Raiding parties of male spider monkeys: insights into human warfare?

Raids into neighboring territories may occur for different reasons, including the increase of foraging and mating opportunities directly or indirectly through the killing of neighboring rivals. Lethal raids have been mainly observed in humans and chimpanzees, with raiding males being reported to search purposefully for neighbors. Here we report on the first cases ever witnessed of raiding parties of male spider monkeys, a species expected to show such a behavioral tendency, given its similarity with humans and chimpanzees in critical socio-ecological characteristics, such as fission-fusion social dynamics and male-male bonding. Despite the high degree of arboreality of spider monkeys, all seven witnessed raids involved the males progressing single file on the ground in unusual silence. This is remarkably similar to the behavior of chimpanzees. The circumstances around the raids suggest that factors such as reduced mating opportunities, number of males relative to that in the neighboring community, and the strength of bonds among males could play a role in the timing of such actions. The raids did not appear to be aimed at finding food, whereas there is some indication that they may directly or indirectly increase reproductive opportunities. Although no killing was observed, we cannot exclude the possibility that spider monkey raids may be aimed at harming rivals if a vulnerable individual were encountered. The similarity of spider monkey raids with those of chimpanzees and humans supports the notion that lethal raiding is a convergent response to similar socio-ecological conditions.