Mammals have hair or fur because they are warm-blooded, and hair helps to keep them warm or cool. Other than helping to regulate internal temperature, mammal hair also has special qualities depending on the species. For instance, porcupine hairs are tipped with barbs as a means of defense against predators.
Some mammals that spend much of their time in water have thick coats of hair to insulate them from the cold. The river otter's fur coat not only repels water, but it also helps the animal to stay buoyant in water. A walrus has hairs or whiskers on its face that are extremely sensitive to touch. The spotted hides on big cats helps them to blend into their surroundings.
Seals, seal lions and polar bears all have hair that functions as a protective layer in water or cold habitats. Even dolphins and whales have some hair, although most grows from the face or sheds as the animals age. These mammals have a thick blubber layer to act as insulation in place of the hair of other mammals.
Not all mammalian hair is the same, but can be guard hairs, bristles, awns, underfur, wool, fur, velli and whiskers. Guard hairs, such as the barbs on a porcupine, overlay the rest of the hair and are protective. Bristles grow in lion or horse manes. Many mammal young are born with velli or a soft down that they usually lose over time.