Chimpanzees are often viewed as highly social and intelligent creatures, capable of complex emotional bonds and cooperative behaviors. However, contrary to this common belief, lethal aggression is a documented and recurring phenomenon among wild chimpanzee populations, involving individuals intentionally causing the death of others.
The Reality of Fatal Aggression
Lethal aggression has been definitively observed across various wild chimpanzee populations. Long-term studies at sites like Gombe, Mahale, and Ngogo in Africa provide extensive evidence. For instance, a 10-year study at Ngogo in Uganda documented 18 observed killings and inferred three others, carried out by a large, male-dominated community. Over 50 years, researchers compiled data from 18 chimpanzee communities, identifying 152 killings across 15 of them, with 58 observed directly.
Victims of lethal aggression include adult males, infants, and sometimes females, while adult males are typically the primary perpetrators. A 2019 study on the Sonso community in Uganda analyzed 33 attacks on 30 victims over 24 years, finding that most attacks were by adult males and victims were often very young, with two-thirds being under one week old.
Underlying Causes of Lethal Conflict
Lethal aggression in chimpanzees arises from competition for resources and social hierarchy. Chimpanzees compete for food, mates, and territory. When resources become scarce, this competition intensifies, often escalating to aggressive encounters. Male chimpanzees are known to compete with rival groups over territory, food, and access to females.
Territorial defense is another significant driver, as chimpanzee communities aggressively protect their boundaries. Expanding territory provides access to more food and increases reproductive opportunities. The desire to establish and maintain social dominance also plays a role, with aggressive tactics sometimes leading to higher social status and reproductive success. Aggressive behaviors, including fatal outcomes, may offer a competitive advantage in securing resources and ensuring reproductive success.
Patterns and Contexts of Killing
Chimpanzee killings are categorized as inter-group or intra-group aggression. Inter-group aggression manifests as territorial raids and border patrols, where male coalitions systematically attack rival communities. During patrols, males travel silently along borders, searching for neighboring groups. Encounters can quickly escalate into violent attacks, leading to fatalities.
Intra-group aggression, while less common, also results in fatalities, including infanticide and power struggles within the same community. Infanticide, often committed by males, can be driven by the sexual selection hypothesis, allowing the mother to become fertile and mate sooner. Females may also commit infanticide, though rarely, sometimes to eliminate competitors for resources. Intra-community killings, such as those observed during community fissions at Gombe and Ngogo, involve former group members turning on each other. These fatal encounters often involve coordinated attacks and mobbing, targeting vulnerable individuals.