The drill (Mandrillus leucophaeus) is one of the most endangered of all African primates. It is a large short-tailed forest baboon, which displays pronounced sexual dimorphism; males can grow up to twice the size of females. 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. There is a white ruff around the head and in mature males this is particularly dense, accentuating the size of his head and chest. Males have pink and lilac coloured testicles and a reddish region around the anus; if an individual is excited these colours are brighter. When females are in oestrus the area around her sexual organs swells up, and during pregnancy this area becomes deep red.