Sexual Signaling in Male Vervet Monkeys (Chlorocebus Pygerythrus); Influences of Hormones, Behaviour, and Parasitism on Scrotal and Penile Skin Colouration

Male secondary sexual characteristics may convey information about the signaller. The red penis and blue scrotum of male vervet monkeys make them a good species to examine variation in signal expression. We quantified colour in vervets at Lake Nabugabo, Uganda from standardized photos. We hypothesized that inter- and intra-individual variation in the expression of the genital colour of males is influenced by parasites, dominance rank, age, androgens and glucocorticoids. Our results indicate that blue scrotal colour between males is predicted by rank and parasites, and red penis colour is predicted by age (or sexual maturity) and parasites. Within males, our findings indicate that short-term variation in blue scrotal colour characteristics and contrast are negatively correlated with measures of parasitism and red penis hue becomes redder (less pink) as males increase in rank. These findings support the Hamilton-Zuk hypothesis and suggest that vervet genitals are a badge of status symbol.