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They are generally endothermic ("warm-blooded"), producing body heat internally. Some species, such as seals regularly also use the sun or other environmental heat sources in addition to the metabolic heat they produce.
Most species give birth to living young - with the notable exception of the monotremes (the platypus and the echidnas) of Australia and the southern Pacific region, which lay leathery shelled eggs.
The approximately 5000 species of mammals range in size from the 100 foot long (30m) blue whale to the 3/4 inch (30-40mm) long Old World hognosed bat.
Various species of mammal can swim, climb, run and fly.
Mammals are distributed worldwide, occurring on all continents and most islands. Even islands that are said to be without mammals will have whales in the nearby waters. One species has even begun to colonize local space off earth.