There's great variety in the natural world, but mammals seem rather drab compared to the colorful birds and reptiles. Most hair and fur colors are earth-toned shades of brown, black and grey, but the tropical plumes and the iridescent scales show off dazzling colors. This stark difference is of evolutionary origin, mainly because of the differences in the eyesight, specialized skin structures, chemical pigments, etc. that have been essential to mammalian physiology for millions of years.