The Ngogo
Chimp War

What happened

Since 1995, researchers from the Ngogo Chimpanzee Project have studied a single large community in Uganda's Kibale National Park — the biggest known group of wild chimpanzees ever documented, peaking at nearly 200 individuals. Males from across the group patrolled together, hunted together, and in 2009 jointly drove a neighbouring group off its territory.

The first sign of fracture came in 2014, when five adult males and one female died in quick succession — more than 10% of the mature male population. Their deaths severed key social ties between the Western and Central clusters. Then, on 24 June 2015, the two clusters met near the centre of their shared range. Instead of the usual reunion, they fought. The Western chimps fled, then returned aggressor. A six-week period of avoidance followed — unprecedented in 20 years of observation.

Network analyses captured what was happening: modularity — the degree to which the group was dividing into separate clusters — jumped sharply in 2015, the single biggest structural shift in 24 years of data. By 2018, the split was permanent. The last cross-group offspring had been conceived in March 2015. Two distinct groups occupied two distinct territories. What had been the centre of a shared range was now a border.

The killing began that same year. Over the next seven years, the Western group launched 24 attacks, killing at least 7 adult males and 17 infants — all from the Central group. Not one Western chimp died. The Western group is numerically smaller (32 adults at the time of the split vs. 69 in Central), yet has won every engagement. Scientists attribute this to cohesion: tight, enduring bonds among Western males appear to outweigh the Central group's numerical advantage.

Source: Sandel, He, Ren, Kei, Mitani et al. · Science, 9 April 2026 · doi:10.1126/science.adz4944

Why did it happen?

No single cause explains the Ngogo fission, but researchers have identified several overlapping factors:

2014 — the social bridge collapses

Five adult males and one female died \u2014 more than 10% of the mature male population. Two likely died of disease; causes of the others are unknown. These individuals had bridged the Western and Central clusters for years. Their loss weakened cross-cluster ties and is the leading candidate for what started the polarisation.

2015 \u2014 alpha male turnover

The Ngogo alpha changed hands in 2015, the same year as the first sustained separation. Both the outgoing and incoming alpha belonged to the Central cluster, though the new alpha had previously been Western. Dominance transitions reliably increase aggression and avoidance in chimpanzees.

2017 \u2014 respiratory epidemic

A respiratory outbreak in January 2017 killed 25 chimpanzees, including 4 adult males and 10 adult females. Polarisation was already underway, but this likely accelerated the final separation. One of the males who died was among the last individuals still connecting the two groups.

Sheer group size

At nearly 200 individuals with 30+ adult males, Ngogo far exceeded the size of any other known chimpanzee group. Large groups strain relationship maintenance. Once key social bridges snapped, there were too many individuals for the network to hold together.

The broader scientific conclusion: you don't need ethnicity, religion, or ideology to produce civil war. Shifting interpersonal relationships alone \u2014 the relational dynamics hypothesis \u2014 are sufficient to fracture a community and Generate lethal collective violence.