Hair are thin growths, that protrude like filaments from the skin of mammals. Different kinds of mammals range widely in their degree of harness. Some have dense growths called are only sparsely haired, such as elephants, and humans. A few mammals, such as the whale, are hairless except for a small number of bristles. Hair is now unique to mammals, although some lizards show hairlike growths, but it probably first appeared on extinct mammal-like reptile ancestors.

Different kind of hair have different functions. Most obviously, the luxuriant furs of mammals in polar and temperate regions serve as body insulation. Insulating furs, often have two coats, a thicker underfur of fine hairs and a coarser cutter coat of guard hairs. More specialized hairs are also seen on many mammals, especially on the faces of carnivores and rodents. These hairs, called vibrissae, are surrounded at their base by sensory nerve fibers. They serve the animals as touch organs, helping them to measure their surrounding environment although in fact all hairs are sensitive to movements.

Coats of hair often serve as camouflage, with some mammals of cooler regions displaying a darker coat in summer and a white coat in winter. Patterns of hair growth also serve as secondary sexual characteristics in most mammals, whether in coloration differences or in such features as the mane of the male lion and the beard of the male human. In addition, hairs in particular body regions may have specific functions. For example, the lashes on eyelids trigger nerve circuits that close the eyelid when they are touched.


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