In a rare, decades-long study, researchers have documented what appears to be the first observed "civil war" in wild chimpanzees. The findings demonstrate that shifting social ties alone can fracture a once unified group and ignite sustained, deadly conflict among former allies. In humans, war and collective violence are often explained by cultural differences that bind groups while fueling hostility towards outsiders. However, this view cannot fully account for conflicts that arise in once-unified communities, as seen in violent rebellions or civil wars. An alternative explanation argues that shifting social relationships and local rivalries alone can fracture groups and produce violence. Despite lacking human cultural systems, chimpanzees – one of our two closest living relatives – exhibit forms of organized aggression and lethal violence against those who were once group companions. However, direct observations of these events in wild populations have remained elusive.
Drawing on 30 years of behavioral observations and demographic data, Aaron Sandel and colleagues describe a rare and well-observed permanent split and subsequent lethal conflict within the Ngogo chimpanzee group of Kibale National Park, Uganda. It's estimated that such events occur only once every 500 years. According to Sandel et al., beginning around 2015, the chimpanzee community began to rapidly split from a single cohesive group into two distinct polarized clusters – a social rupture that was matched by spatial and reproductive separation. By 2018, the split was complete and enduring, with no remaining ties between the two groups. As this division solidified, aggression between the two groups escalated. Following the 2018 split, one of the chimpanzee groups launched sustained and coordinated attacks on the other, marking a clear shift to lethal conflict among former group members. These raids resulted in multiple killings of adult males and, beginning in 2021, expanded to frequent infanticide, averaging several deaths per year. The authors note that the true toll of this violence is likely higher than observed, as many individuals disappeared without clear cause, suggesting additional unrecorded attacks. Chimpanzees who had long cooperated and bonded turned on one another after the split, indicating that group identity can be redefined beyond mere familiarity. Sandel et al. suggest that factors such as unusually large group size, competition over food and reproduction, deaths of key individuals, leadership changes, and disease may have destabilized social ties and contributed to the division. "A hostile split among wild chimpanzees is a reminder of the danger that group divisions can present to human societies," writes James Brooks in a related Perspective. "The study of Sandel et al. also reinforces the importance of maintaining long-term field research sites and of preserving endangered species. Many valuable scientific insights have only been possible because of the commitment and sustained cooperation of those who study and support these species in the wild."
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