The rhesus monkey has possibly been used in more types of research than the fruit fly. With 20 years of ancestral information, therefore, Constance Dubuc and her fellow workers were able to trace sexual selection for facial and “bottom” colours (ie. their “sex skin.”).

The authors worked with the Caribbean Primate Research Center, but also worked at New York University, Duke University, the Universities of Chicago and Puerto Rico (all US) and The Max Planck Institute and Leipzig University, both in Germany. The paper is published in the Proc. Roy. Soc. B as- Sexually selected skin colour is heritable and related to fitness in a non-human primate.

266 facial image, combined with pictures of their genitals and hindquarters were available to this research. The free-ranging population used in the study was situated on Cayo Santiago Island in the Caribbean. The effect of degree of redness was to increase the number of offspring (their fecundity). Both sexes of rhesus macaque, Macaca mulatta, were affected. Males that were darkly coloured and high ranking in macaque society produced many young, while females scored in a similar way. The females however couldn’t be proved to influence male selectiveness. Evidence that the dark skin colouring was inherited by the offspring was also a first for mate choice of red skin ornamentation (inter-sexual selection.) Previously, both sexes had only been proved to show interest in darker faces in. the other sex.

Primates are similar to birds in the sexual displays they show. Age has a strong effect on the skin darkness, while males alone get redder in time. This population had little fighting, with males “queuing” to gain more dominance. Mate-guarding takes place with the more dominant males, the females being more inclined to stay with the darker males. Naturally, the red skin does convey some information about condition (“fitness”) and because of that, suitability for mating.

The use of colour in primates, or rather the lack of colour discrimination also becomes extremely significant for tamarin monkeys in Tamarin’s Colour-blindness.