First Documented 'Civil War' in Wild Chimpanzees
A new study published in Science describes what may be the first thoroughly documented instance of a "civil war" among wild chimpanzees. While chimpanzees are known to engage in lethal aggression with outsider groups, this research highlights a unified group turning against itself.
Cases of neighbors killing neighbors are significant and draw parallels to human conflict dynamics, specifically the ability to cooperate yet also quickly turn on one another.
Initial Observations of Conflict
In June 2015, primatologist Aaron Sandel observed a cluster of Ngogo chimpanzees in Uganda's Kibale National Park displaying nervous behavior upon the approach of other members of their wider group. This observation was later identified as the initial sign of a prolonged conflict within the group.
Group Division and Sustained Attacks
The researchers utilized over three decades of behavioral observations of the Ngogo chimpanzee group, the largest known wild chimpanzee community. The group maintained social cohesion from at least 1995 until 2015. However, a shift in dynamics occurred, leading to the emergence of two distinct groups—the western chimps and the central chimps—by 2018.
Following this division, members of the western group carried out 24 sustained and coordinated attacks on the central group over seven years. These attacks resulted in the deaths of at least seven adult males and 17 infants.
Potential Causes of the Fracture
Researchers propose that changes in social hierarchies contributed to the Ngogo group's fracture. In 2015, the group's alpha male reportedly grunted in submission to another chimpanzee. The deaths of several key older individuals in the years preceding the division also weakened connections among sub-groups.
The abrupt deaths likely made the group vulnerable to polarization, which intensified after the alpha male change. A disease outbreak in 2017 further contributed to or expedited the split.
Scientists suggest a similar rupture and conflict may have occurred in the 1970s within the chimpanzee group in Gombe, Tanzania, observed by Jane Goodall. However, the understanding of chimpanzee behavior at that time was limited.
Conservation Implications and Aftermath
The study indicates that, based on genetic evidence, such "civil wars" among chimpanzees likely occur only once every 500 years. However, human activities like deforestation, the climate crisis, or disease outbreaks could disrupt social cohesion and potentially increase the frequency of such inter-group conflicts, posing a threat to chimpanzee conservation efforts.
The western chimps increased their Darwinian fitness by reducing the survival and reproduction of their competitors. The central chimps subsequently exhibited the lowest survivorship ever documented in a wild chimpanzee community.
Social ties and network connectivity are crucial for group cohesion and can be weakened under specific circumstances, particularly when dependent on a few key individuals.