Mandrillus is a genus of large Old World monkeys distributed throughout central and southern Africa, consisting of two species: M. sphinx and M. leucophaeus, the mandrill and drill, respectively. Mandrillus, originally placed under the genus Papio as a type of baboon, is closely related to the genus Cercocebus. They are characterised by their large builds, elongated snouts with furrows on each side, and stub tails. Both species occupy the west central region of Africa and live primarily on the ground. They are frugivores, consuming both meat and plants, with a preference for plants. M. sphinx is classified as vulnerable and M. leucophaeus as endangered on the IUCN Red List of Threatened Species.

Mandrillus is a genus within the tribe Papionini, which in turn is under the subfamily Cercopithecinae. This subfamily is classified under the family of Old World monkeys (Cercopithecidae) within the infraorder Simiiformes. The Papionini tribe contains six other genera: baboons (Papio), macaques (Macaca), crested mangabeys (Lophocebus), white-eyelid mangabeys (Cercocebus), the highland mangabey (Rungwecebus) and Theropithecus.

Originally, both species were considered part of the Papio genus, as forest baboons, due to superficial similarities such as size and appearance, particularly in facial features. However, studies conducted analysing anatomical and genetic differences between the current Mandrillus and Papio genera showed more differences than similarities resulting in the current taxonomic ranking. Furthermore, the studies showed Mandrillus are more closely related to the white eyed mangabeys, and diverged relatively recently (4 million years ago) from this genus.

Both species of Mandrillus develop extremely large muzzles, prominent nasal ridges and paranasal swelling (swelling in the area adjacent to the nostrils). The size and colour of the paranasal swellings correlate to male dominance and rank, while the size of nasal ridges is a way of attracting mates. Mandrillus teeth consist of two incisors, two premolars, one canine and three molars in each half of the upper and lower jaw, totalling 32 teeth. Furthermore Mandrillus display larger premolars and extended canines; these dental traits are better adapted to crushing hard objects. This is due to a large part of their diet consisting of hard, dry nuts and seeds that require greater crushing power and the use of their teeth in ripping apart rotting wood to search for insects and other invertebrates.

Within the shoulder and upper arm structures of the Mandrillus monkeys a deep scapular, broad deltoid plane, narrow stable elbow region and other skeletal features indicate the use of the forelimbs for climbing and foraging. This is used by the monkeys to climb trees when searching for ripe fruit and in the aggressive foraging of the forest floor in search of food. Mandrillus monkeys have developed an extremely broad and robust ilium, and a rounded tibial shaft. The development of these features can be attributed to the climbing of trees and quadrupedal locomotion. The largest toe is separated from the remaining toes for increased grasping power when climbing trees.

Both species of Mandrillus demonstrate a great degree of sexual dimorphism in weight, anatomy and physical appearance. The mandrill displays the most extreme sexual dimorphism for weight among all primates, with a male-female weight ratio of 3.2 – 3.4 at eight to ten years of age. Similarly, drills are one of the most sexually dimorphic primates for body weight, with a male growing up to 32 kg while a female grows to 12 kg. Sexual dimorphism is also displayed in the growth of the craniofacial bones of both species. The males of each species have longer muzzles, much larger paranasal swellings and longer canines than their female counterparts. In a study of wild drills, female muzzles only grew up to 70% the length of the male muzzles. Furthermore, males have brightly coloured, saturated rumps unlike their female counterparts. Both species also display the greatest visual sexual dimorphism within monkeys. On a scale based on rating the differences in physical features between genders, the mandrill obtained 32 whilst the drill obtained 24.5. These ratings are based on features such as the saturation and colour of the rump (and face for mandrills), the paranasal swelling, the fatted rump and fur colouring.

Mandrillus monkeys have a very localised biographical region located in West central Africa. The two species are often considered allopatric, they occupy non-overlapping regions, and their regions are divided by a physical barrier, the Sanaga river in Cameroon. Mandrillus leucophaeus occupy the area above the river in North western Cameroon and southwestern Nigeria up until the Cross River, and Bioko Island (Equatorial Guinea) which lies off the coast. The mandrill occupies the area below the river line in Cameroon, Río Muni, Gabon and Congo. The Mandrillus species occupy multiple sections of the Guinean forests of West Africa, including Cross–Sanaga–Bioko coastal forests and Cameroonian Highlands forests. The forests the monkeys occupy have a humid, tropical climate and rugged terrain. Deforestation has reduced the habitat of both Mandrillus species, reducing the distribution of each species, especially the drill.

Both Mandrillus species are frugivores, consuming both plants and insects with a preference for fruits and nuts. Mandrillus species spend a large amount of their time foraging through the forest in search of food. In a study conducted in Cameroon, approximately 84% of the faecal matter of mandrills consisted of fruit. Similarly, a study done on drills in southwest Cameroon showed that the mean weight of fruit and seed in faecal matter was equal to or greater than 80%. Seasonal changes can be seen within Mandrillus diet, during peak fruit season (September to March) their diet consisted mostly of fruit, pulp and seeds whilst during the fruit scarce season (June to August) there was a great increase in the consumption of insects, woody tissue and especially nuts. There was also an increase in the variation of the diet during the fruit-scarce season. Important fruit include but are not limited to, the fruit of the bush mango (Irvingia gabonensis), African Corkwood tree (Musanga cecropioides), Grewia coriacea, Sacoglottis gabonensis and Xylopia aethiopica. Invertebrates consumed include crickets, ants, caterpillars and termites. Rarely, Mandrillus monkeys will eat larger animals, such as rats and gazelles when presented with the opportunity.