Romanes (1893) tells us that this statement constituted Darwin's ‘very last words to science’, as they were written shortly before his death, in 1882. Well over a century later, the importance of sexual selection has been amply confirmed and much more is now known about its scope, although the exact processes through which it might operate are still debated. Darwin identified two principal types of sexual selection, intra-sexual competition for access to mates and inter-sexual selection for secondary sexual adornments and displays as enhancers of sexual attractiveness. Both these processes were thought to act mainly upon males, leading to the evolution of larger body size, greater weaponry and aggressiveness, as in the case of red deer stags, or to extravagant masculine adornments and courtship displays, as seen in the peacock. The evolution of extreme body size and canine size sexual dimorphism in the mandrill will be considered first, and then I shall discuss the evolution of the adult male's colourful secondary sexual traits.

Although Darwin was preoccupied with the effects of intra-sexual selection upon males, it is apparent that females might sometimes compete among themselves for mating opportunities, as well as more indirectly for the resources required to nurture their offspring. Inter-sexual selection has also favoured the development of sexually attractive adornments in females of certain species, as well as in males. Indeed, the sexual skin swellings that occur in females of many Old World anthropoids provide powerful evidence for the existence of this type of inter-sexual selection (Dixson, 1983a, 1998a, 2012). The effects of sexual selection upon the evolution of female sexual skin in mandrills, and in other species, will be discussed in this chapter.

Darwin considered that the primary genitalia of both sexes had been moulded by evolution in response to natural selection, rather than as a result of sexual selection. However, it is now known that sexual selection sometimes operates at the level of the gonads (via sperm competition: Parker, 1970) and some other parts of the reproductive system (via cryptic female choice: Eberhard, 1985, 1996), in addition to influencing the evolution of copulatory patterns.