Sexual Skin Color Contains Information About the Timing of the Fertile Phase in Free-ranging Macaca mulatta

Females of several primate species undergo cyclical changes of their sexual skin, i.e., the development of a swelling or a change in color. The relationship between intracycle probability of fertility and the size of sexual swellings is well established, but in the only study to combine an objective measure of color with endocrinological data, researchers found no evidence that swelling color contains such information. To evaluate the role of female skin color in the context of sexual signaling further, we investigated whether changes in sexual skin color contain information about the timing of the fertile phase in rhesus macaques (Macaca mulatta), a species in which adult females do not develop sexual swellings, but do express visually detectable changes in the skin color of the face and hindquarters. Using an objective and quantitative measure of color, along with detailed data on fecal progestogen and estrogen metabolite levels collected from 8 females of the Cayo Santiago colony, we show that the ratio of red to green (R/G) for facial and hindquarter skin significantly varies throughout the ovarian cycle. In addition, facial skin R/G is significantly higher during the 5-d fertile phase versus the 5-d periods immediately before or after this time, but no such pattern occurs in hindquarter R/G. This suggests that skin color change in female rhesus macaques may potentially signal information about the intracycle probability of fertility to male receivers, but that only facial skin color may signal reliable information about its timing.