On a June day in 2015, primatologist Aaron Sandel was quietly observing a small cluster of the Ngogo chimpanzee group in Uganda’s Kibale national park when he noticed something strange. As other members of the chimpanzees’ wider group moved closer through the forest, the chimpanzees in front of him began to display nervous behaviour. They grimaced and touched each other for reassurance, acting more like they were about to meet strangers than close companions.

In hindsight, Sandel said, that moment was the first sign of what would become a years-long bloody conflict between a once close-knit group of chimps.

In a new study published this week in the journal Science, Sandel and his colleagues document what may be the first observed “civil war” in wild chimpanzees. While chimpanzees have long been known to wage campaigns of lethal aggression on outsiders, witnessing a once unified group turn on itself is something new – and very human.

“Cases where neighbours are killing neighbours is more troubling and, in a way, it gets closer to the human condition. How do we have this seeming contradiction within us where we are able to cooperate, but then also very quickly turn on one another?” Sandel said.

“These shifting group identities and dynamics that we see in human civil war rarely have a parallel in other animals, but they do have a parallel in the case of chimpanzees.”

The researchers drew on more than three decades of behavioural observations of the well-studied group of chimpanzees to determine the permanent split in the largest known group of wild chimpanzees in the world. While the chimps had been socially cohesive from at least 1995 until 2015, something shifted in the group’s dynamics, and by 2018 two distinct groups had emerged – the western chimps and the central chimps.

With the two groups solidified, members of the western group made 24 sustained and coordinated attacks on the central one in the seven years that followed, killing at least seven adult males and 17 infants.

Scientists think that a similar rupture and civil war may have occurred in the 1970s within the chimpanzee group in Gombe, Tanzania, observed by the renowned primatologist Jane Goodall. But, at the time, our basic understanding of chimpanzee behaviour was too limited to fully appreciate the rarity of in-group violence.

In the case of the Ngogo chimps, a change in social hierarchies may explain the group’s fracture, researchers said, producing organised aggression and violence. On the day Sandel observed the chimps acting strangely in 2015, earlier that morning, the group’s alpha male had grunted in submission to another chimpanzee. Yet the group’s social structure had also been affected by the death of several key older individuals in the years that preceded the division.

“Their abrupt death likely weakened connections among the neighbourhoods, which then made the group vulnerable to this polarisation that happened when the alpha change occurred,” Sandel said. “Then there was also a disease outbreak in 2017 that probably made the split inevitable, or expedited it slightly.”

That should cause some worry for ape conservation, as chimpanzees are threatened with extinction. The study notes that, based on genetic evidence, these “civil wars” among chimpanzees likely only occur every 500 years. But any human activity that disrupts social cohesion – deforestation, the climate crisis or disease outbreaks – could make such inter-group conflicts more common, Sandel said.

Brian Wood, an evolutionary anthropologist at the University of California Los Angeles, who has also studied the Ngogo chimpanzees but was not involved in the new research, said it was important to consider what one group has to gain by attacking its former community members.

In the theory of Darwinian fitness – a measure of how successful an animal is in passing on its genes – “you can increase your Darwinian fitness by increasing your own survival, increasing your reproduction or by decreasing the survival and reproduction of your competitors,” Wood said.

“And this is what the western chimps have done. The central chimps, after facing the onslaught of the westerners, now have the lowest survivorship that has ever been documented in a wild chimpanzee community.”

Sylvain Lemoine, a professor in biological anthropology at the University of Cambridge, said: “Here we have the first thoroughly reported case of what can be qualified as civil warfare in the species … It shows that, even in absence of cultural group markers, social ties and network connectivity are the cement of group cohesion, and that these ties can be fragilised in specific circumstances, especially when they rely on few key individuals.”

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