Why these chimps have been at war for 8 years

On the last full day of his life, Basie, a large gregarious male chimpanzee, woke up at dawn in a tree nest he’d fashioned from branches and leaves, surrounded by other chimps also dozing in nests, as he’d done nearly every day for 36 years in the forest. 

After an ordinary day of swinging between trees and snacking on ripe figs, danger appeared. A patrol group of about 13 adult chimps from the opposing faction arrived as daylight began to fade. Three adults surrounded Basie, who jumped from a tree. Then 10 chimps attacked him on the ground, piling on him and biting him.

“In the moment, I felt like a war correspondent. I wanted to be there, I wanted to witness it, to document it, and try to understand what’s going on,” says Aaron Sandel, a primatologist at the University of Texas at Austin, who was following the chimps that day and taking notes on his smartphone. “Once I’d written up my notes and shared them with colleagues, that’s when the emotions hit me.”

Basie’s death in 2019 was the second casualty in what scientists call a “civil war” among chimps in Uganda’s Kibale Forest National Park. Sandel co-authored a new study in the journal Science describing an eight-year conflict that continues to this day in a community that used to live in peace.

“The war is ongoing— it’s not finished yet,” says co-author Jacob Negrey, a primatologist at the University of Arizona.

The grisly saga involving one of humanity’s closest living relatives has prompted scientists to ask a simple question: How do former friends become fatal enemies?

(My sleepless night in a chimpanzee nest)

Echoes of Jane Goodall’s observations

Basie once enjoyed a rather tranquil life in the lush Ugandan rainforest. He spent his days among a large community of chimpanzees—at one point around 200 individuals— who lived together in relative harmony for at least 20 years. Scientists closely observed them, learning about their social structures and behaviors. But then the chimpanzee group split into two factions, which researchers called Western and Central. The Western group is smaller but more aggressive.

At first, both sides avoided each other. Then the Western faction began leading violent patrols, marching single-file to find victims at the edge of their territory. So far, these chimps have murdered at least seven adult and 17 infant chimps—their deaths witnessed by biologists—and 14 other chimps have gone missing, possibly killed as well, researchers say.

The conflict is only the second instance of civil war ever observed among chimpanzees. In the mid 1970s, the late primatologist Jane Goodall and colleagues witnessed what they called the Four-Year War, a lethal split among a chimp community at Tanzania’s Gombe Stream National Park. Those years “were the darkest years in Gombe’s history,” Goodall wrote in her memoir, Through a Window. “For several years I struggled to come to terms with … a dark side to their nature.”

(Read Jane Goodall's iconic 1963 article on the chimpanzees of Gombe Stream Game Reserve)

The new research from Uganda adds to Goodall’s early observations about chimp behavior. Scientists documented in detail a series of social disturbances that preceded the current conflict, including the deaths of key individuals who played a role in connecting different chimp “neighborhoods,” a change in dominance or leadership structure, and a disease outbreak.

With the social order unsettled, the group splintered. Hostilities didn’t begin with drama, but with simple avoidance. “When you stop coming together, it’s possible to stop seeing yourselves as part of the same group,” says Negrey. “That can lead to violent consequences in a shockingly short period of time.”

How chimp society crumbles

Basie’s killers were chimps he grew up with. While chimps are known to attack male strangers on the outskirts of their territory, what’s remarkable here is that a previously unified community, called Ngogo, fragmented and went to war with itself.

“While rare, these events show that chimps can change from being friendly to becoming enemies,” says Duke University primatologist Anne Pusey, who was not involved in the study.

It’s impossible to say exactly why splits happen, but researchers have some hunches. In the new study, scientists analyzed more than two decades of “social-network data”—detailed daily logs of who is grooming, feeding, mating or otherwise interacting with whom. These data help determine when a social gulf began to emerge, and what events directly preceded it.

Before the two sides went to war, they first stopped associating with each other— likely because of the deaths of key individuals that had moved easily among neighborhoods. “If you’re not engaging in those daily practices holding everything together, the social glue starts to fall apart,” says Catherine Hobaiter, a primatologist at the University of St. Andrews in Scotland, who was not involved the study.

Study co-authors noted that five older males and one adult female died of a likely respiratory illness in 2014, the year before the two sides started avoiding each other. “Those males were very central,” says Negrey. “To have lost them in a very short time was very significant because they had strong ties throughout the community,” in what the researchers call different “neighborhoods.”

“You can imagine some avuncular figures friendly with everyone who play this bridging social role,” says Joseph Feldblum, a Duke University primatologist who was not involved in the new paper. “If they die, that hardens the divisions between subgroups.” In a previous study, Feldblum and Pusey analyzed data from Goodall’s notebooks to identify a similar pattern of key deaths preceding the Four-Year War.

Around the same time, in 2015, a new alpha male, Jackson, deposed the established alpha. “Whenever there’s a big change or an ambiguity in the ranks, that can lead to more aggression, especially among males,” says Sandel.

Once cracks had already emerged, a second and more widespread respiratory illness in 2017 led to 25 further deaths, and the lines of the cliques hardened. The killings began the next year.

“There can be small unique events that lead to these massive changes in group structure,” says Elizabeth Lonsdorf, an Emory University primatologist who was not involved in the new study. “It was probably a combination of the fact they had change in alpha male, the death of key individuals, and an outbreak of respiratory illness that severed the last remaining ties between the two halves.”

(Meet Tatu and Loulis—the last of the 'talking' chimpanzees)

Acts of kindness

Most of chimp life isn’t violent.

Most of the time they sit around, picking bugs out of each’s fur, scratching themselves, and munching on figs and bananas. Sometimes they lie in what biologists call “grooming piles,” with outstretched arms and feet touching other chimps.

But they aren’t doing nothing. They’re strengthening social ties.

“Spending time together, grooming, sharing food, hunting – all these ordinary activities are associated with stronger social bonds, with making friends and maintaining friends,” says Hobaiter.

And even in times of war, scientists observed acts of caring and tenderness.

After Bassie had been attacked, a 53-year-old chimp named BF approached. They had “a strong relationship for over a decade, something like a friendship,” says Sandel. BF helped Basie walk slowly back to the heart of their territory. At night, Basie lay down, bleeding and clearly weakened, and his pal, though gesturing in distraught ways, remained with him.

When Sandel returned the next morning, BF was still there by Basie’s near-lifeless body. BF extended his hand, appearing to want Basie to follow him.

“Chimps have this reputation as mythic great ape killers,” says Negrey. “They are also capable of incredible kindness to each other. We saw both sides.”