A peaceful community of chimpanzees in Uganda’s Kibale National Park has shocked researchers after splitting into warring factions, with former friends launching deadly coordinated attacks against each other.
For twenty years, scientists watched members of the Ngogo chimpanzee community live harmoniously in their rainforest home, spending time eating, resting, traveling and grooming together. However, this once-stable group eventually broke apart and descended into years of brutal violence that has claimed 28 lives.
According to a new study published Thursday in the journal Science, this represents the first clearly documented case of wild chimpanzees dividing into separate factions, with one group systematically targeting the other through organized attacks.
“Biting, pounding the victim with their hands, dragging them, kicking them – mostly adult males, but sometimes adult females participate in the attacks,” explained Aaron Sandel, a University of Texas primatologist who led the research.
The Ngogo group, which researchers have monitored since 1995, was the largest known wild chimpanzee community anywhere, reaching approximately 200 members at its peak. Most chimp groups typically contain around 50 individuals.
While scientists have previously observed chimpanzees attacking members of neighboring communities, this situation was entirely different because the violence occurred between former allies who had known each other their entire lives.
“It is hard for me to wrap my head around the fact that yesterday’s friend turned into today’s foe. Males in the two groups grew up with each other, knew each other their entire lives and cooperated and collaborated with each other, benefiting in the process,” said John Mitani, a University of Michigan professor emeritus and senior author of the study.
“So why split? Perhaps they became a victim of their own success when the group grew to an intolerably large size,” Mitani added.
Researchers believe multiple factors contributed to destabilizing the community. The group’s unusually large size may have created intense competition for food and mating opportunities among males. Additionally, seven chimpanzees died in 2014 showing signs of illness, which may have disrupted established social bonds and created tensions.
Leadership changes also played a role in the community’s breakdown. Around 2015, when hostilities began emerging, a chimpanzee named Jackson overthrew the previous alpha male in these male-dominated societies.
Initially, the group remained unified despite existing social clusters. However, members of two clusters started avoiding each other in 2015. Following another illness outbreak in 2017 that killed 25 chimpanzees, primarily infants, members of one cluster attacked Jackson, though he survived. By late 2017, two distinct groups had formed – designated as the Western and Central groups.
The Western group initiated the violence against the Central group beginning in 2018. Through 2024, the published research documented 24 deaths – seven adult males and 17 infants. The attacks have continued, with four additional deaths recorded last year and this year, bringing the total to 28. Many other chimpanzees have vanished without explanation, suggesting additional unreported killings.
“They just beat and jump on the victim relentlessly. I’ve witnessed cases that take less than 15 minutes. There’s some biting, and if you examine the bodies of victims, you will see cuts. But nothing that looks like it can cause a fatality. Instead, I’ve always thought that mature victims die due to internal injuries,” Mitani described.
“By contrast, a single mature chimpanzee can snatch an infant from its mother and kill it quickly with a few bites or via blunt force trauma. The latter might include slamming it to the ground,” he continued.
Despite starting smaller in both population and territory, the Western group has now grown larger than the Central group in both aspects and has suffered no known casualties.
Though researchers avoided labeling these events as a civil war – a term with specific meaning in human conflicts – they acknowledged significant similarities.
The team noted one previous example from Tanzania in the 1970s where a chimpanzee community appeared to split with deadly violence between factions. However, that case involved artificial feeding by researchers that altered natural behavior, and observations were limited to feeding locations, leaving many questions unanswered.
While chimpanzees and bonobos are humanity’s closest evolutionary relatives, the researchers warned against drawing direct comparisons between chimpanzee violence and human behavior.
“We are similar in some ways, due to our shared evolutionary history, but we are also fundamentally different because we have changed during the past 6-8 million years, after having split off from them,” Mitani concluded.
