Drill  (Mandrillus leucophaeus)

The drill is one of the most endangered of all African primates (6). It is a large short-tailed forest baboon, which displays pronounced sexual dimorphism; males can grow up to twice the size of females (5). In both sexes the coat is olive brown, with a pale underside and the bare black face, which has an unusual extended muzzle featuring prominent ridges along each side (4). There is a white ruff around the head and in mature males this is particularly dense, accentuating the size of his head and chest (6). Males have pink and lilac coloured testicles and a reddish region around the anus; if an individual is excited these colours are brighter (4). When females are in oestrus the area around her sexual organs swells up, and during pregnancy this area becomes deep red (4). Two subspecies of drill are recognised, both are similar in appearance except for the hairs on the sides of the crown which are ringed yellow and black in the mainland drill (M. l. leucophaeus) and are brownish yellow with a black tip in the Bioko drill (M. l. poensis) (4).