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Sex categorization of conspecific pictures in Japanese monkeys (Macaca fuscata)


We investigated whether monkeys discriminate the sex of individuals from their pictures. Whole-body pictures of adult and nonadult monkeys were used as stimuli. Two male Japanese monkeys were trained for a two-choice sex categorization task in which each of two choice pictures were assigned to male and female, respectively. Following the training, the monkeys were presented with novel monkey pictures, and whether they had acquired the categorization task was tested. The results suggested that while monkeys discriminate between the pictures of adult males and females, discrimination of nonadult pictures was difficult. Partial presentations of the pictures showed that conspicuous and sexually characteristic parts (i.e., underbellies including male scrotums or breasts including female nipples) played an important role in the sex categorization.

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This work was supported by a Grant for Biodiversity Research of the 21st Century COE (A14, Kyoto University) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors are grateful to Nobuo Masataka and Sumiharu Nagumo for support throughout the research. Thanks are also extended to Akemi Kato and Satomi Araya for assistance and the daily care of the monkeys and to Jean-Baptiste Leca for the generous offer of pictures of the Arashiyama troop. The present study complies with the laws of Japan.

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Correspondence to Akihiro Izumi.

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Koba, R., Izumi, A. Sex categorization of conspecific pictures in Japanese monkeys (Macaca fuscata). Anim Cogn 9, 183–191 (2006).

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  • Japanese macaque
  • Sex categorization
  • Conspecific picture
  • Operant conditioning