Scientists studying wild chimpanzees in Uganda have documented a rare and chilling event: a once-united chimpanzee community that split into rival groups and later engaged in deadly attacks against former companions.
The findings come from a decades-long study of chimpanzees in Kibale National Park and were published in the journal Science. Researchers tracked the animals’ social lives for nearly 30 years, using behavioral observations, GPS data and social network analysis to understand how relationships within the group changed over time.
For many years, nearly 200 chimpanzees lived together as a single community–the largest known group of wild chimpanzees ever recorded. But subtle shifts in social relationships began around 2015. Over several years, the community gradually divided into two separate factions, which scientists now refer to as the Western and Central groups.
Researchers describe the process as unfolding in stages. “We identified three key stages: (i) an abrupt shift from cohesion to polarization… (ii) increasing avoidance that led to the formation of two groups; and (iii) lethal aggression between former group members,” the study reports.
By 2018, the split was complete. The groups no longer traveled together, shared territory, or mated across the divide. Soon after, violence began.
Over the next seven years, members of the smaller Western group repeatedly raided the territory of their former companions. “Members of one group made 24 attacks, killing at least seven mature males and 17 infants in the other group,” the researchers wrote.
What makes the discovery particularly striking is that the chimps involved were once close allies. Many had spent years grooming, hunting and defending territory together before the split. Afterward, however, they treated one another as enemies.
The findings may help scientists better understand how group conflict arises–even in humans. Chimpanzees lack the cultural markers that often define human divisions, such as religion, ethnicity or political ideology.
Yet the study suggests conflict can emerge from changing social relationships alone. “In the absence of ethnicity, religion, or political ideologies, social networks can divide, and new group boundaries can emerge, leading to collective violence,” the researchers concluded.
While scientists cannot say exactly what triggered the split, several factors may have contributed, including the unusually large size of the chimpanzee community, the deaths of several key individuals and shifting leadership among dominant males.