By Taylor Nicioli, CNN
(CNN) — Aaron Sandel can pinpoint when it all started.
The codirector of the Ngogo Chimpanzee Project had been observing a group of apes on June 24, 2015, in Uganda’s Kibale National Park, where the project is located, when he suddenly noticed the chimps fall silent. Several began grimacing, a facial expression indicating they were nervous. Others started touching each other for reassurance.
In the distance, more chimps could be heard, but it wasn’t anything unusual. For at least two decades, the Ngogo chimpanzees had formed a considerably large community, with more than 200 individuals living together in harmony at its peak.
But when Sandel saw more chimps appear, the primates did not reunite in their typical fashion of loud screaming, pats on the back and holding hands. Instead, a number of chimpanzees took off running, leaving Sandel and fellow researcher John Mitani puzzled. The once close-knit 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’d studied these chimps for two decades. But we were seeing something new.”
Sandel credits that day as the beginning of the split, when the large group began to organize into two factions now known as the Western and Central chimps. “I think it planted the seeds of polarization, which resulted in the group’s downfall,” said Sandel, who is also an associate professor of anthropology at the University of Texas at Austin.
Since that day, the violence between the two groups has grown, with raids resulting in lethal attacks on adults and infants occurring several times a year. Now, a new study documents what the researchers deem as a chimpanzee “civil war,” a rare occurrence that is estimated to happen every 500 years and has only been observed once before.
The findings, which were published April 9 in the journal Science, provide a unique glimpse into how shifting social ties can cause strife among nonhuman groups of animals, an elusive event in the wild, yet one that could highlight the role of interpersonal relationships in human conflict, researchers say.
‘Civil war’ among primates
Chimpanzees are naturally territorial. Regularly, a group of individuals — typically male — will gather and perform patrols to check for rival group members near the borders. If they find any outsiders, they will attack and sometimes kill the other chimp.
The Ngogo Chimpanzee Project was cofounded in 1995 by John Mitani, who is now an emeritus professor of anthropology at the University of Michigan. From the start, experts have debated whether the unusually large group of chimps would split. Researchers did not initially believe they would, since there were no signs of fracture at the time. The forest also was well equipped to support the big group, as the protected area they occupied was rich in food and trees, said lead author Sandel.
But after that day in 2015, the chimps quickly split themselves into the Western and Central clusters, named for the territories the chimps have divided within. Now, they patrol to keep one another away.
The Western chimps are more aggressive than the Central chimps; between 2018 and 2024 the group organized up to 15 patrols every four months and killed an average of one adult and two infants per year from the Central group, accordi