For many years in Uganda’s Kibale National Park, two groups of chimpanzees coexisted, engaging in grooming, socializing, and territory patrols within their communities.

Then, in a shocking turn of events, one group violently attacked the other, igniting years of conflict likened to human civil wars.

When the violence erupted in 2015, John Mitani, a professor emeritus of anthropology at the University of Michigan with over 20 years of research on chimpanzees, described the chaos: “It was just chaos. They started screaming and chasing each other.”

In the three years following the outbreak, Mitani and his colleague Aaron Sandel, an associate professor of anthropology at the University of Texas, documented how the chimpanzees’ social networks began to erode. By 2018, the two factions known as Western Ngogo and Central Ngogo chimpanzees “stopped sharing territory and began engaging in aggressive behavior, even killing each other,” Mitani reported.

At least 28 chimpanzees, including 19 infants, have been killed in this period, according to the initial research published in Science.

Mitani remarked, “Individuals who once aided each other now view one another as enemies.”

This marks the second observed instance of chimpanzee factions splitting and turning violent. Given that chimpanzees and bonobos are humans’ closest genetic relatives, the findings may offer insights into human behavior.

“Civil wars afflict people. How can you turn a neighbor against you? This study of chimpanzees highlights how group identity evolves and how lethal aggression arises,” Sandel noted.

Infants were torn from their mothers and killed.

Following the social collapse, one-sided violence escalated. The Western chimpanzees, initially a minority group, were responsible for all subsequent attacks post-split in 2018. While their numbers rose from 76 to 108, the Central chimpanzee population has steadily decreased.

Attacks have been brutal, with a Western chimpanzee reportedly tearing an infant from its mother’s arms and killing it.

Sandel highlighted that chimpanzees often utilize gang violence when targeting adult or adolescent males.

“Five or ten chimpanzees will overwhelm an individual, holding them down, biting, beating, and dragging them,” he explained. “The violence can be horrifying.”

Mitani remarked, “It’s distressing to witness.”

“The situation deeply troubles me,” he expressed.

Researchers seek answers regarding the reasons behind the collapse of social structures.

Since 1995, chimpanzees at Ngogo have been closely monitored, with structured documentation of their behavior. The recent study utilized 10 years of GPS tracking, 30 years of demographic data, and 24 years of detailed observations.

The team examined chimpanzee social networks by observing individual males for an hour, recording proximity, interactions, and grooming behaviors. They noted recurring patterns of overlapping social dynamics that ultimately led to a breakdown.

Mitani and Sandel propose that the group’s large size might have played a role in the violent divide. While typical chimpanzee groups consist of around 50 individuals, the Ngogo community boasted about 200, possibly straining social connections and heightening competition for resources.

Additionally, the death of five adult males from disease prior to the split could have disrupted critical social ties. Following this, a new alpha male emerged in 2015, further complicating social dynamics.

“That’s a significant factor,” Mitani explained, noting that such shifts typically occur every 6-8 years, often increasing aggression and altering relationships.

Decades ago, Jane Goodall witnessed similar violence

Approximately 50 years ago, the late Jane Goodall and her team observed a series of assaults that led to group fragmentation in Tanzania’s Gombe National Park. The main group hunted down and killed all males from the splinter group.

Researchers subsequently termed this conflict the “Four Years’ War.”

Anne Pusey, a professor emeritus of evolutionary anthropology at Duke University, studied these interactions until 1975, and noted that conditions preceding the killings were “remarkably similar” to those seen in Ngogo.

In Gombe, changes in alpha leadership, a shortage of mating females, and the deaths of amicable older males precipitated violent behavior.

Pusey remarked, “These social bonds deteriorated, leading to hostility.”

Joseph Feldblum, an evolutionary anthropologist with experience studying Gombe, stated that the recent findings align with historical observations.

“Such behavior is infrequent, yet exists within the natural repertoire of chimpanzees,” he noted.

Mitani expressed concern over the future of the Ngogo central group, suggesting they may be “doomed” based on past events at Gombe.

“The signs are evident,” he stated.

With the ongoing violence against infants and exclusion of females, Mitani concluded, “We might be witnessing an extinction event.”

Impact on humans

What can we learn from the violent behaviors displayed by our closest relatives?

Sandel emphasized that while cultural differences are often blamed for human warfare, this explanation does not apply to chimpanzees.

“Chimpanzees lack ethnicity, religions, and political ideologies, which are often identified as causes of human conflicts, especially internal strife like civil wars.”

Instead, researchers believe the violence is rooted in the breakdown of friendships and rivalry escalation. Sandel suggested that these dynamics may play a more pivotal role in human civil wars than commonly recognized, proposing that small gestures of reconciliation could be vital for peace.

Mitani reminded us that humans diverged from chimpanzees 6 to 8 million years ago. He cautioned against viewing violence against neighbors as an inherent human trait simply because it is observed in chimpanzees.

“We have evolved,” Mitani asserted. “As a species, we have become increasingly cooperative and socially inclined, often helping not only our neighbors but even strangers. This capability is not shared by chimpanzees.”