Mammals are a group of warm-blooded vertebrate animals that belong to the class Mammalia of the animal kingdom. They are characterized by several unique features, including:

• Hair or fur: All mammals have hair or fur, which helps to regulate their body temperature and protect their skin.

• Mammary glands: Female mammals have mammary glands that produce milk to feed their young.

• Unique jaw structure: Mammals have a unique jaw structure that is made up of a single pair of bones that articulate in a unique way with the skull.

• Three middle ear bones: All mammals have three middle ear bones, which help to transmit sound waves to the inner ear.

• Warm-blooded: Mammals are warm-blooded, which means they can regulate their own body temperature via a region in the brain called the neocortex.

Mammals are divided into three groups: monotremes, marsupials, and placentals. Monotremes are the platypus and echidnas, and the females lay soft-shelled eggs. Marsupials give birth to small, poorly developed young and most female marsupials, such as kangaroos, wallabies, and the koala, have pouches. Placental mammals, like humans, whales, rodents, and bats, differ from monotremes and marsupials in that they generally give birth to well-developed young. There are more than 5,500 species of living mammals, arranged in about 125 families and as many as 27–29 orders.