Mammals are distinguished from other animals by having mammary glands and complex hair follicles, and by the structure of their skulls, which feature highly modified jaw bones. Mammals are also warm-blooded and have a number of adaptations to allow for high metabolism.
Perhaps the most obvious difference between mammals and other animals is hair. Even mammals that do not have hair as adults, such as whales, have hair as tiny embryos, and later lose their hair as they continue to develop. Hair primarily functions to keep in body warmth, but it also serves as camouflage. Some mammals, like porcupines, have highly modified hair that is used as a defense.
The feature for which mammals are named is the mammary gland, which is a modified sweat gland that produces milk, a nutritious liquid that helps young mammals grow. The evolution of milk is probably linked to diphyodonty in mammals. Diphyodonty means that mammals have two sets of teeth throughout life. The first teeth, called baby teeth in humans, are relatively weak, as young mammals do not need to chew their food. Most other animals continually grow and replace their teeth throughout their lives.
Mammalian jaws are very different from other animal jaws in other ways. In reptiles, the lower jaw is made up of multiple bones. In mammals, the lower jaw is a single bone. The remaining jaw bones in mammals have evolved to be very tiny and serve as a hearing apparatus inside the ear.
Mammal adaptations for warm-bloodedness include a four-chambered heart, which is able to move blood around the body more efficiently and keep oxygen-poor blood separate from oxygen-rich blood. This keeps more oxygen flowing to the rest of the body, to allow for a higher rate of metabolism and a higher body temperature.