Face recognition of the same and different species by squirrel monkey (Saimiri sciureus)

Face serves for both person identification and species recognition in humans. Do monkeys utilize their faces to identify other monkeys as humans do? We explored the face recognition by squirrel monkey (Saimiri sciureus) under three conditions, i.e. identification of individual monkeys of their own species (discrimination between two squirrel monkeys), of other species (discrimination between two Japanese macaques), and of two different species (discrimination between a squirrel monkey and a Japanese macaque). METHOD: The face stimuli consisted of six photographs: three faces of unfamiliar squirrel monkeys and three faces of unfamiliar Japanese macaques. We trained three squirrel monkeys to distinguish between two kinds of stimuli simultaneously presented in pairs for each condition. Monkeys were reinforced when they responded to the photograph of one monkey and were not reinforced when they responded to the photograph of the other monkey. RESULT: Two of three subjects could discriminate between two different species. But they could not identify individual monkeys of their own species and other species. DISCUSSION: The results suggest that squirrel monkeys may utilize their faces to discriminate between species, but may not utilize their faces for identification of individual monkeys of their own species and other species. From the evolutional viewpoint, we may consider that the cognitive ability of the identifying individual monkeys through the use of their facial features might have been acquired in the latest stage of evolution of primates.