Underfur

Underfur, also known as the undercoat, is the dense layer of soft fur closest to a mammal’s skin. It lies beneath the longer, coarser guard hairs that form the outer coat. In most animals with a two-layer pelage, the underfur is shorter and woolly, but highly effective at trapping air and reducing heat exchange with the environment.

Its primary function is insulation. The air pockets created by the underfur slow heat loss in cold conditions, helping animals maintain body temperature. The guard hairs above provide protection from the elements and wear, while the underfur remains relatively fine and pliable to maximize insulation.

Seasonal molt: Many species grow a thicker, longer underfur in autumn as preparation for winter and shed it in spring. Domestic animals such as dogs and cats with a double coat also experience seasonal shedding of the undercoat, particularly in temperate climates. Some tropical or desert species have little or no underfur.

Variation among species: The density and length of underfur vary greatly; arctic and high-altitude mammals often possess dense, long underfur, while other mammals retain only a light undercoat or none at all. The underfur is distinct from the guard hairs, which are longer and serve other functions such as weatherproofing and camouflage.

In humans, underfur is not present; the term is mainly used in zoology and veterinary contexts. In animals kept for fur, the underfur contributes to warmth and is a factor in grooming and molt cycles.