Chimpanzees are acting like humans and killing their own in civil wars, study finds

A new study has documented what may be the first observed “civil war” in wild chimpanzees. Primatologist Aaron Sandel wrote in the research published in the journal Science, that the first time he saw the chimps turning on their own group members and fearing them was in 2015. Sandel witnessed the moment in Uganda’s Kibale National Park where members of the Ngogo chimpanzee group began acting terrified of their own kind. Instead of welcoming approaching group members, they paced and sought comfort from one another as if facing dangerous strangers. This behaviour marked the beginning of a violent, years-long civil war that would eventually tear the once-unified chimpanzee colony apart. Researchers say that chimpanzees are known to turn aggressive against outsiders. However, now seeing them turn against their own group is worrying and very human-like. "Cases where neighbours are killing neighbours are 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.

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The chimpanzee group broke into two factions

The study states that it is rare to see this human civil war parallel in other animals, but now they have an example in the case of chimpanzees. They witnessed this collapse in the world's largest known wild chimpanzee colony after observing them for over 30 years. The group had remained a tight-knit community since at least 1995, but a shift in social dynamics began in 2015. By 2018, this internal friction resulted in a permanent split, leading the original population to split into two rival factions: the western and central chimpanzees. In the following seven years, members of the western group carried out 24 sustained and coordinated attacks on the central group and killed at least seven adult males and 17 infants. Scientists think that something similar was likely witnessed by primatologist Jane Goodall in the 1970s within the chimpanzee group in Gombe, Tanzania. But since chimp behaviour was not as well known at the time, it went unnoticed.

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The deaths of influential "elder" chimpanzees triggered instability

Researchers in the latest study say that this organised aggression and violence against their own could be a result of a change in social hierarchies. The signs of instability were visible even before the 2015 incident. Earlier that same morning, the group's alpha male uncharacteristically signalled submission to another chimp, hinting at a collapse in hierarchy. But it wasn't just a single leadership dispute. In the years leading up to the split, the deaths of several influential "elder" chimpanzees—who had long acted as social anchors—left a power vacuum and weakened the group’s overall stability, ultimately paving the way for the civil war. “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. A disease outbreak in 2017 probably "made the split inevitable, or expedited it slightly."

The scientists noted that genetic evidence shows that these “civil wars” among chimpanzees likely only occur every 500 years. However, human activity, like deforestation, climate change or disease outbreaks, could make such inter-group conflicts more common, Sandel said.