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What are mammals?

Is a kangaroo a mammal or a marsupial; is a rat a rodent or a mammal; is a bat a placental, a marsupial, a mammal or a bird? First off, they are all mammals because unlike snakes, birds, centipedes, squids etc they all share the following:

  • Milk-making glands (mammary glands) from which their newly-born young take nourishment. The whole group (Mammals/Mammalia) takes its name from these glands.
  • Hair that partially or wholly covers the animal’s body (although in some groups like whales, dolphins and dugong the hairs are either reduced to bristles, or absent).
  • A lower jaw comprised of two single bones (‘joined’ at the front). In other vertebrate groups each side of the jaw is comprised of more than one bone.
  • A three-boned middle ear containing the stapes (stirrup), incus (anvil) and malleus (hammer).
  • A left-curving aortic arch. In birds, the main artery leaving the heart curves to the right becoming the aortic arch. In other vertebrates more than one main artery leaves the heart.
  • A diaphragm. No other animal group has this sheet of muscle and tendon that separates the heart and lungs from the liver, kidneys, stomach and intestines.

Having these features, kangaroos, rats and bats are all primarily sorted together as mammals (we say they are members of the Class Mammalia) and can then be further grouped according to other shared features; kangaroos going into the Subclass Marsupialia, Order Diprotodontia; rats and bats together into the Subclass Eutheria (placental mammals), then the rats into the Order Rodentia (rodents) while the bats go into the Order Chiroptera.
