Mandrills are noted as being exceptionally colorful by mammalian standards. Charles Darwin wrote in The Descent of Man: "no other member in the whole class of mammals is colored in so extraordinary a manner as the adult male mandrill's".[20] The bright colors of mandrills are indeed not produced conventionally (no mammal is known to have red and blue pigments), being derived from light diffraction in facial collagen fibers.[21][22]