Seven Characteristics of a Mammal

Warm Blooded

• Like birds, animals in the Class Mammalia are warm blooded. Being warm blooded means that mammals can regulate their body temperature, which allows them to live and hunt in vastly different climates, from the polar bear in the Arctic to the fennec fox in the Sahara. From an evolutionary perspective, mammals inherited the warm-blooded trait from their Triassic ancestors that resembled rodents and hunted during the cold nighttime, when predators were less of a threat than during the day.

Hair

• All mammals have hair at some stage in their development. Hair acts as an insulator and helps the warm-blooded animals maintain a constant body temperature. Because it costs a high amount of energy for mammals to stay warm, many of them are covered completely with hair. Sometimes the amount of hair is small and fleeting. Baby dolphins, for example, are born with a mustache that helps them feel their mother and locate the source of milk while they are nursing. The mustache sheds naturally after the first few days.

Diaphragm

• Mammalia is the only class in which the animals' bodies include a diaphragm, a wall of muscle that separates the heart and the lungs from the liver, stomach, kidneys and other organs. Because of the high energy demand created by being warm blooded, the heart and lungs were sectioned off in mammals' bodies through years of evolution to help them process mass amounts of air. The muscular diaphragm pumps oxygen throughout the body and also pumps carbon dioxide out.

Specialized Jawbones and Inner Ear Bones

• A mammal's lower jaw consists of a single bone on either side. The jaws of all other vertebrates, or animals with backbones, have more than one bone on each side. The three-boned middle ear, made up of the stapes, the incus and the malleus, is also unique to mammals. These specializations exist because two bones that were once part of the jaw evolved to become part of the middle ear, improving hearing in mammals over time.

Advanced Brains

• Animals in the Class Mammalia have the most advanced brains in the animal kingdom. The neocortex, a brain region found only in mammals, allows for conscious thought and spatial reasoning. Advanced brain capacity helped mammals become the dominant animals on Earth through years of evolution. According to Discovery.com, mammals' affinity for learning and being able to react to their environment based on that learning, give them advantages over other animals that rely on genetic programming.

Mammary Glands

• Mammary glands are modified sweat glands, found only in mammals, that produce milk instead of perspiration. Before babies in the Class Mammalia are able to digest other foods, they feed solely on milk from their mother. Some mammalian mothers produce milk through patches on their bellies, though most have developed advanced feeding organs, known as nipples. During the nursing period, the mother's milk not only nourishes the mammalian baby, but also builds its immune system through the transfer of antibodies.

Live Young

• Five living species of mammals, called monotremes, lay eggs: the platypus and four types of echidna. The rest of the 5,400 species of mammals bear live newborns. Within the live-bearing mammals are two groups. Marsupials, which make up about half of Australia's mammals, carry their young in pouches on their bellies. The second, larger group are placentals, which nourish their young inside the womb through a pouch that is delivered from the body during the birth of the baby.