A community of around 200 chimpanzees living in Uganda's Kibale National Park has fractured into two warring factions, with one group launching a years-long campaign of lethal attacks against the other.
The Ngogo chimpanzees have been studied continuously for three decades, but in recent years scientists have watched as a violent split unfolded in slow motion.
From around 2015, what had been a single cohesive group began to polarise. Social ties frayed, neighbourhoods within the community hardened into distinct factions and shared territory became a contested border. By 2018, the break was permanent.
What followed was remarkable – and troubling. The smaller of the two groups – the Western chimps – began making targeted raids into the territory of the larger Central group. Over the next six years, they killed at least seven adult males and 17 infants.
And that figure is likely an undercount. A further 14 adolescent and adult Central males disappeared or died unexpectedly between 2021 and 2024, none of whom showed any signs of illness beforehand.
Today, the Western group has surpassed its rival to become the dominant force in the jungle.
The findings, published in Science, have prompted comparisons to civil war. Unlike inter-group conflict between strangers, what happened at Ngogo involved former companions, groomers, and long-term social partners turning on one another.
The scientists behind the study estimate such conflict occurs only once every 500 years.
“One of the most compelling aspects of this is that we really are seeing this shift from what you might call 'from friend to enemy',” lead author Prof Aaron Sandel of the University of Texas at Austin told BBC Science Focus. “With that, we're getting a window into the chimpanzee mind that's really rare.”
The study supports the idea that group identities can shift and override even deep social bonds without any of the ethnic, religious or ideological markers typically thought to underpin most collective violence.
"We almost excuse war between groups," Sandel said. “But we're troubled by civil war. We're really upset by cases where neighbours turn on neighbours."
He added that the chimp data could help researchers develop testable hypotheses about what drives humans towards – or away from – that kind of breakdown.
"If we zoom in on interpersonal relationships and conflict management," he said, “we might find more effective interventions for peace.”
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