Mammals are the vertebrates within the class Mammalia, a clade of endothermic amniotes distinguished from reptiles by the possession of a neocortex, hair, three middle ear bones, and mammary glands. Females of all mammal species nurse their young with milk, secreted from the mammary glands.Mammary Glands

Mammals got their name from this evolutionary adaptation. Mammary glands are only found on mammalian species, and no other species. Mammary glands evolved from modified sweat glands to produce milk for offspring. Only female mammals produce milk, an adaptation that requires an investment of time and energy to care for the newborn offspring.

Hair

Hair on mammals is an adaptation that provides insulation to keep the animal warm, provide protection to the animal's skin, provides camouflage through color patterns and helps the animal by providing greater sensory input through touch.

Middle Ear Bones

All mammals have three middle ear bones, commonly called the hammer, anvil and stirrup. Their scientific names are malleus, incus and stapes. The hammer and anvil evolved from the jaw bones from primitive mammal ancestors. These bones work together to transmit vibrations to the eardrum.

Sweat Glands

Sweat glands, while being unique to mammals, aren't present in all mammals. For example, the whale has no sweat glands, but doesn't need them because it lives in the ocean.

Temperature Regulation

While most mammals are warm-blooded, not all are able to regulate their temperature, which is the definition of being warm-blooded. Bats and mole rats can't regulate their body temperatures; whereas bees, birds and the hawk moth can.