Aaron Sandel can tell you when it all started.
Ngogo chimpanzee project co-director was observing a group of monkeys on June 24, 2015 in Kibale National Park, Uganda, where the project is located, when he suddenly noticed that the chimpanzees had gone silent. Many began to smile, facial expressions indicating they were nervous. Others began to touch each other for reassurance.
In the distance, several chimps could be heard, but this was not unusual. For at least two decades, the Ngogo chimpanzees formed a large community of more than 200 individuals living together at their peak.
But when Sandel saw more chimpanzees emerge, the primates did not regroup in their typical style of loud screeching, patting on the back and holding hands. Instead, many of the chimpanzees began to run, perplexing Sandel and fellow researcher John Mitani. A once close group of chimps were suddenly treating each other like strangers.
“I remember asking John, ‘What’s going on?’ He said, ‘I don’t know,'” Sandel recalled. “And that also stuck with me, because this is one of the world’s experts on chimps. He studied these chimps for two decades. But we were seeing something new.”
Ngogo chimpanzees smile and reassure each other while listening to other chimpanzees in 2015. – Aaron Sandel
Sandel credits that day as the beginning of the split, when the large group began to organize into two groups now known as the Western and Central Chimps. “I think it planted the seeds of polarization, which 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 raids leading to deadly attacks on adults and infants several times a year. Now, a new study documents what researchers consider a chimpanzee “civil war,” a rare event estimated to occur every 500 years and observed only once before.
The findings, published April 9 in the journal Science, provide a unique glimpse into how changing social relationships can lead to conflict between nonhuman groups of animals, an elusive phenomenon in the wild, but one that could highlight the role of interpersonal relationships in human conflict, the researchers say.
‘Civil War’ Among Primates
Chimpanzees are territorial by nature. Regularly, groups of individuals – usually men – would gather and perform patrols near the border to check for rival group members. If they find any outsiders, they will attack and sometimes kill another chimpanzee.
The Ngogo Chimpanzee Project was founded in 1995 by John Mitani, now professor emeritus of anthropology at the University of Michigan. From the beginning, experts have debated whether an unusually large group of chimpanzees would split. Researchers initially didn’t believe they would, as there was no sign of a fracture at the time. Because the protected area they occupied was rich in food and trees, the forest was also well equipped to support large groups, said lead author Sandel.
But after that day in 2015, the chimps immediately split into the western and central clusters, named after the regions the chimps split into. Now they are patrolling to keep each other away.
Western chimps are more aggressive than central chimps; Between 2018 and 2024, the group conducted 15 patrols every four months and killed an average of one adult and two infants per year from the central group, according to the study. Western chimpanzees appear to have an advantage over central chimps, possibly because of their early sociability, Sandel said.
The first fatal attack was in 2018 on a young adult male named Errol. The chimp was attacked by five adult western males feeding on a fig tree in the middle of the Ngogo area. When Sandell joined the project in 2012, Errol was 10 years old and the subject of his dissertation.
Before the split, the chimps were able to cross the entire territory, but now their land is split in two, bordering near the center, Sandel said. The frontier is always shifting, he added, and it appears that western chimps are currently pushing it further east.
The second fatal attack, in 2019, occurred when Sandel and other researchers were feeding several chimpanzees inside a large tree. A group of western chimpanzees burst in and startled them, causing chaos.
The central chimps scattered as the western chimps climbed the tree. The researchers, unaware that the group was permanently divided, observed three adult males surround a chimpanzee from the central group and begin attacking it. Sandel immediately recognized the victim as Bassey, a 33-year-old member of the Ngogo group.
As the chimps piled on top of him, an adult female chimp, Aretha, tried to protect Basie from her attackers, but was quickly chased away. When the chimps finally relented, Basie was brought back home by a male chimp in his 50s named BF, who seemed to have grown close to Basie over the years. Basi died the next day.
BF (left) was the last male to pass between central and western chimpanzees and was closely related to Basi. – Aaron Sandel
So far, the death toll is seven adults and 17 infant chimps from the central cluster, with another 14 chimps missing that may have been victims of the fatal attack, the study found.
“It’s really sad to see these chimpanzees kill each other, especially when I see chimpanzees being killed that I know very well. I sometimes feel like a war correspondent,” Sandel said. While researchers are currently studying acts of violence, they’re also getting opportunities to study other chimp emotions, such as empathy, as well as acts of heroism and friendship, he added.
“I think we’re really tapping into what it means to be a chimpanzee,” Sandel said. “Seeing these relationships change so dramatically, we’re gaining insights into chimps that we wouldn’t normally have from observation alone, and a window into their minds and their emotions.”
What Chimpanzees Can Teach Us About War
The late primatologist Jane Goodall first witnessed the chimpanzee ‘civil war’ during her research on chimpanzees in Tanzania’s Gombe National Park in the 1970s. Suddenly, chimps that had grown up together were splitting up and killing each other in what Goodall and colleagues dubbed the “Four Years’ War” and the darkest time in Gombe’s history.
While Ngogo researchers aren’t sure why the war broke out between their groups, they have a few theories. Similarly, in Goodall’s group of chimps, the community experienced a change in dominance hierarchy, which seemed to immediately affect how the chimps interacted with each other, Sandel said. Ngogo researchers speculated that several chimpanzee deaths from unknown causes in 2014, a shift to alpha males in 2015, and a respiratory epidemic in 2017 weakened social bonds and led to group breakups.
“Careful documentation of this rare phenomenon over years provides invaluable insight into intergroup conflict,” said Katie Slocombe, a comparative psychologist and professor of psychology at the University of York in the United Kingdom. Slocombe was not involved with the new study.
“This was the largest chimpanzee community, so maintaining effective relationships with many individuals can be challenging for community members,” Slocombe said in an email. She added that this new information on chimpanzee groups could add to our understanding of how interpersonal relationships and other environmental factors contribute to human conflict.
The study’s authors argued that because chimpanzees lack cultural markers that are largely attributed to causing human warfare, such as religion or ethnicity, studying chimpanzees could be beneficial for learning more about our own species and the role of relational dynamics in human warfare, Sandel said.
There are two possible possibilities for how the war ends, Sandel added. The first is that the central group will organize itself in a way that allows them to defend their territory and borders against the western group, and deadly attacks will become less frequent. The second possibility is similar to what Goodall observed in Gombe: the stronger group will kill all members of the weaker group.
“There is a third one, which seems extremely unlikely, but there could be some reunification between the groups,” Sandel said. “For everything I know about chimp behavior, I don’t see how that’s possible, but I also know enough about chimps to never wonder what they’re capable of.”
Taylor Nicioli is a freelance journalist based in New York City.
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