In the mid-1970s, more than a decade into her research on chimpanzees in Tanzania's Gombe National Park, the late and legendary primatologist Jane Goodall witnessed something that horrified her.
The group of chimps she and her colleagues were studying broke into two factions and turned on each other. It looked very much like a civil war. Chimpanzees that had intermingled peacefully and grown up together were systematically killing each other.
It changed Goodall's view of one of humanity's closest relatives.
"I used to think, 'Well, they're very [much] like people but nicer,'" she told the public radio program Fresh Air in 1993. "And then I realized that when opportunity arises, they have this nasty, brutal side to them just like we do."
Asked what precipitated the war, Goodall said it was hard to say. It was the first one that researchers had ever seen. "We shan't be very sure until it happens again," she said.
Now, in the journal Science, a team of researchers has described a second brutal and ongoing "civil war" that has permanently divided the largest known group of wild chimpanzees in the world.
"I was struck by some of the similarities of what they've described to what we observed in Gombe," said Anne Pusey, a retired primatologist who worked with Goodall in Tanzania and wasn't involved in the new study.
"It's rather uncomfortably familiar seeing how these relationships can break down and then lead to antagonisms between groups that weren't there before."
The new study draws from more than 30 years of observations of the Ngogo chimpanzee group in the western forests of Uganda. At its peak, nearly 200 individuals were in the Ngogo group, living cohesively in smaller subgroups that the researchers labeled as "clusters." Males and females from different clusters intermingled. They mated, hunted together and worked together to fight off other outside groups. Researchers took videos of males from different clusters holding hands.
Then, in 2015, the researchers started seeing signs that something was off.
"I can even pinpoint it to one particular day when there was a really big change," said Aaron Sandel, the lead author of the new study and a primatologist at the University of Texas at Austin.
On that June day, Sandel was observing a large number of chimpanzees from the Western cluster while they were in their territory. At one point, they heard other chimpanzees nearby, presumably from the larger Central cluster.
The Western chimpanzees quieted all of a sudden. "They started touching each other in this reassurance, like they were really nervous," Sandel said. "And to me, this seemed like they were acting as if they were hearing outsider chimps."
Instead of reuniting and intermingling like they normally would, the Western chimpanzees fled and the Central chimpanzees chased them.
"Nothing really like that had ever been observed before — and then they avoided each other for six weeks," Sandel said. "So this was very clear, like on the ground, something big has just happened."
Over the next few years, polarization increased, and by 2018 the clusters were essentially completely separate groups. Then the killing started.
The victim of the first observed lethal attack was an adolescent male from the Central cluster that the researchers had named Errol. Sandel had watched him grow up.
"I'm just trying to observe as objectively as possible and really just document everything," he said. "In some respects, I feel like a war correspondent. I'm trying to understand this really rare behavior. … Like what's causing this?"
Over the next seven years, the Western group killed at least six other adults and 17 infants from the Central cluster. The fighting continues to this day. Why the Ngogo group split and why its members turned on each other is still unclear. In the paper, Sandel and his co-authors suggest several factors that may have contributed: the size of the group, competition for food and male-to-male competition. The natural deaths of five adult males and one adult female in 2014, before the intergroup divisions took root, may have weakened social networks.
"I think it's clear from this study and from other studies of chimps and other animals that you can get these kinds of conflicts without a lot of things that we think about as being the source of conflict in humans," said Michael Wilson, a primatologist at the University of Minnesota, who wasn't involved in the study. "Lions don't have religion and political parties or ideologies. Neither do wolves or ants for that matter."
Neither do chimpanzees, the authors of the new study note.
To Sandel, that's a reason for optimism.
"If in chimpanzees, we can see this conflict and lethal violence occur in the absence of all these aspects of human behavior that we often attribute to civil war, then I wonder to what extent are the interpersonal relationships and behaviors actually more important than we realize in humans," he said.
Perhaps, he added, strengthening our social bonds and letting old grudges die can help prevent larger violence.
"Like with the chimps: If you act like a stranger, you become a stranger," Sandel said. "I want to avoid that in my own life."
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