Once they were allies, now they’re killing each other: The world’s largest known wild chimpanzee group in Uganda’s Kibale National Park has split into two camps that fight each other to the death. In addition to adult males, the victims also include numerous young animals. A long-term study that has been following the animals since 1995 provides insights into the social dynamics that have turned former friends into enemies. The results show that war can arise regardless of cultural, political or religious ideologies when the social structure is in upheaval.
When we humans wage war, it is often about economic interests, different political and religious ideals or cultural and linguistic divides. The different self-images of the groups promote both cohesion within the community and hostility towards outsiders. But even within previously united societies, divisions and even civil war can occur. In this case, the hypothesis of different group identities as triggers falls short. “An alternative model suggests that changing interpersonal bonds and local rivalries are enough to divide groups and generate collective violence, regardless of cultural characteristics,” explains a team led by Aaron Sandel from the University of Texas at Austin.
Sandel and his colleagues have now observed an extraordinary war in chimpanzees that supports this second hypothesis. Chimpanzees normally defend their territory against outsiders, but stick together within their group. But in Ngogo in Uganda’s Kibale National Park, a huge chimpanzee society has split into two parts that are fighting each other bitterly. Researchers have been observing the Ngogo chimpanzees since 1995 and the Netflix documentary series Chimp Empire, published in 2023, also portrayed this group.
The big break
To understand the social dynamics behind the split and the subsequent armed conflict, Sandel and his team evaluated 30 years of behavioral and demographic data and conducted social network analyses. According to this, the animals formed changing subgroups over the course of two decades, but they all reproduced among themselves, defended their territory together and united in changing alliances. However, the first signs of a split into a western and a central group emerged after several adult males and a female died in 2014 and there was a change in the alpha male in 2015.
The new alpha male originally came from an alliance of males that later formed the western group, but as he rose in the hierarchy he joined the central group. There was a disruption in the previously united chimpanzee population. From then on, only a few animals maintained contact between the two groups. One of the last of these connecting individuals died of illness in 2017. By 2018, this division had become so widespread that the previously united society had emerged into two different groups that avoided each other and no longer reproduced with each other.
From social division to war
The newly formed western group included ten adult males and 22 adult females as well as several juveniles and was therefore significantly smaller than the central group with 30 adult males and 39 adult females. But despite the numerical inferiority, it was the Western group that repeatedly carried out deadly attacks on the central group starting in 2018, including one in which the alpha male was seriously wounded. From 2021, members of the western group also began to kill the young animals of the central group.
“From 2018 to 2024, members of one group carried out 24 attacks in which at least seven adult males and 17 young animals from the other group were killed,” the researchers report. 14 other previously healthy individuals also disappeared without a trace and may also have been victims of the aggression. “It is particularly striking that the chimpanzees kill former members of their own group,” says Sandel. “The new group identities override cooperative relationships that had existed for years.” Photos document that males who had defended their territory together against strange groups of chimpanzees in 2015 killed one of their former comrades just four years later because he belonged to the other half of the group.
Dynamics of violence
According to Sandel and his colleagues, such permanent group splits are extremely rare in chimpanzees. “Genetic evidence suggests that something like this only occurs about once every 500 years on average,” the researchers write. The only other case reported so far occurred in the 1970s in Gombe, Tanzania, where primate researcher Jane Goodall observed a war between former members of the same group. However, because the chimpanzees in Gombe were fed by the researchers, it was controversial to what extent the observed lethal aggression was actually part of their natural behavior.
“Taken together, these events show how networks can break down in the face of diverse demographic and social changes,” the researchers write. “They provide evidence that changing relationships, regardless of cultural characteristics, can divide a community and trigger collective violence.”
What does this reveal about human conflict?
According to Sandel, chimpanzee war is not directly comparable to human civil wars, but it can provide clues about fundamental dynamics. “The polarization and collective violence we observed in these chimpanzees could give us insights into our own species,” he says. “If relationship dynamics alone can lead to polarization and deadly conflict in chimpanzees, which lack language, ethnicity, or ideology, then these cultural traits may also be secondary to something more fundamental in humans.”
From the perspective of James Brooks of the German Primate Center in Göttingen, who was not involved in the study, the hostile division of the Ngogo chimpanzees is also a reminder of the danger that group divisions can pose to human societies. “However, people also relate, form bonds, and work together on a variety of levels across overlapping groups,” he writes in a commentary on the study. “This flexibility enables deep cooperation, but also provides the basis for acts of violence. Humans can learn from studying the group behavior of other species, both in times of war and in times of peace, while always remembering that their evolutionary past does not determine their future.”
Source: Aaron Sandel (University of Texas, Austin, USA) et al., Science, doi: 10.1126/science.adz4944