Mandrills (Mandrillus sphinx) are among the most visually striking primates, easily recognised by the vivid red and blue ridges on their snouts. Unlike many animals that flush red when agitated, mandrills show an unusual bright-blue colouration whenever they are emotionally aroused—especially when angry or asserting dominance. This effect is caused by blood rushing under the skin and microscopic light-scattering structures that reflect blue wavelengths.

The colour change is most intense in males, directly correlating with testosterone levels and overall strength. A bluer male is seen as more dominant, often avoiding fights by signalling power visually. In huge social groups (sometimes 800 individuals), this mechanism maintains order without constant aggression.

Blue hues also play a role in mate selection—females prefer more vibrantly coloured males, interpreting the brightness as a sign of fitness. Mandrills are often confused with baboons but are closer to drills, differing in size, colour, and group dynamics.

Sadly, mandrills are listed as Vulnerable by the IUCN, facing threats from habitat loss and hunting, making conservation vital for this remarkable primate.