A number of physiological characteristics must be present in an animal for it to be classified as a mammal. These traits include a four-chambered heart, a lower jaw made of a single bone, and three middle ear bones. Pigs meet all of these requirements.
According to About.com, pigs are one of about 225 species belonging to the even-toed hoofed mammal classification, a category known scientifically as Artiodactyla. Other key characteristics that are required of all mammals include the presence of a diaphragm, mammary glands, hair and an endothermic metabolic system. Mammals come in all sizes, from the 3-centimeter-long bumblebee bat to the blue whale, which can grow to be longer than 100 feet.