Cheek pouches open beside the lower teeth and extend down the side of the neck. They can hold the equivalent of a stomach load of food, leaving hands and feet free for running and climbing. The mandrill's range follows the moist evergreen rainforest in central West Africa, where bulk forage throughout the year sustain it while fruit is scarce. They are found in the countries of Cameroon, Congo, Equatorial Guinea, and Gabon. Mandrills live in groups, mainly in a harem structure, with a dominant male.
Excess males live alone or forage with the group.
Groups can range in size from a few to as many as 50 individuals. They are diurnal, arboreal, and terrestrial. They sleep in trees at a different site each night. The nasal folds reflect the age and condition of the male mandrill, allowing other individuals to make an immediate assessment of his rank. The brighter the colors, the more dominant the male in the social hierarchy.