Aaron Sandel can pinpoint exactly when it all started.
On June 24, 2015, the co-director of the Ngogo Chimpanzee Project was observing a herd of great apes in Uganda’s Kibale National Park, where the project is based, when he suddenly noticed that the chimpanzees were silent. Some of the people started frowning, their expressions showing they were nervous. Some started touching each other for reassurance.
I could hear more chimpanzees in the distance, but that was nothing unusual. For at least 20 years, Ngogo chimpanzees formed sizable communities, with over 200 living together at their peak.
But when Sandel saw more chimpanzees appear, the primates didn’t reunite in the typical way of yelling, patting each other on the back and holding hands. Instead, many of the chimpanzees started running, perplexing Sandel and fellow researcher John Mitani. A once-close-knit group of chimpanzees suddenly started treating each other like strangers.
“I remember asking John, ‘What’s going on?’ and he said, ‘I don’t know,'” Sandel recalled. “And that stuck with me, too, because he’s one of the world’s experts on chimpanzees. He had been studying these chimpanzees for 20 years. But we were seeing something new.”
Sandel believes that day marked the beginning of the split, when the large group began to organize into two factions, now known as Western Chimpanzees and Central Chimpanzees. “I think that planted the seeds of polarization and ultimately led to the collapse of the group,” said Sandel, who is also an associate professor of anthropology at the University of Texas at Austin.
Since that day, violence between the two groups has escalated, with attacks involving deadly attacks on adults and children occurring several times a year. Now, a new study has documented what researchers believe to be a chimpanzee “civil war,” a rare event estimated to occur once every 500 years and only observed once before.
The findings, published April 9 in the journal Science, offer a unique glimpse into how changes in social connections can drive conflict among groups of non-human animals, and the researchers say they may highlight the role of interpersonal relationships in human conflict, an elusive event in the wild.
Chimpanzees are naturally territorial. Periodically, multiple individuals (usually men) come together and conduct patrols near the border to check for members of rival groups. When they spot an outsider, they attack and sometimes kill other chimpanzees.
The Ngogo Chimpanzee Project was co-founded in 1995 by John Mitani, currently professor emeritus of anthropology at the University of Michigan. Experts have debated since the beginning whether this unusually large group of chimpanzees will split. At first, researchers didn’t believe it would happen because there were no signs of a fracture at the time. Lead author Sandel said the forest was well-equipped to support large groups because the reserves they occupied were plentiful with food and trees.
But after that day in 2015, chimpanzees quickly split into western and central groups. The name comes from the territory divided by chimpanzees. Now they are on patrol to keep each other away.
Western chimpanzees are more aggressive than central chimpanzees. According to the study, between 2018 and 2024, the group organized up to 15 patrols every four months, killing an average of one adult and two infants per year from the central group. Western chimpanzees appear to have an advantage over central chimpanzees, Sandel said, perhaps due to early cohesion.
The first fatal attack occurred in 2018 against a young adult man named Errol. The chimpanzee was attacked by five adult Western males that were feeding in a fig tree near the center of Ngogo’s territory. Errol was about 10 years old when Sandel joined the project in 2012 and was the subject of his doctoral thesis.
Before chimpanzees split, they were able to traverse their entire territory, Sandel said, but now the territory is split into two, with a border near the center. Borders are constantly changing, he added, and it now appears that western chimpanzees are succeeding in pushing the border further east.
The second fatal attack in 2019 occurred while Sandel and other researchers were observing several chimpanzees feeding in a large tree. A herd of Western chimpanzees rushes in and surprises them, causing chaos.
When the western chimpanzee climbed the tree, the central chimpanzees scattered. The researchers, unaware at the time that the group had permanently split, observed three adult males corner the central group of chimpanzees and begin attacking them. Sandel immediately recognized the victim as Basie, 33, a member of the Ngogo group.
When the chimpanzee piled on top of Basie, Aretha, an adult female chimpanzee, tried to protect Basie from the attackers, but was quickly chased away. When the chimpanzees finally relented, Basie returned home, accompanied by a male chimpanzee named BF, over 50 years old, who had apparently been friends with Basie for many years. Basie died the next day.
Research has so far confirmed the deaths of seven adult chimpanzees and 17 infants in the central cluster, with 14 more chimpanzees missing who may have been victims of the deadly attack.
“It’s really sad to see chimpanzees killing each other, especially chimpanzees that I know very well. I feel like a war correspondent sometimes,” Sandel said. Researchers are currently studying violent behavior, but they also have the opportunity to study other chimpanzee emotions such as empathy, heroism and friendship, he added.
“I feel like we’re experiencing something that gets to the core of what it means to be a chimpanzee,” Sandel said. “By watching these relationships change dramatically, we can gain insights into chimpanzees and into their minds and emotions that we normally wouldn’t get through observation alone.”
The late primatologist Jane Goodall observed the first chimpanzee “civil wars” while studying chimpanzees in Tanzania’s Gombe National Park in the 1970s. This war, which Mr. Goodall and others called the “Four Years’ War,” was the darkest period in Gombe’s history.
Ngogo researchers are not sure why the war started between their groups, but they have several hypotheses. Like Goodall’s chimpanzee group, this community also experienced a change in dominance hierarchy, which seemed to have an immediate impact on how the chimpanzees interacted with each other, Sandel said. Ngogo researchers hypothesize that the unexplained deaths of several chimpanzees in 2014, an alpha male shift in 2015, and a respiratory disease outbreak in 2017 weakened social bonds and caused the group to split.
Katie Slocombe, a comparative psychologist and professor of psychology at the University of York, UK, said: “Carefully documenting this rare event through many years of longitudinal data provides valuable insight into intergroup conflict.” Slocombe was not involved in the new study.
“It was the largest known chimpanzee community, which may have made it difficult for community members to maintain effective relationships with so many individuals,” Slocombe said in an email. She added that this new information about chimpanzee groups could further deepen our understanding of how interpersonal relationships and other environmental factors contribute to human conflict.
The study authors argued that studying chimpanzees could be useful for learning more about our own species and the role of relational dynamics in human warfare, because chimpanzees lack cultural markers, such as religion or ethnicity, that are thought to be the main drivers of human warfare, Sandel said.
Sandel added that there are two possibilities for how the war will end. The first is that the central group will be organized in such a way that it will be able to better defend its territory and borders against Western groups, resulting in less frequent deadly attacks. The second possibility is similar to what Goodall observed in Gombe. That is, the stronger group will kill all members of the weaker group.
“It’s very unlikely that there will be a third round, but there may be a reunion between the groups,” Sandel said. “With everything I know about chimpanzee behavior, I don’t know how that’s possible, but I know enough about chimpanzees that I’m not really surprised by their abilities.”
Taylor Nicioli is a freelance journalist based in New York City.
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