Study: Lethal conflict after group fission in wild chimpanzees (DOI: 10.1126/science.adz4944)
After the large Ngogo chimpanzee group in Kibale National Park, Uganda, split into two, individuals in one group attacked and killed more than 20 members of the other group-chimps who had been their former friends and allies.
Prior to its split, the Ngogo group was unusually large. Chimpanzee groups typically comprise about 50 members, according to John Mitani, professor emeritus of anthropology at the University of Michigan. When he first began observing the Ngogo group, there were well over 100 members. Over time, the group grew to about 200 individuals. He and Yale University professor David Watts started the Ngogo Chimpanzee Project in 1995.
From 1998 to 2014, the chimpanzees lived together in a single group. In 2015, aggression between individuals of two subgroups living in the center and west erupted, with members in each isolating themselves from each other. In 2016, the western group sent a territorial patrol to the central group, and the males fought. Then, in 2018, members of the western group killed a young adult male from the central group. After that event, individuals in the two groups separated and stopped interacting socially, spatially and reproductively. Over the next seven years, members of the western group killed seven mature males and 17 infants.
Researchers have long known that chimpanzees will attack and kill their neighbors. But this finding, published in Science, was surprising because these chimpanzees were killing their former friends and allies, says Mitani, senior author of the study. For him, it highlights a key difference between chimpanzees and humans.
"Chimpanzees appear to consider outsiders as the enemy no matter who they are. One stark difference that exists between chimpanzees and humans is that we are an unusually prosocial and cooperative species," Mitani said. "We go out of our way to help and aid neighbors, some of whom may be total strangers.
"While aggression and wars break out among humans from time to time, for the most part, we live peaceably side-by-side with others now in a world of over 8 billion people. This difference between chimpanzees and humans is something that gives me hope, especially in this time of polarization."
Why the split? It's complicated
Several factors could have led to the 2018 split, Mitani said. The group's unusually large size is likely to have led to both increased feeding and reproductive competition. The deaths of several males in 2014 before aggressive interactions broke out in 2015 could have altered social dynamics within the group, creating hostilities that weren't there previously. But none of these events in isolation explain the break to the researcher. In sum, a combination of factors most likely led to the split.
For Mitani, the split is difficult to come to grips with because for many years the Ngogo chimpanzees thrived due to their large group size. Previous studies found that they used their large numbers to work together collectively and cooperatively, benefiting in the process: They dominated their neighbors, took over areas previously occupied by those neighbors, gained more food, and obtained reproductive advantages.
"Given this, why did yesterday's friend become today's foe?" Mitani said. "It's been hard to watch chimpanzees that I have studied for so long, know, and love turn on each other like this."
Aaron Sandel, lead author of the study and anthropologist at the University of Texas, says that their findings may tell us something about the evolution of human warfare, challenging the hypothesis that human warfare, including civil war, is driven primarily by cultural markers of group identity such as ethnic or religious differences.
"In humans, we often attribute violent conflicts, including civil wars, to tensions that arise from ethnic, religious or political differences," Sandel said. "We know this can't be the case with chimps. They don't have ethnicity or religion or political institutions. So this complex, violent conflict must be driven by more basic relational dynamics: Enduring social bonds, cliques, shifting alliances, rivalries."
Lessons from long-term data
Another unusual aspect of the split and its associated aggression is that the attacks and killings have been asymmetric, with members of the initially smaller group-the western group, with 76 members-attacking and killing members of the larger central group, which exited the split with 116 members. Typically, individuals in larger groups of chimpanzees use their numerical superiority to attack those in smaller groups. Members of the central group, however, failed to retaliate when the western chimpanzees started their killing spree.
"With these killings, the western group has flipped the tables on the central group," Mitani said. "At the outset of hostilities in 2015, the members of the smaller western group were quite afraid of the members of the larger central group, but something happened. They realized they could kill the central chimps and began doing so, to an obvious extreme."
As of December 2025, the western group had 108 individuals while the central group's numbers fell to 76.
"Findings like those in this paper are derived from long-term data collected over many, many years and have been supported by federal funds. I hope funds from our government will continue to be available for projects like this," Mitani said. "The Ngogo chimps have many more important and surprising stories to tell us."
Co-authors include U-M doctoral student Sharifah Namaganda and U-M alums Rachna Reddy, now of the University of Utah and the Utah Natural History Museum, and Kevin Langergraber of Arizona State University.
