Spectrocolourimetry visualized differences in sexual skin colouration in macaques

The females of some catarrhines develop conspicuous sexual skin transformations in their hind limbs. Among macaques (one of the radiated and adapted catarrhine groups with diversified sexual skin transformations), differences in sexual skin colouration between the Japanese macaque Macaca fuscata and the rhesus macaque M. mulatta have not been quantitatively analysed. In this study, the sexual skin colouration of these macaques was spectrocolourimetrically measured in the non-mating season (NMS) and the mating season (MS) and represented in a CIELAB space with the variables L*, a* and b*. The variables L*, a* and b* represent positions on the light-dark, red/magenta-green, and yellow-blue axes, respectively. In the Japanese macaques the average ± SD of L*, a* and b* was 53.61 ± 3.31, 11.51 ± 4.57 and 6.66 ± 2.25 in the NMS and 46.60 ± 2.78, 19.97 ± 2.99 and 8.80 ± 1.34 in the MS, respectively, while in the rhesus macaques the average ± SD of L*, a* and b* was 60.09 ± 3.96, 5.99 ± 4.59 and 5.83 ± 2.37 in the NMS and 52.70 ± 6.54, 13.62 ± 6.86 and 8.07 ± 1.43 in the MS, respectively. The sexual skin of the Japanese macaques was consistently much redder (larger a*) and darker (smaller L*) than that of the rhesus macaques. The smaller L* suggested a greater dermal melanin content in the Japanese macaques. These closely related macaque species have similar but distinct sexual skin colourations. Spectrocolourimetry is thus useful to suggest the histophysiological background of the colouration.