Mammals

Mammals are separated from other animals by over 300 million years of evolution. During that time, several traits evolved that distinguish them from other vertebrates — namely hair, sweat glands, mammary glands, a modified middle ear and an advanced brain. In mammals, two bones that were once part of the jaw became part of the middle ear; so, a mammal's middle ear has three bones to aid in hearing instead of just one.

There are around 5,400 species of mammals divided into two main categories: those that lay eggs (monotremes) and those that bear live young (marsupials and placentals). There are only five species of monotreme: the platypus and four species of echidna. Marsupials, which carry their young outside the womb through early infancy, account for 334 species, including roughly half of all Australian mammals (kangaroos, koalas, wombats, etc.) as well as some New World animals, like opossums. All other mammals — rabbits, rodents, shrews, moles, anteaters, sloths, armadillos, badgers, pangolins, bats, primates, whales, elephants, manatees, camels, cattle, deer, antelope, goats, etc. — are placental. Their young are nourished in the womb by a placenta, which delivers nutrients to the developing fetus.

Hair is part of what sets mammals apart from other animals. All mammals have hair, even if it happens to be sparse and in out-of-the-way places like in whales and dolphins. Hair helps regulate body temperature, which in warm-blooded mammals must remain high and constant. Mammals have high energy requirements in order to stay warm, which means they must eat nutritious food, and plenty of it — much more than their cold-blooded relatives.

Mammals are able to regulate their own body temperature, a trait they share with birds. This allows them to hunt and forage in cold temperatures that most other animals can't tolerate. In the mammal line, warm bloodedness evolved during the Triassic period when dinosaurs became the dominant land predators. At the time, the direct ancestors of mammals were small, rat-like creatures. They only came out at night when it was relatively safe to feed. To keep from losing heat, these small-bodied, nocturnal creatures developed the ability to raise their metabolic rate; this allowed them to maintain a high, constant body temperature despite the cold, nighttime temperatures. All mammals inherited this highly useful trait