For years, they lived side by side—grooming, hunting, and raising young together in one of the most studied chimpanzee communities on Earth. Then, something changed.
In the thick forests of Uganda’s Kibale National Park, a once-unified group of chimpanzees split into two rival factions. What followed was an unsettling campaign of lethal violence between former companions.
The phenomenon, described by researchers as extraordinarily rare, is now giving a disturbing window into the roots of conflict, not just in animals, but potentially in ourselves.
A new study published in Science documents what may be the clearest example yet of a chimpanzee “civil war,” a term researchers use cautiously but one that illustrates the scale and intensity of the breakdown.
Drawing on three decades of observations, the research reveals how shifting social ties—not culture, ideology, or identity—can break down even the most cohesive societies.
“I would caution against anyone calling this a civil war,” lead author and professor of anthropology at UT Austin, Dr. Aaron Sandel, said in a press release. “But the polarization and collective violence that we have observed with these chimpanzees may give us insight into our own species.”
From Unity to Division
The Ngogo chimpanzee community, the largest ever recorded, had long exemplified a hallmark of chimpanzee society. Individuals formed temporary subgroups but remained part of a larger, interconnected social network. For nearly two decades, this system remained in place. But around 2015, researchers began to notice slight changes.
According to the study, interactions between the two major subgroups, the Western and Central clusters, began to decline. Individuals who once mingled freely began avoiding one another. What once had been a dynamic but cohesive system began to polarize.
By 2018, the split was complete. What had previously been a single, interconnected chimpanzee community had become two separate groups occupying distinct territories, with the social bonds that once linked them effectively gone. This was not a temporary falling-out or the usual fluid reshuffling seen in chimpanzee society. It represented a lasting break.
The Violence That Followed
What happened next sets this case apart from nearly every other documented primate split. Rather than reducing tension, as group fissions often do in other species, the division triggered an escalation of aggression. Territorial patrols increased. Encounters grew more hostile. Then came the killings.
Between 2018 and 2024, members of the Western group carried out 24 attacks on chimpanzees from the Central group, killing at least seven adult males and 17 infants. Researchers say the true toll may be higher, since some Central chimpanzees disappeared without explanation and may have been victims of unobserved attacks.
Significantly, the victims were not strangers. They were former allies—individuals who had shared food, territory, and social bonds for years.
“What’s especially striking is that the chimpanzees are killing former group members,” Dr. Sandel said. “The new group identities are overriding cooperative relationships that had existed for years.”
Why Did a Chimpanzee Civil War Happen?
Chimpanzees do not have religion, politics, or ethnic identity. Yet, the violence observed at Ngogo closely resembles forms of human conflict often attributed to those very factors. This is what makes these findings so significant.
The study challenges the long-standing idea that human warfare is chiefly driven by cultural divisions. Instead, it supports what researchers call the “relational dynamics hypothesis”—the idea that changing social relationships alone can fracture groups and lead to violence.
In other words, conflict may not emerge from who we are, but from how we relate to one another.
The researchers identified several possible triggers behind the split. The Ngogo group had grown unusually large, nearly 200 individuals, possibly straining social cohesion. Competition for food and mates may have increased. The deaths of important individuals in 2014, including several adult males, may have weakened the social “bridges” that held the group together.
A change in leadership may have added to the tension, and researchers say the situation was likely further destabilized by a respiratory epidemic in January 2017 that killed 25 chimpanzees. But the social fracture itself appears to have begun earlier, around 2015, before the final split set into open conflict. From there, the breakdown accelerated.
A Rare Event With Big Implications
Permanent group divisions in chimpanzees are exceedingly rare, with genetic evidence suggesting they occur at median intervals of about 500 years.
The only previously reported comparable case, observed in the 1970s at Gombe, Tanzania, has long been debated because researchers provisioned the chimpanzees with food, and important details were obscured by restricted observations.
What makes the Ngogo case different is the depth of observation. Researchers tracked the community continuously for nearly 30 years, recording the full arc from cohesion to collapse.
That level of detail allows scientists to test broader theories about the origins of conflict. The findings show that even in the absence of cultural identity, groups can rapidly redefine “us” versus “them.” Once that boundary is drawn, cooperation can give way to hostility with startling speed.
What a Chimpanzee “Civil War” Means for Humans
The parallels to human conflict are difficult to ignore.
Civil wars, insurgencies, and internal divisions often arise within previously unified societies. While these conflicts are typically framed in terms of ideology or identity, the Ngogo study hints at something deeper.
If chimpanzees, our closest living relatives, can descend into lethal conflict without language, politics, or culture, then the roots of human violence may lie in more fundamental social processes.
That doesn’t make conflict inevitable. In fact, the researchers suggest the opposite. If relationships can fracture and lead to violence, they can also be repaired.
The study suggests that the same relationship dynamics that can drive groups apart may also provide a way toward holding them together, stressing the idea that stability can be reinforced through everyday social repair and renewed connection.
“If relational dynamics alone can drive polarization and lethal conflict in chimps without language, ethnicity, or ideology, then in humans, those cultural markers might be secondary to something more basic,” Sandel says. “If that’s true, then we may have the potential to reduce societal conflicts in our personal lives, and that gives me hope.”
“Focusing entirely on these cultural factors, however, overlooks social processes that shape human behavior—processes also present in one of our closest animal relatives,” researchers conclude. “In some cases, it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com