Capuchins recognize familiar faces

Although growing neural evidence suggests that human and nonhuman primates share a similar face-processing mechanism, evidence from behavioural research has been mixed. If primates share a similar face-processing system, one would expect to observe similar behavioural effects in both human and nonhuman primates. A particularly robust effect observed in humans is the familiarity effect; humans are better able to recognize familiar as opposed to unfamiliar individuals. Here, we used a matching-to-sample paradigm to examine capuchin monkeys' ability to discriminate conspecific faces across three degrees of familiarity: individuals living in one's own social group ('in-group'), individuals living in one's neighbouring group ('out-group') and completely unfamiliar individuals ('unfamiliar'). We hypothesized that if capuchins utilize their knowledge of familiar individuals to help them discriminate photos of conspecific faces, then performance on the recognition task would increase with the familiarity of the individual. Capuchins were better able to individuate familiar in-group members and familiar out-group members compared to unfamiliar individuals, suggesting that familiarity affects capuchins' ability to discriminate conspecific faces, as it does that of humans and apes. However, there was no significant difference between in-group members and out-group members, suggesting that the concept of 'familiarity' may extend to individuals living in neighbouring groups that one interacts with regularly. This would be a fitness advantage for social species, like capuchins, which compete with neighbouring groups over access to food and mates. It may be an advantage for males in particular as they emigrate to neighbouring groups when they reach maturity.