All mammals are endothermic (warm-blooded), have some fur or hair on their body, and feed their young milk. They have a bony skeleton with a backbone, and their lower jaw, made of one bone, hinges directly onto the skull. Mammals breathe using lungs. A few mammals lay eggs, and some carry their young in pouches, but most have a placenta and give birth to live young. Mammals are found all over the world, on land, in the air, and in water.
There are about 4,500 species of mammal in a total of 21 orders, of which the following are a selection.
(duck-billed platypus, echidna)
Features: lay eggs, short legs, small head, tiny eyes
The duck-billed platypus closes its eyes, ears, and nose when diving and finds its way using sense receptors around its bill. The platypus lays eggs. It does not have nipples, so when the young hatch, they suck milk from the fur around the openings of the milk glands. It lives by rivers in Australia and Tasmania.
Dolphins, like whales, spend their entire life in the water, but must still surface to breathe air through their lungs. Their fat reserves, called blubber, keep them warm in cold seas.