one adult female in 2014 may have weakened social networks in the Ngogo group. In June 2015, primatologist Aaron Sandel observed that chimpanzees in the Western cluster quieted and touched each other for reassurance when they heard chimpanzees from the Central cluster nearby. The Western cluster chimpanzees then fled as Central cluster chimpanzees chased them. The two clusters avoided each other for six weeks following the incident, and by 2018 they had become essentially separate groups.

The first observed lethal attack victim was an adolescent male from the Central cluster named Errol. Over the next seven years, the Western group killed at least six adults and 17 infants from the Central cluster. Intergroup fighting continues as of 2026. The study's authors suggested that group size, competition for food, and male-to-male competition may have contributed to the split and violence.

"If in chimpanzees, we can see this conflict and lethal violence occur in the absence of all these aspects of human behavior that we often attribute to civil war, then I wonder to what extent are the interpersonal relationships and behaviors actually more important than we realize in humans," said Sandel, a primatologist at the University of Texas at Austin. "I want to avoid that in my own life."

"I think it's clear from this study and from other studies of chimps and other animals that you can get these kinds of conflicts without a lot of things that we think about as being the source of conflict in humans," said Michael Wilson, a primatologist at the University of Minnesota. "Lions don't have religion and political parties or ideologies. Neither do wolves or ants for that matter."

The Ngogo split echoes an earlier episode in chimpanzee research. In the mid-1970s, primatologist Jane Goodall and her colleagues witnessed a chimpanzee group in Gombe National Park split into two factions and turn on each other. Chimpanzees that had previously intermingled peacefully were systematically killing each other. "I used to think, 'Well, they're very much like people but nicer,' and then I realized that when opportunity arises, they have this nasty, brutal side to them just like we do," Goodall said.