What is a mammal?
A mammal is a warm-blooded animal with hair or fur that feeds its young with milk. Most mammals give birth to live young, and they breathe air with lungs.
Mammals share several features. Females have mammary glands that produce milk, which gives the group its name. All mammals have hair at some stage of life, even whales, which have a few bristles. They have three small bones in the middle ear and a lower jaw made of a single bone. Mammals are warm-blooded, keeping a steady body temperature.
There are about 6,500 known species of mammals. They live on land, in the sea and in the air, and range from the tiny Etruscan shrew, weighing about two grams, to the blue whale, the largest animal ever known. Humans are mammals too.
Three groups
Placental mammals, like dogs, elephants and humans, give birth to well-developed young. Marsupials, like kangaroos, raise tiny newborns in pouches. Monotremes, the platypus and echidnas, are mammals that lay eggs, then feed their hatchlings with milk.
Did You Know?
Rodents are the largest group of mammals, making up about 40 percent of all mammal species. Bats are the second-largest group.