What is a mammal?

Mammals are a class of animal. They have certain traits that distinguish them from animals in other classes, such as fish, reptiles, and amphibians. All mammals share two characteristics: they all feed their young with mammary gland milk, and they all have hair. Almost all of them are warm-blooded, which means they try to keep the inside of their bodies at a constant temperature. They do this by generating their own heat when they are in a cooler environment, and by cooling themselves when they are in a hotter environment. Unlike reptiles, who sit in the hot sun to regulate their body temperature, mammals wake up and are ready to go! In general, mammals spend much more time raising and training their young than other animals do. Some examples of mammals include apes, bats, lions, mice, moose, aardvarks, beavers, elephants, gorillas, pandas, hamsters, dogs, cats, horses, whales, and dolphins. There are three types of mammals: placental mammals, monotremes, and marsupials. Placental mammals are those whose young are born live and at a relatively advanced stage. Before birth, the young are nourished through a placenta. Like the human placenta, it is a specialized embryonic organ that is attached to the mother’s uterus and delivers oxygen and nutrients to growing young. Most mammals are placental mammals, including cats, dogs, and horses. The monotremes are egg-laying mammals. These include the echidnas (spiny anteaters) and the duck-billed platypus. Marsupials give birth to their young in an immature state, and most female marsupials have pouches in which to carry and nurse their young. Some marsupials include the koala, kangaroo, and the numbat. Some mammals, such as cows, horses, and pandas, are plant eaters—called herbivores. Others, including tigers, lions, and whales, are meat eaters—called carnivores. Other mammals, including bears, eat a combination of plants and meat.