Carl Zimmer reports:

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. [Continue reading…]