Since 1995, scientists have tracked a huge group of chimpanzees living in the forests of Uganda in east Africa. 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. A fortnight ago, 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 April 3, has cast doubt on whether the research will be able to 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, US, 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 ever met up, they would have a pleasant reunion.

“They start grooming each other, they start socialising, they start acting as one,” said 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 neighbouring group of chimpanzees, driving them away and expanding the Ngogo range.

But on June 25, 2015, the scientists witnessed something strange.

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,” 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, 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.

The Ngogo conflict showed that what Goodall had not witnessed was not a fluke. It also showed that chimpanzees could fight on a far bigger scale.

Sandel, who now teaches at the University of Texas at Austin, US, became obsessed with documenting the Ngogo conflict and understanding how it happened. “I feel like a war correspondent,” 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. 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.)

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 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,” Sandel said.

And in 2014, five such adult males in the Ngogo group died.

NYTNS