Ruminants are a group of hooved animals with a special digestive system for eating plants (grasses, roots, herbs, leaves, flowers, or even twigs). Ruminants lack the enzymes to digest the hard cell walls of plant material and must rely on a multi-step process that includes some help from resident bacteria. First, square, flat molars at the back of their mouths rub together to break open plant cells and release the nutrients inside. Ruminants move their jaws from side to side to create grinding action. Chewed food then travels to the first (the rumen) of several stomach chambers, where bacteria begin to digest it. The ball of food is regurgitated back to the mouth and re-chewed like an old piece of gum (chewing the cud). The ball eventually passes back through the rumen, then a second, third, or even fourth stomach chamber. At each step, more digestion and absorption of nutrients occurs.

All mammals have hair, outgrowths of the skin, during at least some part of their development. While it evolved to insulate animals in cold climates, mammal hair has various functions. Long, stiff hairs (vibrissae, or whiskers) are found in all mammals but humans. Whether on the face, legs, or tail, vibrissae extend an animal's sense of touch. Some mammals, such as porcupines, have enlarged hairs that function as defensive spines. Defense is also achieved by raising individual hairs to make an animal look larger (such as an angry cat). Muscles in the skin make the hair stand up or lie down. Raising and lowering hair changes how much air is trapped under it, which also makes it more or less insulating. Mammals in cold climates tend to have an extra layer of insulating fur, or extra fat under their fur. Mammals in climates that change seasonally shed and replace their hair to create a thicker winter coat and a thinner summer coat.