For more than two decades, the Ngogo chimpanzees of Uganda lived together as a massive, unified society. But after a sudden rupture in social ties permanently fractured the community, former allies descended into a brutal and sustained “civil war”.
According to a landmark new study published in the journal Science, researchers have documented the first definitive instance of a wild chimpanzee community permanently splitting in two, followed by years of lethal violence between the newly formed factions.
Drawing on 30 years of behavioural observations from Kibale National Park — a population recently made famous by the Netflix documentary series Chimp Empire — the findings reveal that shifting social relationships alone can ignite deadly conflict among former friends.
A society divided
In many primate species, large groups regularly split to reduce competition for food and resources. However, permanent fissions among chimpanzees are extraordinarily rare, with genetic evidence suggesting such events occur only once every 500 years.
For the first 20 years of observation, the Ngogo community was highly cohesive. Individual apes regularly moved between flexible subgroups, maintaining cooperative social ties across the entire territory.
But in 2015, scientists noticed a severe polarisation taking hold. The community began to rapidly divide into two distinct clusters — Western and Central — which increasingly avoided each other. Researchers believe this destabilisation was triggered by a combination of an unusually large group size, disease, leadership changes, and the deaths of several key adult males who had previously acted as social “bridges” holding the community together.
By 2018, the rupture was complete. The chimpanzees were cleanly divided into two separate groups with distinct territories and zero reproductive overlap.
Lethal betrayal
As the division solidified, the once-peaceful neighbours turned violently against one another. Following the 2018 split, the Western group launched a series of sustained, coordinated attacks on members of the Central group.
Between 2018 and 2024, researchers observed or inferred with high confidence seven lethal attacks on adult males. The violence expanded into frequent infanticide beginning in 2021, resulting in 17 fatal attacks on infants. Furthermore, scientists note that the true death toll is likely much higher, as many individuals simply disappeared without a clear cause.
Dr Aaron Sandel, an associate professor of anthropology at The University of Texas at Austin and the study’s lead author, highlighted the shocking nature of the betrayal.
“What’s especially striking is that the chimpanzees are killing former group members. The new group identities are overriding cooperative relationships that had existed for years,” Sandel said.
While a similar event was reported in the 1970s at Gombe, Tanzania, during Jane Goodall’s famous long-term study, that case remains heavily debated because researchers actively provided the animals with food. The Ngogo chimpanzees were never provisioned, offering a pristine, unmanipulated view of wild behaviour compiled over three decades by Dr John Mitani of the University of Michigan and a large team of Ugandan field staff.
Dark origins of human warfare
The violent fracture of the Ngogo community poses a major challenge to traditional anthropological theories regarding human warfare. Historically, collective violence and civil wars are explained by cultural differences — such as ethnic, religious, or ideological divides — that bind groups together while fuelling hostility towards outsiders.
However, the chimpanzee conflict proves that advanced cultural markers are not a prerequisite for organised, deadly violence.
“If relational dynamics alone can drive polarisation and lethal conflict in chimps without language, ethnicity, or ideology, then in humans, those cultural markers might be secondary to something more basic,” Sandel explained. “If that’s true, then we may have the potential to reduce societal conflicts in our personal lives, and that gives me hope. As our paper concludes, it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace.”
James Brooks, who authored a related Perspective for the American Association for the Advancement of Science, noted that the apes serve as a vital mirror for our own species.
“A hostile split among wild chimpanzees is a reminder of the danger that group divisions can present to human societies,” Brooks wrote.
