Chimpanzee group’s violent rupture hints at evolutionary roots of war

A formerly unified group of wild chimpanzees has fractured into two distinct groups, resulting in persistent conflict and increased violence. This division, researchers suggest, implies that warfare may be an intrinsic aspect of human nature, rather than a development of more complex cultural systems.

Aaron Sandel from the University of Texas at Austin and his team have spent 24 years studying social networks, 10 years on GPS-based movement tracking, and 30 years gathering demographic data on the chimpanzees (Pan troglodytes) in Uganda’s Kibale National Park.

“We want to be especially cautious with the words we use,” says Sandel. “These are chimps. War and civil war are terms that have a special significance for humans. What we saw isn’t civil war. But it does have important parallels. Notably, the shifting group identities that are underlying the lethal conflict.”

Chimpanzees are notorious for inflicting severe violence on one another, but this aggression is usually directed towards outsiders or infants sired by competing males.

The population of Ngogo chimpanzees, numbering between 150 and 200, is among the largest of these primates, which, alongside bonobos (Pan paniscus), are humans’ closest relatives.

From 1995 to 2015, the group was seen as cohesive, functioning as a collaborative unit with fission-fusion dynamics, according to scientists. This means, like all chimpanzee groups, they form temporary alliances during the day while roaming a shared territory, regrouping in the evening.

While females typically leave their group at adolescence, males remain for life. Before 2015, adult males in Ngogo would socialize with females, hunt together, and participate in joint territorial patrols.

However, on 24 June 2015, a meeting in the group’s central territory led to the central cluster of Ngogo chimps driving away the western cluster.

From that day, unity began to deteriorate; by 2018, the groups had permanently separated. Between 2018 and 2025, the western group conducted 24 attacks, resulting in the deaths of at least seven adult males and 17 infants from the central group.

According to Sandel, it remains uncertain which group initially sparked the conflict, even though the central group first chased the western group. “Both the western and central groups were actively involved in territorial behaviour as the new groups emerged and the split was complete,” says Sandel. “But the western group became the aggressors, and they are responsible for all of the lethal attacks.”

Researchers propose several factors that might have contributed to the group’s disintegration. These include disputes over food resources, the deaths of five key males and a female in 2014 which likely weakened social ties, a change in the alpha male, and a respiratory illness outbreak.

This illness claimed the lives of 25 Ngogo chimp members in January 2017, including the last two males bridging the western and central groups. This tragedy seemingly extinguished any remaining hope for reconciliation.

Sandel and his team believe that the unfolding of this conflict could offer insights into the evolutionary origins of human warfare. Current human conflicts are often explained by ethnic, religious, or political differences, but this focus on cultural factors might overlook social dynamics apparent in our closest animal relatives, they suggest.

“In some cases, it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace,” the team writes in their research paper.

Maud Mouginot at Boston University in Massachusetts notes two main theories about how human warfare evolved. One suggests war is a relatively recent development tied to agriculture and nation-states, while the other posits that its origins are much older. “I think the Ngogo data make a strong contribution to the deep-rooters’ case,” says Mouginot.

“This study demonstrates that the social dynamics of group fissioning and subsequent war can happen without any of the cultural markers that we often attribute human war to – differences in beliefs, language, religion, dress,” says Luke Glowacki, also at Boston University.