One more brutal war ( Between Chimpanzee groups ! )

What plunged these chimps into civil war? A new study traces the breakdown 

From a Science news feature  by Jon Cohen ( April 2026) 

('Science' is one of the most reputed journals in the world) 

Fascinating article about a research study that has so many layers and nuances and possibly some learnings for us humans ! 

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Chimpanzees regularly fight viciously over food, mates, and rank, but only rarely do these brawls spill over into a broader civil war. Now, a study tracing 30 years of chimp behavior in Kibale National Park in Uganda reveals how and why such internecine violence erupts. The study, described today in Science, shows how in chimps—and perhaps humans—tensions in once-peaceful groups can grow into deadly violence, even without resource shortages or cultural divisions to fuel them.

“This study demonstrates beautifully the analytical power gained through sustained research,” says Roman Wittig, a primatologist at the Max Planck Institute for Evolutionary Anthropology who studies chimps in Ivory Coast’s Taï National Park. Primatologist Richard Wrangham, who in 1987 kicked off his own study of a neighboring chimp community in Kibale, says the new study is “terrific” and both clarifies motivations for human warfare and spotlights how we differ from one of our closest relatives.

Researchers in 1995 began to study the Kibale chimpanzees in a densely forested area called Ngogo, carefully tracking their movements and social networks. At one point, there were more than 200 individuals, the largest community of chimps ever studied. They lived in two main social groups, designated as Central and Western, that peacefully intermingled, with many cross-group matings.

But on a fateful day in June 2015, some chimps from the two clusters met up near the center of their territories, and the Central chimps chased the Western ones away. Afterward the two clusters avoided each other, and reproduction between the groups stopped. Western males regularly began to patrol in Central territory, looking to expand their domain.

In 2017, the tensions boiled over. A group from the Western cluster attacked and injured the Central group’s alpha male. Between 2018 and ’24, the researchers estimate, males in the Western group killed seven adult males and 17 infants in the Central group. Yet even though they were larger in number, the Central group males curiously never ganged up to kill any of the Western chimps.

What sparked the violence? Animals that turn from friend to foe are often competing for scarce food. But at Ngogo, “there was still a lot of food in this forest,” says Aaron Sandel, a primatologist at the University of Texas at Austin and the study’s first author. 

One contributing factor, says co-author John Mitani, a primatologist at the University of Michigan who helped establish the Ngogo research site, is that “the Ngogo chimps were victims of their own success. The group continued to grow and grow and grow, and it reached the size that individuals couldn’t pull together anymore.”

The social bonds between the males may also have frayed as the community lost some of its critical peace brokers: In 2014, five adult male chimps died within about 1 month of one another, possibly from disease. “Some of those adult males were important connectors,” Sandel says.

Reproductive competition could have played a role in the fission. The Central group was larger, and for unknown reasons, those males appeared to have lost access to females in the Western cluster before the war broke out. “I can easily imagine a situation that, if you’re a Central male, you’re saying to yourself, ‘Wait a second, we’ve been cut off from these females, maybe now’s the time to try to do something about that,’” Mitani says. “But those guys badly miscalculated, because they’re the ones who have been victimized and have suffered all the killings, which is another unusual aspect of the story.”

As the researchers note in their report, chimps aren’t divided by religion, language, politics, and ethnicity. “You do not need ideology to generate hostilities,” says Wrangham, an professor emeritus at Harvard University. “The motivations for warfare are much more concerned with our biology than people would have believed a long time ago.”

In the early 1970s, Wrangham, whose book, Demonic Males: Apes and the Origins of Human Violence, explores the link between apes and the origin of human violence, helped document a similar fission in the chimp community in Gombe National Park in Tanzania that Jane Goodall made famous. That led to what researchers called “the 4-year war,” in which one group killed six males and one female from a second group. The lead-up to violence in Kibale “fits entirely in my mind with what happened in Gombe,” Wrangham says, “but it’s much more informative in many ways.”

Critics of the Gombe work have noted that the researchers gave the chimps bananas at a feeding station, altering their behavior—and some have contended that it drove the lethal attacks. Wrangham acknowledges the provisioning had a “muddling impact” on the results. But he maintains that the division of the Gombe community into two warring factions “was very clear” and catalyzed by a rift between two ranking males—which he suspects played an important role in Ngogo, too.

Primatologist Catherine Crockford, who co-directs the Taï Chimpanzeee Project with Wittig, cautions that the “impressive and insightful study” at Ngogo does not rule out the possibility that cultural differences increased tensions between the groups. Even in the absence of language, chimpanzees have distinctive ways to communicate, which Crockford studies. As she has shown, groups of chimps can learn specific pant hoots that can reinforce bonds, and she wonders whether shared vocalizations might have gradually “fed increasing hostilities.”

The lessons for human conflict only go so far. Wrangham notes that unlike humans, chimps do not seem to commit revenge killings, likely because they don’t have language. “In humans, the first thing that happens when a member of your community, your band, your village, gets killed, everybody gets together and says, ‘OK, well, what are we going to do about it?’” Wrangham says. “You don’t have revenge killings in chimps, because in order to be able to conduct revenge, you need to discuss a plan.”

Mitani says the study ultimately helps explain chimp behavior more than our own. “One of the unusual things about us as humans is that we’re an incredibly pro-social and cooperative species,” he says. “Instead of attacking our neighbors, we go out of their way to help them, even if they are complete strangers. That’s the lesson I learned from all this. I try to be optimistic, especially in these times as the world becomes increasingly polarized.”