Mammals, unlike other animals, have hair, mammary glands and give birth to live young. Mammals have a regular body temperature and a four-chambered heart.
The mammal group is one of the largest on the planet, including such organisms as humans, primates, cats and even whales. All female mammals nurse their young with milk produced by their body. Mammals can also be identified by the presence of sweat glands, which assist in keeping a regular body temperature. They also possess such characteristics as a middle ear bone, a jaw joint and replacing teeth.
Mammals, with few exceptions, have a skeletal system, a nervous system, a respiratory system, an integumentary system and a reproductive system. Due to the need of maintaining a body temperature, mammals must maintain a constant diet, and intake a large amount of calories depending on size. Mammals are one of the most diverse groups on Earth, and come in all shapes and sizes. Some can be as small as the bumble bee bat while others as large as the blue whale. Mammals are also not restricted to live on the ground. Some mammals, like small primates, live in trees while some have evolved to live in the ocean like dolphins.