Since 1995, scientists have tracked a huge group of chimpanzees living in the forests of Uganda. The sustained research, featured in the 2023 documentary “Chimp Empire,” has led to profound insights about our closest living relatives and, by extension, our own ancestors.
In one line of research, the scientists studied deep bonds among male chimpanzees in the Ngogo group, named for a hill in the Kibale National Park where they live. The males spend years hunting together and patrolling the boundaries of their range. The female Ngogo chimps, scientists discovered, may experience menopause, never previously documented in primates aside from humans.
Now scientists are finding darker parallels to humans in the lives of the Ngogo chimpanzees. On Thursday, a group of researchers reported that the Ugandan chimps are locked in a primate version of civil war. Two factions split about a decade ago and have been engaged in a highly lethal conflict ever since.
Scientists have never seen such widespread, long-running bloodshed among chimpanzees. Further studies may shed light on the roots of warfare in our own species, although the Trump administration’s proposed budget, released on Friday, has cast doubt on whether the research will continue.
When scientists first started tracking the Ngogo chimpanzees, the first thing that struck them was the sheer number of apes: over 100 across a territory of about 10 square miles.
“They were everywhere,” said John Mitani, a primatologist at the University of Michigan and one of the founders of the Ngogo Chimpanzee Project.
The group continued to grow over the years, ultimately rising to 200 apes who wandered the forest in small bands in search of food. If two bands met up, they would have a pleasant reunion.
“They start grooming each other, they start socializing, they start acting as one,” said Dr. Mitani.
A chimpanzee might leave one band and join another several times a day. But each chimpanzee had its closest ties to one of three communities — known as the Western, Central or Eastern clusters.
Originally, these clusters were a unified group. Males and females from different clusters mated. The males all formed border patrols and hunting parties. They even fought together against a neighboring group of chimpanzees, driving them away and expanding the Ngogo range.
But on June 25, 2015, the scientists witnessed something strange.
Dr. Mitani was following chimpanzees from the Central cluster when they suddenly ran down a slope. He caught up with them, along with Aaron Sandel, a graduate student who had been following apes from the Western cluster.
The two researchers watched the two bands reunite. It was not a happy scene. “All hell broke loose,” Dr. Mitani recalled.
The apes screamed and fought. The Western chimpanzees ultimately fled, with the Central chimpanzees in hot pursuit.
At first the researchers thought that confrontation was a fluke. But over the next few years, Dr. Mitani and his colleagues witnessed more violence between the Central and Western clusters.
It became so common that young chimpanzees got nervous just hearing the calls of mature males in the distance. By 2018, the confrontations were turning deadly.
Only once before had primatologists observed a group of chimpanzees so violently split.
Jane Goodall and her colleagues documented confrontations among about two dozen chimpanzees in the mid-1970s in Tanzania. But in the years that followed, no one saw other such conflicts.
Researchers wondered if the battles among Dr. Goodall’s chimpanzees were unnatural. Some noted that she had provided bananas to the animals for years. Perhaps they had fought over the food in a way they wouldn’t have if left alone.
The Ngogo conflict showed that Dr. Goodall had not witnessed a fluke. It also showed that chimpanzees could fight on a far bigger scale.
Dr. Sandel, who now teaches at the University of Texas at Austin, became obsessed with documenting the Ngogo conflict and understanding how it had happened. “I feel like a war correspondent, in a way,” he said. “I want to be there to see it, but it’s sad. I’ve seen so many dead bodies of chimps.”
The researchers found that the earliest hints of tensions had emerged in 2014. Over the next few years, the chimpanzees in the Western and Central clusters had interacted less and less. They only mated within their own clusters. By 2018, the clusters were occupying different parts of the forest.
Then the killing started, and it has not stopped since. Dr. Mitani said that researchers have now observed 28 deaths among the Ngogo apes, including those of 19 infants. The numbers far exceed anything seen before among chimpanzees.
What makes the conflict even more extraordinary is how lopsided it is.
At the beginning, the Central cluster was much bigger than the Western cluster. But chimpanzees from the Western cluster have become far more aggressive, and every victim of the conflict so far has belonged to the Central cluster.
(The Eastern cluster is allied with the Central cluster but seems to be sitting out the fight.)
Dr. Sandel and his colleagues have no idea when the battles will simmer down. It is conceivable that the Western cluster may ultimately eliminate the Central cluster.
And the researchers are still trying to figure out what set off the conflict in the first place. “All of a sudden, yesterday’s friend becomes today’s foe,” said Dr. Mitani.
Within any group of chimpanzees, violence will flare from time to time — when apes converge on a tree full of fruit, for example, or when lower-ranked males vie to replace an old alpha male.
But this aggression can be dampened by the friendships that form over years. Some chimpanzees are especially social, jumping between many cliques. “They’re these important social bridges,” Dr. Sandel said.
In 2014, five adult males died, perhaps because of disease. Dr. Sandel speculated that these deaths ripped away some of the bridges that previously held the Ngogo groups together. Low-level conflicts blew up into something akin to civil war.
The Ngogo conflict could offer a glimpse at the kind of violence that might have flared up in our ancient forebears, given that chimpanzees and humans descend from common ancestors that lived about six million years ago.
“These findings tell us indeed that these civil-war-like types of conflicts were possible in the course of human evolution,” said Sylvain Lemoine, a primatologist at the University of Cambridge who was not involved in the study.
The Ngogo chimpanzees show how our ancestors could have gotten dragged into years of lethal fighting without ideology or cultural identity — let alone the language to talk about them.
Instead, shifting social bonds might have been enough to light the fire.
As bleak as that lesson may be, Dr. Sandel sees a silver lining: Caring for social bonds can help counter violence. “If that’s the case, then conflict management in our own lives becomes a civic duty to bring about a more peaceful world,” he said.
But Dr. Sandel and his colleagues may not be able to continue exploring ideas like these by following the Ngogo chimpanzees.
In the White House’s proposed budget for 2027, funds for the National Science Foundation would be cut by more than half.
The agency itself has released a detailed budget request that would eliminate the division that provides $150 million to studies of behavioral and cognitive sciences, including the Ngogo Chimpanzee Project. Congress often ignores the administration’s budget proposals and in the past has continued to fully fund science agencies.
But if the proposed cuts at the National Science Foundation came to pass, “the loss of long-term field sites like Ngogo would undo decades of investment, work and scientific progress,” said James Brooks, a primatologist at the German Primate Center who was not involved in the new study.
A spokesman for the foundation declined to comment on the proposed cuts. The budget request cites a “constrained fiscal environment” and “woke and weaponized grant programs.”
Carl Zimmer covers news about science for The Times and writes the Origins column.
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