Osman Hill (1970) has provided a detailed account of the comparative anatomy of both species of Mandrillus. My intent here is to focus on those facets of the mandrill's structure that will provide a useful background for further consideration of its behaviour, reproductive biology and evolution. Of special concern are those traits that are sexually dimorphic, including the secondary sexual traits that exhibit such spectacular development in adults of this species. The degree of similarity of these traits between the mandrill and drill must also be discussed, as well as some interesting differences between the two species.

External features

Mandrills are notable for their thickset bodies, large heads, long snouts and stumpy tails; Plate 1 shows an adult male and a female displaying the marked differences in body size and secondary sexual traits that are so characteristic of this species. Adult males weigh more than 30 kg and females 9–10 kg (Setchell et al., 2001; Wickings and Dixson, 1992a). Data on the weights and crown–rump lengths of male and female mandrills, ranging in age between one year and 11 years, are provided in Table 3.1.

The mandrill's pelage is brown dorsally and on the flanks, and consists of long soft hairs, each of which is annulated with alternating light and dark pigmented bands. The hair on the abdomen is sparser and lacks pigment. In some animals (especially some adult males), a longitudinal fringe of white hair is visible on the ventrum (the epigastric fringe: Plate 3). In the adult male, longer hair forms a crest on the crown of the head and nape of the neck; this crest is erected during some facial displays. The adult male mandrill does not have a well-defined cape of long hair on the shoulders and upper body in the same way as do male hamadryas baboons or geladas. Nonetheless, some mature mandrill males have longer hair on the neck and upper body, so that it resembles a mane. The same is true of the adult male drill.