The average body mass for an adult male mandrill is between 21 and 28 kilograms, and for the female it is between 11 and 12 kilograms (Hill, 1970). The mandrill has a dental formula of 2:1:2:3 on both the upper and lower jaws (Ankel-Simons, 2000). The incisors of this species are broad and high-crowned (Caldecott et al., 1996). This species has pronounced maxillary ridges. The mandrill has a relatively short tail. The pelage color ranges from dark brown to charcoal-gray. The penis of the male is colored red and the scrotum has a lilac color. The face also has bright coloration like the genitalia and this develops in only the dominant male of a multi-male group (Dixson et al., 1993; Wickings and Dixson, 1992). Females and juveniles have a duller blue snout and a buffy colored beard (Rowe, 1996). The dominant male also tends to have a relatively heavier rump, larger testes, and higher plasma testosterone levels (Dixson et al., 1993; Wickings and Dixson, 1992).
The mandrill is found in the countries of Cameroon, Congo, Equatorial Guinea, and Gabon. This species is found in dense primary rainforest and sometimes secondary rainforest, gallery forest and coastal forest (Harrison, 1988; Rowe, 1996). Mandrills also can be found on plantations, examples being cassava, Manihot utilissima, and banana, Musa paradisiaca (Sabater Pi, 1972). Mandrills were found to be found in plantations during the dry season (Sabater Pi, 1972).
The mandrill is a frugivorous species, but also eats leaves, seeds, nuts, shoots, arthropods, crabs, fish, small vertebrates, and cultivated crops like manioc and oil-palm fruits. Tutin et al. (1997) found that in Lope Reserve, Gabon, the diet of the mandrill was composed of 50.7% fruits, 26% seeds, 8.2% leaves, 6.8% pith, 2.7% flowers, 4.1% animals, and 1.4% other. In Lope Reserve, Gabon, this species was found to avoid unripe fruit, even those of preferred food trees (Rogers, 1996). In captivity, individuals have been known to catch and eat house sparrows, Passer domesticus, that happen to fly in the cage (Mellen et al., 1981). The mandrill has also been observed hunting a young bay duiker, Cephalophus dorsalis (Kudo and Mitani, 1985). In Lope Reserve, mandrills will forage more in gallery forests and savanna-forest edge during the dry season (Rogers, 1996). This foraging strategy might be because in gallery forests and savanna-forest edge, fruit resources might be available were in the continuous forest they may be depleted (Rogers, 1996). This species mainly forages on the forest floor, but can feed in any level of the canopy (Estes, 1991; Hoshino, 1985; Harrison, 1988).
The mandrill has a unimalesocial system, with the leader male receiving most of the copulations. These small groups come together with other groups to form troops of up to 250 individuals, but sometimes a group does not form a troop with other one-male groups (Hoshino et al., 1984). Rogers et al. (1996) found that in Lope Reserve, Gabon, the social system was multimale-multifemale. Hoshino et al. (1984) found that multi-male groups are found during the minor fruiting season and one-male groups are found during the major fruiting season. During the major fruiting season the fruiting trees are patchily distributed in the forest and it might be easier for mandrills to move in smaller groups (Hoshino et al., 1984). In multi-male groups there tends to be one or a few more dominant males, with a hierarchy existing amongst the males (Kudo, 1987; Wickings et al., 1993). Wickings et al. (1993) found that in multi-male groups mating may be restricted to dominant males, those that have the prominent coloration on the face and the genitalia. Multimale-multifemale groups may form because increased predation protection and increased foraging efficiency (Rogers, 1996). Solitary males also occur in this species (Hoshino et al., 1984; Sabater Pi, 1972). Groups are made up of adult males, subadult males, adult females, and juveniles, with immature members comprising more than 50% of the group population (Hoshino et al., 1984). Males disperse from their natal group (Hoshino et al., 1984; Harrison, 1988). Groups stay in close together during the day, maintaining cohesion by use of the two-phase grunt and the crow calls (Hoshino et al., 1984). Hoshino et al. (1984) suggests, "the integration of a group of mandrills may be maintained by the members of a group focusing attention on adult males." This is not a territorial species, with smaller groups coming together easily without antagonism to form larger groups (Hoshino et al., 1984). Adult males initiate movement of the group (Sabater Pi, 1972). The mandrill was found to form mixed-species associations with Cercopithecus cephus and Cercopithecus pogonias (Sabater Pi, 1972).
The mandrill gives birth to a single offspring. During estrus the perineum of the female swells and turns red. Estrus synchrony amongst the females in a group has been found in a semi-free population of this species (Feistner, 1990). In semifree-ranging individuals the birth season was from July to October (Feistner et al., 1992). The mean interbirth interval for the mandrill is 17.3 months (Feistner et al., 1992). Dominant males will guard females when the females are in estrus (Dixson et al., 1993). Sometimes an adult female will avoid or flee an adult male that is attempting to guard them (Dixson et al., 1993). Dixson et al. (1993) found that in a semi-free ranging group that the heavy rumped males fathered all of the infants and the solitary/periphery males did not father any. In captivity, mandrills can reproduce before reaching adult weight and dental complement (Feistner et al., 1992).