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Mammals are a type of animal. Mammal females produce milk from special glands called mammary glands to feed their babies. All 4,150 species of mammals have backbones, are warm-blooded, have a skin covering of hair or fur, and need to breathe air. The largest mammal is the blue whale, which is over 30 m long and weighs 91 metric tons. The smallest is the Kitty hog-nose bat. It is only 3.1 cm long and weighs 2 g!