Colour and contrast of female faces: attraction of attention and its dependence on male hormone status in Macaca fuscata

Colour signals play a major role in social and sexual communication in a broad range of animal species. Previous studies on nonhuman primates showed that intense female skin coloration attracts male attention. We investigated (1) whether sexually active male Japanese macaques are attracted by intensely coloured female skin, (2) whether a preference for intense skin coloration results from the increased colour contrast between the skin area and its surroundings irrespective of the red chromaticity, and (3) whether the endocrine status of sexually active males affects their attentional selectivity (or preference) for salient female sexual skin coloration. We conducted two behavioural experiments in two consecutive mating seasons. First, we presented two female face images coloured in a natural range of red skin coloration on monitors. Second, we presented the same faces dissociated from the red chromaticity while maintaining their initial colour contrast properties. In both experiments we analysed male selective visual attention and approaches as a function of stimulus type. Faecal samples were collected after each experiment to analyse focal males' cortisol and testosterone excretion rates. We found that female facial skin coloration triggered selective behaviour in social-living male Japanese macaques. Variances in colour contrast also triggered males' selective orienting towards an intensely coloured face image but the red chromaticity remained essential to induce prolonged male interest. Furthermore, elevated cortisol facilitated male preferences for the intensely coloured female faces, sociosexual stimuli that are presumably highly relevant during the mating season. Future studies may pursue the principle of colour contrast in male attentional behaviour with respect to subtle colour changes expressed by females throughout the reproductive cycle. Cortisol-related physiological processes should be considered in studies on mating-relevant selective attention.