Primatologist Aaron Sandel remembers the day he noticed something was wrong within the largest known chimpanzee community in the world. It was more than ten years ago, in June 2015. On the green hill of Ngogo in Kibale National Park, Uganda, he was observing a part of this primate group when he saw monkeys agitating as other clan members approached.
“They seem nervous. They touch each other, as they usually do to reassure themselves when they hear other chimpanzees (from outside their clan). At that moment, I remember being completely perplexed. I was focused and thought, ‘What is happening?'”, he told the digital journal 404 Media.
This was the first sign of tension between two factions of the chimpanzee community. Aaron Sandel had been studying this united group of over 200 monkeys for years. The Ngogo Chimpanzee Project was launched in 1995 by his colleague, American primatologist John Mitani, to study the long-term social relationships and cooperation among these primates.
However, cooperation was no longer possible after that day in June 2015. From then on, when they met, the two groups of monkeys chased each other and shouted. They made calls usually reserved for confrontations with foreign groups, remembers John Mitani, “but I couldn’t imagine they would split,” he added.
The split became irreversible within three years. The scientists witnessed the clan splitting into two distinct groups. Fiercely opposing and irreconcilable, these two factions would confront each other for years. After the effective split of the group in 2018, hostilities persisted between the two camps for the following seven years. Researchers recorded 24 attacks by one group on the other, resulting in the death of seven adult males and 17 young monkeys.
This story of a fratricidal war is documented in an article published in the journal Science on April 9. They reported a rare and permanent split within the largest known group of wild chimpanzees. They described a transition from cohesion to polarization in 2015 and the emergence of two distinct groups in 2018.
This conflict is the first time a deadly conflict between groups once socially connected has been observed outside the human world. These results indicate that group identities can evolve and deteriorate into deadly hostility in one of our closest living relatives, without the cultural markers often considered necessary for human warfare, noted the scientists.
The researchers are still uncertain about the causes that triggered this war. Several hypotheses are being considered, such as intensified food and reproductive competition, rejection of a new alpha male, or a respiratory epidemic that killed about twenty chimpanzees in 2017, weakening the clan.
“If chimpanzees can go through such a complex process in the absence of ethnicity, language, religion – causes often attributed to human warfare – it must be their interpersonal social links, daily conflicts, reconciliations, evictions – all these dynamics, are at play,” analyzed Aaron Sandel.
According to the study authors, based on genetic data, this kind of permanent split among chimpanzees is extremely rare, occurring only once every five hundred years. Aaron Sandel warned that any human activity disrupting social cohesion – deforestation, climate crisis, epidemics – risks making such intergroup conflicts more frequent.
