The amount of hair present varies between species of mammals. This is an adaptation to a specific way of life. Sometimes hair has been adapted for defense, as in the African-crested porcupine whose quills are really stiffened hairs. How females provide milk for their young may also vary among mammals. If you are lucky, you may see a mammal nursing her young, especially during the springtime, at the zoo.
Mammals live everywhere in and on the earth.
Seals are aquatic, spending their lives in the water, while sea lions, also aquatic, have well-developed flippers that also enable them to maneuver on land. Moles and prairie dogs live underground. Some of these subterranean animals have poor eyesight but a keen sense of smell and find protection in the tunnels and burrows they dig in the soil. Arboreal mammals live in trees, which give them protection from ground-level predators. Some, like sloths, eat, sleep and bear their young hanging from branches and rarely venture to the ground.