Seven major characteristics distinguish the 4,500 unique species of mammals from other animals. Mammals are air-breathing, warm-blooded and have a backbone, but these traits alone do not set them apart them from all other animals. Mammals are uniquely capable of regulating their body temperature via their metabolism and sweat glands.

With the exception of some primitive mammals--like a duck-billed platypus--called Monotremes, mammals give birth to live young. Female mammals produce milk containing water, carbohydrates, fat, protein, minerals and antibodies that provide nourishment to feed their young. Milk is produced by mammary glands, which defines the class of animal, giving it the name "mammal."

All mammals possess hair during at least one part of their life cycle. Hair follicles have nerve endings that respond to touch, which adds to the mammal's awareness of its surroundings. A coat of hair is called a pelage, and it protects mammals from the environment. There are two major types of pelage: undercoat hairs are small short hairs that provide a dense layer of insulation, and guard hairs are longer, providing color and protection from the elements.

The lower jaw in mammals is a single bone. This characteristic is unique to mammals; all other vertebrates have more than one bone on each side of the jaw. The mammal middle ear contains three bones, including the stirrup (stapes), anvil (incus) and hammer (malleus). During the early evolution of mammals, these bones were part of the jaw, but they changed jobs and became a part of the hearing function instead.

Mammals have four chambers in their hearts. In mammals, the main artery of the heart curves to the left when leaving the heart, becoming the aortic arch. This main artery curves to the right in birds, and all other vertebrates have more than one main artery. Only mammals have a diaphragm: a sheet of muscle and tendon that separates the body cavity. Heart and lungs are in an upper section of the body cavity, and the liver, stomach, kidneys, intestines and reproductive organs are in the lower section.

Mammal brains are larger than other animals. This is particularly true of the cerebellum, the portion of the brain that controls memory and learning. Mammalian brains also possess a unique region of the brain called a neocortex.The neocortex functions as the area of the brain that handles sensory perception, motor commands and spatial reasoning. Conscious thought and human language are also processed in the neocortex.