[Visually-guided discrimination and preference of sexuality in female macaque monkeys]

Visual information about face and body including facial expression and bodily behavioral patterns has been known to play an important role in social and emotional communication in monkeys. Its involvement in sexual activity has also been demonstrated in male monkeys but it is poorly understood in female monkeys. In the present study, visually-guided discrimination and preference of sexuality were investigated in female macaque monkeys performing operant bar-press tasks in an experimental cage which had a transparent panel facing a display. In the sex discrimination task, two rhesus monkeys (Macaca mulatta) were trained to discriminate sex of a monkey shown in a picture which was randomly selected from six photographs (three males and three females) and was presented on the display. The monkey pressed a right or left bar for male or female monkey, respectively, to get water as a reward. Under this discrimination task, the monkeys could discriminate the sexes of monkeys shown in newly presented pictures. When choice bars were reversed, correct responses significantly decreased below chance level. In the sex preference task, three rhesus monkeys and three Japanese monkeys (M. juscata) were used. The monkeys voluntarily pressed the bar to watch the video movie showing either male or female rhesus monkeys. The movies were presented as long as the subject kept pressing the bar. The same movie was continued when the monkey pressed the bar again within 10s after the previous release of the bar, while it was changed to the other when 10s passed after the subject released the bar. The total duration of the responses in daily sessions was measured. In this visual preference task, four out of six monkeys showed sex preference. Three adult Japanese monkeys (6-8 y) pressed the bar to watch the video movie of male monkeys which was taken in breeding season with longer duration than that of female monkeys taken in the same season. The other two adult rhesus monkeys (7 8 y) did not show the sexual preference. The one remaining youngest rhesus monkey (5 y) preferred to watch the female movie which was taken in non-breeding season. The visual preference was observed not only in winter but also in summer when tested. The present results suggest that visual information alone is sufficient to discriminate sex and produce sex preference in some female monkeys. The seasonal changes in visual appearance of objects seem to be more determinant factors for the preference than the endogenous factors of subjects.