In a landmark study published this week in the journal Science, researchers have documented what may be the first observed case of a 'civil war' among wild chimpanzees. The conflict, which took place within the Ngogo chimpanzee group in Uganda's Kibale National Park, saw a once-unified community split into two factions that engaged in sustained, coordinated attacks over several years.
The study, led by primatologist Aaron Sandel, describes how the group's social cohesion began to fray in 2015. On a June day that year, Sandel noticed chimpanzees displaying nervous behaviour, grimacing and touching each other for reassurance as other group members approached. This was the first sign of a rupture that would lead to a permanent split by 2018, forming a western group and a central group.
Over the following seven years, the western chimpanzees launched 24 coordinated attacks on the central group, killing at least seven adult males and 17 infants. The researchers drew on more than three decades of behavioural observations to understand the split, which they attribute to a change in social hierarchies following the death of several key older individuals and a disease outbreak in 2017.
While chimpanzees have long been known to wage lethal aggression on outsiders, witnessing a once-unified group turn on itself is unprecedented. The researchers note that a similar event may have occurred in the 1970s in Gombe, Tanzania, observed by Jane Goodall, but at the time the understanding of chimpanzee behaviour was too limited to fully appreciate its rarity.
The study warns that human activities such as deforestation, climate change and disease outbreaks could make such inter-group conflicts more common, posing a threat to chimpanzee conservation. Brian Wood, an evolutionary anthropologist at UCLA, noted that the western chimpanzees' attacks increased their Darwinian fitness by reducing the survival and reproduction of their competitors, with the central group now experiencing the lowest survivorship ever documented in a wild chimpanzee community.
