Intergroup lethal gang attacks in wild crested macaques, Macaca nigra

Lethal gang attacks, in which multiple aggressors attack a single victim, are among the most widespread forms of violence between human groups. Gang attacks are also frequent in some other social mammals, such as chimpanzees, Pan troglodytes, wolves, Canis lupus, spotted hyaenas, Crocuta crocuta, and meerkats, Suricata suricatta. So far, species in which gang attacks have been observed share one or more of these socioecological features: territoriality, fission–fusion, cooperative breeding or coalitionary bonds. However, the scarcity of data in other taxa makes it challenging to determine whether one/all of these socioecological features is necessary and sufficient to drive the evolution of gang attacks. Here we describe the first reports of intergroup gang attacks in the crested macaque, using data on three groups collected over 13 years, with the joint observation times for the three groups summing to 37 years. Crested macaque gangs attacked outgroup conspecifics when aggressors were numerically superior to victims. Adult females were the most frequent age/sex category to attack outgroup conspecifics. The victims were mostly adult females and infants. We propose that coalitionary bonds, hostility towards outgroup individuals and the ability to estimate numerical odds may suffice to trigger intergroup gang attacks when the conditions favour an imbalance of power between victims and attackers.