In the 1970s, the late Jane Goodall observed a community of chimpanzees in Gombe, Tanzania, breaking into two factions; the males in one group ended up killing all the males in the rival group over the next four years, along with one female chimp. But the case was considered an anomaly, although there is genetic evidence suggesting this kind of split is a rare event occurring every 500 years or so. Now researchers have observed the largest known community of Ngogo chimpanzees in Uganda also permanently splitting into two rival groups with a similar outbreak of violence, according to a new paper published in the journal Science.
"What's especially striking is that the chimpanzees are killing former group members," said co-author Aaron Sandel, an anthropologist at the University of Texas, Austin. "The new group identities are overriding cooperative relationships that had existed for years. I would caution against anyone calling this a civil war. But the polarization and collective violence that we have observed with these chimpanzees may give us insight into our own species."
The authors analyzed 24 years' worth of data from social networks, 10 years of GPS tracking, and 30 years of demographic data on the Ngogo chimps in Uganda's Kibale National Park. They identified three distinct phases to the split. First there was an abrupt shift as chimp relationships became polarized into two distinct clusters: Western and Central. The chimps then spent the next two years increasingly avoiding those in their rival cluster; there were very few interactions across clusters, and Western male chimps started patrolling their territory, showing increased aggression toward Central males. By 2018, the fissure had become permanent.
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