Mammals are one of the classes of vertebrate animals. The distinguishing characteristic of mammals is that the mother nourishes newborn young with milk produced by special glands.
Mammals also have several other distinguishing features. Hair is common among mammals and can take many forms, including whiskers, spines and fur. All mammals breathe air, so even those living in water must surface to take a breath. Two types of mammals—the platypus and the echidnas—lay eggs, but all others give birth to live babies.
Like birds, all mammals are warm-blooded. That means they generate their own body heat through metabolism, so they can stay warmer than their surroundings, day or night, sun or shade. Mammals can be carnivores, herbivores or omnivores.
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