Fur

Fur is a soft, thick growth of hair that covers the skin of almost all mammals. This covering is often called pelage, which refers to an animal's complete coat. While many people use the word "fur" casually, it can also describe animal pelts processed into leather with hair still attached. The primary biological purpose of fur is thermoregulation. This is the process of managing body temperature to keep an animal stable. Beyond temperature control, fur provides protection, sensory input, waterproofing, and camouflage.

Modern mammalian fur often works through a complex, layered system. The innermost layer is called down hair, also known as underfur or undercoat. These hairs are short, numerous, and wavy or curly. Their main job is to insulate a layer of dry air directly against the skin. The middle layer is made of awn hair, which acts as a hybrid. Awn hairs grow like guard hairs at first, but they become thin and wavy halfway through their growth. This allows them to help with insulation while also shedding water.

The outermost layer consists of guard hair, or overhair. These hairs are longer, coarser, and have nearly straight shafts. They protrude through the softer layers to form the visible part of the coat. Guard hairs are vital because they repel water and block ultraviolet radiation from the sun. They can also protect the skin from cuts and scratches. Some mammals, like cats and dogs, use a pilomotor reflex to raise these hairs when they feel agitated. This can serve as a threat display to others.

There are many different specialized types of hair in the animal kingdom. Vibrissae are sensory hairs, most commonly known as whiskers. Spines are stiff guard hairs used for defense, such as on a porcupine. Bristles are long hairs used for visual signals, like a lion's mane. Some mammals have velli, or "down fur," which insulates newborn animals. There is also wool, which is long, soft, and often curly. These variations allow different species to thrive in specific environments.

Coloration plays a massive role in survival through selective pressures. Camouflage helps animals hide from predators or prey. For example, a leopard's coat helps it remain an effective ambush predator. Some animals use aposematism, which is a warning signal. The black-and-white pattern of a skunk or a honey badger warns others that they can defend themselves. Other species use seasonal color changes to match their surroundings. The arctic fox and the snowshoe hare change from brown in summer to white in winter.

Temperature regulation is also tied to coat color and density. Darker coats can absorb more heat from solar radiation, which helps small mammals like voles stay warm in winter. Conversely, the white, pigmentless fur of a polar bear may reflect solar radiation onto the skin. Density is just as important as length. The muskox has extremely dense fur and guard hairs that create an airtight coat. This allows it to survive in extreme cold. Interestingly, some desert animals like camels use dense fur to block solar heat, keeping their skin cool.

While fur is a defining trait of mammals, some species have evolved to be "naked" or hairless. Aquatic mammals like whales and hippopotamuses evolved hairlessness to reduce resistance in the water. The naked mole-rat is also hairless, likely due to its underground lifestyle. Humans are unique among primates for having undergone significant hair loss. This may be linked to the loss of functionality in a gene called KRTHAP1, which helps produce keratin. This mutation is very old, as it was also present in Altai Neandertal and Denisovan peoples. Through artificial selection, humans have even created hairless breeds like the Sphynx cat.