Lethal intergroup aggression by chimpanzees in Kibale National Park, Uganda

Area of Science:

• Primate Behavioral Ecology
• Ethology
• Evolutionary Anthropology

Background:

• Chimpanzees (Pan troglodytes) exhibit frequent hostile intergroup relations across their range.
• Intergroup aggression involves male coalitions attacking neighboring communities during foraging and patrols.
• Lethal aggression risk is highest for adult males and infants, and lowest for sexually receptive females.

Purpose of the Study:

• To document and analyze fatal intergroup coalitionary aggression in chimpanzees.
• To add new data to the understanding of intercommunity violence in chimpanzees.
• To test the "imbalance of power" hypothesis in the context of chimpanzee aggression.

Main Methods:

• Direct observation of four fatal/near-fatal attacks on adult males and one on a juvenile.
• Inference of a fifth attack based on forensic and behavioral evidence.
• Summary of additional intercommunity attack data from Ngogo and Kanyawara communities in Kibale National Park, Uganda.

Main Results:

• Three fatal attacks on adult males and two near-fatal attacks (one on an adult male, one on a juvenile) were documented.
• Attacks were carried out by male coalitions from habituated, unprovisioned communities.
• Observations align with the "imbalance of power" hypothesis.

Conclusions:

• Lethal coalitionary intergroup aggression in male chimpanzees is likely an evolved behavioral strategy.
• Such aggression may enhance resource acquisition, improve survivorship, and increase female fertility for the attacking group.
• The findings support the adaptive significance of intergroup violence in chimpanzee societies.