Fur

Fur is the soft, thick growth of hair that covers the skin of almost all mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket that keeps the animal warm.

The fur of mammals has many purposes: protection, sensory, waterproofing, and camouflaging, with the primary purpose being thermoregulation. The types of hair include

definitive, which may be shed after reaching a certain length;

vibrissae, which are sensory hairs and are most commonly whiskers;

pelage, which consists of guard hairs, underfur, and awn hair;

spines, which are a type of stiff guard hair used for defense in, for example, porcupines;

bristles, which are long hairs usually used in visual signals, such as the mane of a lion;

velli, often called "down fur", which insulates newborn mammals; and

wool, which is long, soft, and often curly.

Hair length is unimportant in thermoregulation, as some tropical mammals, such as sloths, have the same fur length as some arctic mammals but with less insulation; and, conversely, other tropical mammals with short hair have the same insulating value as arctic mammals. The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur; for example, the muskox has guard hairs measuring 30 cm (12 in) as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of −40 °C (−40 °F). Some desert mammals, such as camels, use dense fur to prevent solar heat from reaching their skin, allowing the animal to stay cool; a camel's fur may reach 70 °C (158 °F) in the summer, but the skin stays at 40 °C (104 °F). Aquatic mammals, conversely, trap air in their fur to conserve heat by keeping the skin dry.

Mammalian coats are colored for a variety of reasons, the major selective pressures including camouflage, sexual selection, communication, and physiological processes such as temperature regulation.