Among the mammals, sexual selection has sometimes resulted in the evolution of extreme sex differences in body size, weaponry and secondary sexual adornments. Nowhere is this observation more apposite than in the case of the mandrill (Mandrillus sphinx), as it offers numerous examples of the effects of sexual selection, especially in adult males. At over 30 kilogrammes in weight, the mature male mandrill is the largest of all the Old World monkeys, and it is more than three times the size of the adult female. The male's enormous jaws are equipped with long, dagger-like canine teeth, notably in the upper jaw. Most extraordinary, however, is the mandrill's colouration. Adult males of this species display large areas of bright blue and red skin (the so-called ‘sexual skin’) on the face, rump and genitalia. As young males transition to sexual maturity, boney paranasal swellings enlarge on each side of the snout, and cobalt blue sexual skin overlies these swellings in a series of ridges, flanking the scarlet mid-nasal strip and fleshy tip of the nose. Add to these extraordinary secondary sexual traits the possession of a yellow beard, a crest of hair on the scalp and nape of the neck, a mane, a large sternal cutaneous gland and marked enlargement of the colourful rump owing to deposition of fat, and the male mandrill ranks as the most visually striking of all primate species. Although adult female mandrills are certainly much less brightly coloured than the males, they are not lacking in secondary sexual adornments. Thus, there is a female sexual skin covering the perineal and genital areas, and this undergoes marked changes in swelling and colouration during the menstrual cycle.

Biologists have long speculated as to why the mandrill should exhibit such an extreme expression of so many sexually dimorphic traits. When Charles Darwin (1871, 1876) was formulating his ideas concerning evolution by sexual selection, he observed that ‘no other member in the whole class of mammals is coloured in so extraordinary a manner as the adult male mandrill’. Darwin regarded the male mandrill as the mammalian equivalent of the peacock, suggesting that its bright colouration had also evolved to ‘serve as a sexual ornament and attraction’ to females.