Japanese monkeys (Macaca fuscata) discriminate between pictures of conspecific males and females without specific training

We investigated whether Japanese monkeys can discriminate pictures of conspecific males and females using a visual paired comparison (VPC) task. Whole-body pictures of adult and nonadult monkeys were used as stimuli. The monkeys were first familiarized with pairs of pictures of different monkeys from one sex category (the familiarized sex). Pairs of novel pictures of a member of the familiarized sex and the opposite sex (novel) were then presented in test. The monkeys showed a preference for novel-sex pictures of both adult and nonadult individuals, indicating that they perceive the differences between familiarized- and novel-sex pictures. These results suggest that monkeys discriminate between pictures of males and females without specific training.