Faces and Behinds: Chimpanzee Sex Perception

Six adult chimpanzees (Pan troglodytes) trained on computerized matching-to-sample were shown a sample behind (anogenital region) of a chimpanzee and rewarded for selecting a corresponding facial image. If the two faces were of the same sex, and one belonged to the same individual as the behind, subjects made the correct association for familiar individuals but not unfamiliar ones, suggesting whole-body knowledge of group mates. If the two faces were of opposite sex, subjects selected the same-sex face as the behind at first only for familiar individuals when face and behind belonged to the same individual. During subsequent exposures, however, they learned to associate the same-sex face with the behind even if the behind was "generic" male or female (i.e., unmatched to any known individual) provided the depicted individuals were familiar. This suggests that sex perception is aided by a "gender construct" derived from firsthand experience with group mates, and which construct is unavailable for unfamiliar individuals.