It is not entirely clear why the once closely-knit chimpanzee community of Ngogo in Uganda's Kibale National Park is now in conflict, but since 2018, scientists have recorded 24 killings, including 17 infants.
"These were chimpanzees that used to hold hands. Now they are trying to kill each other," said lead author Aaron Sandel.
The study, published in the journal Science, suggests that the intensity and duration of the violence could help in understanding the development of early human conflicts. Sandel, an anthropologist at the University of Texas in the U.S. and co-director of the Ngogo Chimpanzee Project, says chimpanzees are "highly territorial" and have "hostile interactions with members of other groups."
"It's like a fear of strangers," he told the Science podcast.
However, for several decades, nearly 200 chimpanzees in Ngogo lived in harmony. They were divided into two groups, which researchers call the Western and Central groups, but essentially functioned as a single community.
Sandel said he first noticed their polarization in June 2015, when chimpanzees from the Western group fled and the Central group chased them.
"Chimpanzees are a bit melodramatic," he explained, saying that after a conflict, there would usually be "screaming and chasing," and then later they would groom and cooperate with each other.
But after the 2015 conflict, researchers observed a six-week period of mutual avoidance between the two groups, with interactions becoming increasingly rare.
When interactions did occur, Sandel says they were "more intense and aggressive."
After two separate groups formed in 2018, members of the Western group began attacking chimpanzees from the Central group.
The study shows that in 24 targeted attacks since the split, at least seven adult males and 17 infants from the Central group have been killed, although researchers believe the actual number of victims is higher.
Researchers believe multiple factors may be at play, such as group size and competition for resources, as well as "male-male competition" for reproduction. However, they highlight three likely triggers.
The first was the deaths of five adult males and one adult female in 2014 for unknown reasons, which may have disrupted social networks and weakened bonds between subgroups.
The following year, there was a change in the alpha male, coinciding with the first period of separation between the Western and Central groups.
"Changes in dominance hierarchy can increase aggression and avoidance in chimpanzees," the study notes.
The third factor was the death of 25 chimpanzees, including four adult males and 10 adult females, due to a respiratory epidemic in 2017, a year before the final split. One of the deceased males was "among the last linking the groups," the paper states.
Sandel and his colleagues suggest that their findings encourage people to reconsider their understanding of conflict and warfare.
"In the case of the Ngogo split, individuals who had lived, foraged, groomed, and patrolled together for years became targets of lethal attacks simply because they belonged to a new group," they wrote, as reported by the BBC.
If chimpanzees, one of the genetically closest species to humans, can act this way without human constructs like religion, ethnicity, and political beliefs, then "interpersonal relationships may play a greater causal role in human conflicts than is often assumed," they add.
James Brooks, a researcher at the German Primate Center, said this is "a reminder of the danger that group divisions can pose to human societies."
Commenting on the study in Science, he wrote: "Humans must learn from the study of group-based behavior in other species, both in war and peace, while remembering that their evolutionary past does not determine their future."