A long-term study at Ngogo in Kibale National Park shows a single large chimpanzee group split into two and then engaged in lethal violence. The Ngogo Chimpanzee Project began in 1995 under John Mitani and David Watts; the new findings appear in Science and raise questions about chimpanzee social behaviour.
When observers first began work at Ngogo there were well over 100 members and the group later grew to about 200. From 1998 to 2014 the chimpanzees lived as one unit. In 2015 aggression rose between two subgroups, called central and western, and the two began to isolate. In 2016 the western group sent a territorial patrol to the central group and males fought. In 2018 western chimpanzees killed a young adult male. Over the next seven years members of the western group killed seven mature males and 17 infants. At the split the western group numbered 76 and the central group 116; typically larger groups use numerical advantage to attack smaller ones, but the central group did not retaliate. As of December 2025 the western group had 108 individuals while the central group had fallen to 76.
Mitani, senior author, said chimpanzees treat outsiders as enemies regardless of past relationships and contrasted this with humans, who often cooperate with neighbours and strangers. Aaron Sandel, lead author, argued the case challenges ideas that human warfare is driven mainly by cultural markers like ethnicity or religion and instead points to basic relational dynamics such as shifting alliances and rivalries.
Mitani noted that several factors could have caused the split, including the group's very large size and the earlier deaths of several males in 2014, and he said a combination of events is the most likely explanation. He also stressed the value of long-term, federally supported research.
- Additional coauthors: University of Michigan
- University of Utah
- Utah Natural History Museum
- Arizona State University
- Source: University of Michigan
Difficult words
- lethal — causing death or able to cause death
- territorial — relating to land defended by animals or people
- patrol — a group sent to watch or guard an areaterritorial patrol
- retaliate — to make an attack in return for an attack
- outsider — a person or animal not part of a groupoutsiders
- aggression — hostile or violent behaviour toward others
- alliance — a relationship where people or groups cooperatealliances
- rivalry — competition or continued conflict between groupsrivalries
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Discussion questions
- Which of the possible causes Mitani mentions (very large size, earlier male deaths, or a combination) do you find most convincing? Explain your reasons.
- Do you agree with Sandel that shifting alliances and rivalries can explain violence between groups, including humans? Give examples or reasons.
- What is the value of long-term research like the Ngogo Chimpanzee Project for understanding animal social behaviour?
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Connie Nshemereirwe is an educational measurement specialist and former engineer who promotes Africa-led research, better science communication and stronger ties among scientists in the global South. She also directs the Africa Science Leadership Program.