Why Do Mandrills Have Colorful Faces? Color and Status

What makes the colors?

The mandrill’s face combines two very different ways of making color.

• Red: produced by blood flowing through a dense network of vessels just beneath bare skin. More blood flow and more vessels mean a deeper red.
• Blue: a structural color, not a pigment. In 2004, researchers studying mandrill skin found that collagen fibers in the deeper skin layer are arranged in tightly ordered arrays. These arrays scatter blue wavelengths of light back to the eye, much as the structure of some bird feathers creates blue.

In females, darker faces get their color from melanin, the same pigment that colors human skin and hair. The pattern of the male face is supported by bony ridges along the snout, which make the blue grooves stand out even more.

Color shows rank

The mandrill is one of the most striking examples of sexual selection, the evolution of traits that help an animal win mates rather than survive. Adult males compete for rank, and color is closely linked to that competition.

A study of a semi-free-ranging colony in Gabon, published in 2001, followed males as their status changed. When a male became the alpha, the highest-ranking male, his face and rump turned redder, his testes grew and his testosterone rose. When an alpha lost his position, the red faded somewhat. The blue was more stable, but high-ranking males showed stronger contrast between red and blue.

These changes make the face a kind of living badge. Other males can judge a rival’s strength at a glance, and many conflicts are settled by displays of color, head-bobbing and threat stares instead of dangerous fights. Adult males have canines up to about 4.5 cm (1.8 in) long, so avoiding serious fights benefits everyone.

Fatted and non-fatted males

Color is one part of a whole package of status signals. Dominant males also put on fat along their flanks and rump and produce more scent from a gland on the chest. Scientists call them “fatted” males.

Subordinate adult males, called “non-fatted,” stay slimmer and duller, and in some the development of these traits seems to be held back while a dominant male is nearby. They spend more time on the edges of the horde or alone. If the chance arises, a non-fatted male can develop the full colors and condition of a fatted male.

What do females prefer?

Females play a role too. Observations in the Gabon colony showed that females were more likely to approach, groom and present to brighter males when they were fertile. Dominant males also guard fertile females closely, so it is hard to separate female choice from male competition, and both probably favor bright colors.

Females have their own signals. Their facial redness varies with age and reproductive state, and they develop red sexual swellings on the rump when fertile. Males may also judge a female’s condition using scent.

How the colors develop with age

Mandrills are not born colorful. Newborn infants have pinkish, mostly bare skin and a dark cap of hair, and their faces darken over the first few months. Young males show only hints of red and blue for years.

Males mature slowly. Their bodies and colors become more distinct from about four years old, but they do not reach full adult size and coloring until around nine or ten years. Females, by contrast, start breeding years earlier, at about four to eight years old. This long delay means a male’s colors reflect years of growth, health and social success, which may be one reason they are such a reliable signal.

Why such bright colors in a dark forest?

Mandrills live on the floor of dense rainforest in west-central Africa, where light is dim and greenish. Red and blue contrast strongly with green leaves, which may help mandrills spot each other in thick vegetation. Hordes can contain hundreds of animals, and colorful rumps may help members follow one another as the group moves.

Bright colors could attract predators such as leopards, but adult males are big and powerful enough to defend themselves, and the reproductive benefits of being colorful seem to outweigh the risks.

Myths and facts

• Myth: all mandrills are equally colorful. Color varies a lot. Infants have pale faces, females are much duller than males, and among adult males the colors track rank and change over time.
• Myth: the blue is paint-like pigment. It is structural color produced by collagen, as described above.
• Fact: Charles Darwin discussed the male mandrill in The Descent of Man (1871) as one of the most remarkably colored mammals and an example of sexual selection.
• Fact: the mandrill is the largest monkey in the world, and large size in males evolved under the same pressures that produced their colors.