Violent encounter between two groups of Ngogo chimpanzees
Aaron Sandel
A harmonious group of wild chimpanzees has split into two, leading to unbearable conflict and violence. Researchers say the rift shows that human wars are a deep part of our nature, not something that has emerged only recently as our culture has become more complex.
Aaron Sandel At the University of Texas at Austin and colleagues analyzed 24 years of social networks, 10 years of GPS-based ranging, and 30 years of demographic data on chimpanzees (pan troglodytes) in Kibale National Park, Uganda.
“We wanted to be particularly careful about the words we use,” says Sandel. “These are chimpanzees. War and civil war are words that have special significance for humans. What we saw is not a civil war. But there are important similarities. In particular, the changing group identities that are behind the deadly conflict.”
Chimpanzees are known to inflict horrific violence on each other, but this is usually reserved for outsiders or infants born to rival males.
The Ngogo chimpanzees, with a population of 150 to 200, were among the largest known groups of primates, along with bonobos (pan paniscus), are humans’ closest relatives.
Scientists say that between 1995 and 2015, the group was thought to be cohesive, living as a cooperative unit and exhibiting fission-fusion dynamics. This means that, like all chimpanzee populations, they form temporary associations as individuals throughout the day and move to a shared common area before coming back together in the evening.
Females mostly disperse in adolescence, while males remain in their group throughout their lives. Before 2015, adult males in Ngogo associated with females in groups, hunting together and cooperating in territorial patrols.
Then, on June 24, 2015, group members met in the middle of their territory. One group of Ngogo chimps, known as the central group, drove out another group, known as the western group.
From this day onwards harmony began to break down; By 2018, the two groups had permanently split. Between 2018 and 2025, the Western group carried out 24 attacks, killing at least seven mature men and 17 infants from the other group.
Chimpanzees of the Western Group on patrol
Aaron Sandel
Sandel says it is unclear which group initiated the conflict, even if it was the central chimpanzees who first pursued the western chimpanzees. Sandel says, “As new groups emerged and partition became complete, both western and central groups were actively involved in regional behavior.” “But Western groups became aggressive, and they are responsible for all the deadly attacks.”
Researchers suggest that several factors may be responsible for the breakdown. The first may have been conflict over food resources, then the deaths of five important men and one woman in 2014, which likely weakened social bonds. After this there was a change in the alpha male. The final blow to the prospects for peace was an outbreak of respiratory disease.
The disease resulted in the death of 25 members of the Ngogo chimpanzee group in January 2017, including the last two males spanning both the western and central groups. In the wake of this tragedy, it appears that the last hopes for reconciliation have vanished.
Sandel and his colleagues say the way the conflict unfolded could have implications for understanding the evolutionary roots of human conflict. Polarization and war among humans today is usually attributed to ethnic, religious or political divisions. But focusing solely on these cultural factors ignores social processes that also exist in our closest animal relatives, the researchers say.
“In some cases, small, daily acts of reconciliation and reunion between individuals provide our opportunities for peace,” the team writes in their paper.
Maud Mouginot Boston University in Massachusetts says there are broadly two camps when it comes to predicting how warfare evolved and originated among humans. The first argues that war is a relatively recent innovation rooted in human culture that emerged with the rise of agriculture and nation-states. The other camp argues that the roots of war go much further back in our evolution. “I think Ngogo makes a strong contribution in terms of data deep-rooters,” says Mouginot.
“This study demonstrates that the social dynamics of group division and subsequent warfare can occur without any of the cultural markers to which we often attribute human warfare – differences in belief, language, religion, dress,” says. Luke GlowackiAlso at Boston University.
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