The primate most famously known for its spectacularly colored posterior is the Mandrill (Mandrillus sphinx), an Old World monkey species. The striking coloration, which features shades of red, pink, blue, and purple, is a secondary sexual characteristic most pronounced in adult males. This highly visible display serves an important function in the complex social life and reproductive success of the species.
Identifying the Mandrill
The Mandrill is the largest monkey species, exhibiting extreme sexual dimorphism. Adult males significantly outweigh females, weighing up to 119 pounds and reaching about 32 inches in height, while females are roughly a third of that size. The species is instantly recognizable for its colorful rump and vivid facial markings. These markings feature thick, ridged blue flanges along the muzzle and a prominent red stripe down the center of the nose.
This primate is native to the dense tropical rainforests and forested savannas of west-central Africa, primarily inhabiting Gabon, Cameroon, and Equatorial Guinea. Mandrills spend much of their day foraging on the forest floor, making their bright posterior a constant visual element. Their thick, olive-green fur covers most of their body, contrasting sharply with the brilliant, hairless skin patches on their face and rump. These colorful patches help differentiate the Mandrill from its close relatives, the Drills, which have predominantly black faces.
The Physiological Cause of the Color
The intense red and blue coloration on the Mandrill’s skin results from two distinct biological mechanisms. The red and pink hues come from a high density of superficial blood vessels located beneath the skin’s surface. This rich vascularization means the red color is essentially a flush, similar to blushing in humans. Its intensity can fluctuate based on the monkey’s emotional state and physical condition.
The blue and violet shades are not caused by pigment or blood flow but by structural coloration. This effect occurs when light scatters off the organized collagen fibers within the skin’s structure, a rare mechanism for color display in mammals. The vibrancy of all these colors is directly regulated by circulating sex hormones, particularly testosterone. Males with higher testosterone levels exhibit brighter, more saturated colors, signaling their physiological state.
The Role of Coloration in Social Structure
The Mandrill’s flamboyant coloration functions as a highly effective visual signal within its large and complex social system. Mandrills live in groups known as “hordes,” which can number in the hundreds and sometimes exceed a thousand individuals. In this crowded environment, the bright colors act as a long-distance advertisement of an individual’s status and reproductive quality.
The intensity of a male’s red coloration strongly correlates with his dominance rank within the group hierarchy. The most brightly colored males are typically the dominant, or alpha, males who monopolize mating opportunities. When a male achieves the top rank, his testosterone levels rise, and his facial and posterior coloration becomes significantly more vibrant over months.
This vivid display is an outcome of sexual selection, where females prefer the most brightly colored males. The vibrant red and blue patches signal genetic fitness, good health, and competitive ability, making the most colorful individuals the preferred mates. Conversely, if a dominant male loses his rank, his color intensity and testosterone levels decrease. This may help subordinate males avoid conflict with dominant individuals.