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Aquatic and Land Animal Diversity

Mammals

What characterizes a mammal?

The class Mammalia includes more than 5,000 species in about twenty-six orders (depending on which classification system you use), which are found on all continents and in all oceans and seem to have a disproportionately large ecological role compared to their abundance—mainly because their metabolism is so high, they have to eat more. In general, most mammals have several characteristics—especially three not found in other animals: The females produce milk using modified sweat glands (called mammary glands, which can vary from two to a dozen or more); have hair (mostly made of a protein called keratin, similar to what makes up human fingernails and mostly developed to insulate the mammal against extreme environments); and have three middle ear bones (the malleus, incus, and stapes, allowing mammals to better hear sounds).



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