Sloths move only when necessary and even then very slowly; they have about a quarter as much muscle tissue as other animals of similar weight. Sloth fur exhibits specialized functions: the outer hairs grow in a direction opposite from that of other mammals. In most mammals, hairs grow toward the extremities, but because sloths spend so much time with their legs above their bodies, their hairs grow away from the extremities to provide protection from the elements while the sloth hangs upside down. In most conditions, the fur hosts two species of symbiotic algae, which provide camouflage.