A violent encounter between the two factions of Ngogo chimpanzees
Aaron Sandel
A once harmonious group of wild chimpanzees has split into two, leading to intractable conflict and escalating violence. Researchers say the rift suggests that human wars are a deeply rooted part of our nature, rather than something that emerged recently as our culture became more complex.
Aaron Sandel at the University of Texas at Austin and his colleagues analysed 24 years of social networks, 10 years of GPS-based ranging and 30 years of demographic data on the chimpanzees (Pan troglodytes) in Kibale National Park, Uganda.
“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 well known for perpetrating horrific violence on each other, but typically this is reserved for outsiders or infants born of rival males.
The Ngogo chimpanzees, with a population ranging from 150 to 200, were among the largest known groups of the primates, which, along with bonobos (Pan paniscus), are the closest relatives of humans.
Between 1995 and 2015, the group was regarded as cohesive, living as a cooperative unit and displaying fission-fusion dynamics, say the scientists. This means, like all chimpanzee populations, they form temporary associations throughout the day as individuals and move over a shared common territory before coming back together in the evening.
Females mostly disperse at adolescence, while males remain in their group for their entire lives. Prior to 2015, adult males at Ngogo associated in groups along with females, hunted together and cooperated in territorial patrols.
Then, on 24 June 2015, members of the group met in the middle of their territory. One cluster of the Ngogo chimps, known as the central group, chased away the other, known as the western group.
From this day on, cohesion began to break down; by 2018, the two groups had split permanently. Between 2018 and 2025, the western group made 24 attacks, killing at least seven mature males and 17 infants in the other group.
Chimpanzees from the western group on patrol
Aaron Sandel
Sandel says it is unclear which group initiated the conflict, even though it was the central chimps that first gave chase to the western chimps. “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.”
The researchers suggest that several factors may have led to the breakdown. The first may have been conflict over food resources, then the deaths of five important males and a female in 2014, which probably weakened social bonds. This was followed by a change in the alpha male. The final blow to the prospects for peace was a respiratory illness outbreak.
This illness resulted in the deaths of 25 members of the Ngogo chimps in January 2017, including the last two males that straddled both the western and central groups. It was in the wake of this tragedy that the last hopes for reconciliation appear to have been lost.
Sandel and his colleagues say the way the conflict unfolded may have implications for understanding the evolutionary roots of human conflict. Polarisation and war occurring among humans today are typically attributed to ethnic, religious or political divisions. But focusing entirely on these cultural factors overlooks social processes that are also present in our closest animal relatives, say the researchers.
“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 says there are broadly two camps when it comes to speculating how war evolved and arose among humans. The first contends that war is a relatively recent innovation rooted in human culture that emerged alongside 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 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.
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