Violent coalitionary attacks and intraspecific killing in wild white-faced capuchin monkeys ( Cebus capucinus)

During 12 years of observation, we have observed three confirmed and two inferred lethal coalitionary attacks on adult male white-faced capuchins ( Cebus capucinus) by members of two habituated social groups at Lomas Barbudal Biological Reserve, Costa Rica. In one case, an alpha male was badly wounded and evicted from his group, and when later found by his former groupmates he was attacked by several of them and died less than 24 h later. In two other cases, lone extra-group males were mobbed by adult and immature males of a bisexual group. One victim's abdomen was torn open and he died less than 24 h later. A second victim was quite badly bitten but may have escaped. The fourth and fifth cases resulted from intergroup encounters. One victim lost the use of both arms but may have survived, whereas the other died of unknown causes within an hour of the attack. The observed death rate from coalitionary aggression at our site is approximately the same as that reported for eastern chimpanzees. Because at least three of the five observed incidents involved large coalitions attacking lone victims, they support the general hypothesis that imbalances of power contribute to intraspecific killing in primates. However, the occurrence of lethal coalitional attacks in a species lacking fission-fusion social organization poses a challenge to the more specific version of the imbalance-of-power hypothesis proposed by Manson and Wrangham in 1991 to explain chimpanzee and human intergroup aggression.