The Ngogo chimpanzee "civil war"

The world's largest known group of wild chimpanzees — about 200 individuals in Kibale's Ngogo area — fractured into two factions called the Central and Western clusters, and the Western group began killing former group members. It's an exceptionally rare event: scientists estimate a community split like this happens, on average, only once every 500 years. Scientific American

What caused it?

Researchers identified several overlapping factors rather than a single trigger:

1. Group size. Most chimpanzee groups involve around 50 animals, but Ngogo had some 200, which may have stretched members' ability to maintain social connections and heightened competition for food and mates. NBC News
2. Loss of key social "bridges." Before the split, five adult males died, possibly from illness, severing key social connections between the subgroups. National Geographic
3. A change in alpha male leadership. A new alpha male emerged around 2015 — something that happens perhaps once every six to eight years — which disrupts social relationships and can increase levels of aggression across the community. NBC News
4. Eroding social bonds, not resource scarcity. Crucially, the study shows tensions grew into deadly violence even without resource shortages or cultural divisions to fuel them — it was the breakdown of social relationships itself that drove the split. Science

How the violence unfolded

In 2015, the Western cluster was first observed running away from the Central cluster, and a pattern of avoidance emerged. By 2018, the groups were permanently split — socially, geographically, and reproductively. NewsNation Between 2018 and 2024, Western adults killed seven males and 17 infants from the Central group, and 14 additional males disappeared under suspicious circumstances. Scientific American

The broader significance

The lead researcher noted that this gives scientists "a window into the chimpanzee mind that's really rare," and suggests that group identities can shift and override even deep social bonds without any ethnic, religious, or ideological markers — the kind of markers typically thought to underpin collective human violence. BBC Science Focus Magazine

The war is still ongoing, with further attacks recorded in 2025 and 2026. Live Science It's only the second chimp "civil war" ever observed — the first was Jane Goodall's famous "Four-Year War" at Tanzania's Gombe in the 1970s.