The pigmentation comes from two sources: red from blood in dense superficial capillaries and blue from structural scattering in collagen fibres of the skin. Both respond to hormonal state, so the display can brighten or dull without any change in the underlying anatomy.
Long-term study of a semi-free-ranging colony in Gabon showed that the brightest males held the highest rank, sired the most offspring, and lost colour intensity after being deposed, sometimes within a few weeks. Subordinate males remain duller throughout. Females and juveniles carry a much muted version of the same pattern.
The name misleads. Mandrills and drills were long called baboons, but they sit in their own genus, Mandrillus, and molecular work places them closer to the mangabeys of the genus Cercocebus than to true baboons in Papio. Adult males can exceed thirty kilograms, more than any other monkey.