Aaron A. Sandel, Yixuan He, Junpeng Ren, Yik Lun Kei, Kevin Lee, Isabelle R. Clark, Rachna Reddy, Jacob D. Negrey, Charles Birungi, Blessing A. Apamaku, Diana Kanweri, Davis Kalunga, Christopher Aliganyira, Sebastián Ramírez-Amaya, Phionah Nakayima, Raymond Katumba, Brian Kamugyisha, Daniela Acosta-Florez, Bas van Boekholt, Godfrey Mbabazi, Erone Akamumpa, Sharifah Namaganda, Alfred Tumusiime, Samuel Angedakin, Gesine Reinert, Oscar Hernán Madrid Padilla, Mihai Cucuringu, David Wipf, Kevin E. Langergraber, David P. Watts, John Mitani
Territorial conflicts in animals can inform aspects of human warfare, but civil war, with its shifting group identities, has not been previously observed. We report a rare, permanent fission in the largest-known group of wild chimpanzees ( Pan troglodytes ). Using 30 years of behavioral observations and network analyses, we describe a transition from cohesion to polarization in 2015 and the emergence of two distinct groups by 2018. Over the next 7 years, members of one group made 24 attacks, killing at least seven mature males and 17 infants in the other group. These findings indicate that group identities can shift and escalate into lethal hostility in one of our closest living relatives in the absence of the cultural markers often thought necessary for human warfare.