Hair is a filamentous outgrowth from the skin, found mainly in mammals.
The hair of non-human mammals is also known as pelage, fur or wool. Some of the various forms of hair that make up the pelage are guard hairs (such as the defensive spines of porcupines) and bristles (which make up the manes of lions). The polar bear has a form of pelage where transparent and unpigmented hairs transport sunlight, particularly ultraviolet light, through the dense fur to the skin.
Not all mammals have thick fur. Many aquatic mammals, such as whales and dolphins, have little or no body hair (though life in the water does not necessarily lead to hair loss, as the pinnipeds and otters demonstrate). A general inverse trend exists between body size and the thickness of body hair (measured in hair follicles per square centimeter). Large animals such as elephants and rhinoceros that have a higher ratio of body mass to surface area (and that need to keep from overheating) also have little or no body hair.