For two decades, researchers observed members of the Ngogo chimpanzee group in Uganda's Kibale National Park spending their days eating fruit and leaves, resting, traveling, and caring for each other in their rainforest habitat. But this stable community then fell apart and plunged into years of deadly violence.
Researchers are now describing the first clearly documented example of a group of wild chimpanzees splitting into two separate factions, with one launching a series of coordinated attacks against the other. Adult males and young were targeted, with 28 deaths.
"Biting, repeated hitting with hands, dragging, kicking - mostly adult males, but sometimes females also participate in attacks," said primatologist Aaron Sandel of the University of Texas, lead author of the study published Thursday in the journal Science.
Researchers began studying the Ngogo chimpanzees in 1995. This was the largest known group of wild chimpanzees in the world, reaching about 200 members at its peak. Typically, chimpanzee groups have about 50 members.
Scientists have long known that chimpanzees attack and kill members of neighboring groups – i.e. strangers – but this was different.
"It's hard for me to understand how yesterday's friend became today's enemy. The males in both groups grew up together, have known each other their entire lives, and have cooperated for mutual benefit," said primatologist John Mitani of the University of Michigan.
"So why the split? Maybe they became victims of their own success, when the group grew to an unmanageable size," he added.
The researchers said a combination of factors may have destabilized the group. The large initial size may have increased competition for food and mating among males. The deaths of seven chimpanzees in 2014, with signs of illness, may have disrupted social relationships and created hostility.
Chimpanzee communities are dominated by males. Around the time tensions began in 2015, there was a shift in the alpha male – the highest-status individual – when a chimpanzee named Jackson dethroned another male from the position.
Before the split, the group was a close-knit community, although social clusters existed. In 2015, members of the two clusters began to avoid each other. A few months after an illness in 2017 that killed 25 chimpanzees, mostly young ones, members of one group attacked Jackson, although he survived. In late 2017, two distinct groups formed, called the Western and Central groups.
The subsequent violence was carried out by the Western group against the Central one, starting in 2018.
The study includes observations up to 2024, with seven adult males and 17 cubs killed, for a total of 24. The violence has continued. Last year and this year, one adult male, one adolescent male and two cubs were killed, bringing the total number of victims to 28. Many chimpanzees have disappeared without a clear cause, suggesting other unrecorded killings.
"They just beat and jump on the victim without stopping. I've seen cases that last less than 15 minutes. There are bites and cuts on the victims' bodies, but nothing that seems to cause death. I think the adults die from internal injuries," Mitani said.
"In contrast, an adult chimpanzee can snatch a young one from its mother and quickly kill it with a few bites or by slamming it hard to the ground," he added.
The Western group was initially smaller in numbers and territory, but later surpassed the Central group in both respects. This group seems to have suffered no losses.
Although scientists prefer not to call this a "civil war," a term with specific meaning for human conflicts, they see important similarities.
The researchers cited a previous case in Tanzania in the 1970s, where a community of chimpanzees split and one faction resorted to deadly violence against the other. In that case, the chimpanzees were regularly fed by researchers, which altered their natural behavior and left many questions unanswered.
Chimpanzees and their close cousins, the bonobos, are our closest evolutionary relatives. However, researchers caution against direct comparisons between chimpanzee violence and human behavior, writes Reuters.
"We are similar in some ways because of our shared evolutionary history, but we are also fundamentally different because we have changed over the 6–8 million years since our separation from them," Mitani said.
