Many mammals have fur the color of brown and black. Why don't they have more exotic colors, like purple and neon pink?

Most mammals wear coats of brown, black, and gray, while parrots flash brilliant reds, reef fish shimmer in electric blue, and chameleons shift between greens and golds. This disparity is not random.

Mammals are not especially diverse. Roughly 6,800 mammal species are known to exist, compared with about 8,800 species of amphibian, 11,000 species of bird and 12,500 of reptile. Yet when most people ...