A polar bears skin is black, and its hairs are hollow and translucent, which means that they only appear white because they reflect visible light. The long, coarse guard hairs that protect the plush thick undercoat are hollow and transparent, while the thinner hairs of the undercoat are not hollow, but they, like the guard hairs, are colorless. The black skin of polar bears works as built-in insulation and helps them retain heat and keep warm in the harsh temperatures of the Arctic.