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.

Mammals are able to make enough internal body heat to keep their bodies at a relatively constant, high temperature. Endothermy (endo= inside; thermic= heat) makes mammals less dependent on outside temperatures, freeing them to be active in a wide range of conditions. Mammals may be active at night (nocturnal) or in the day (diurnal), in climates as extreme as the cold poles or the hot tropics. Adaptations to warm up or cool off, such as shivering and sweating, allow mammals to maintain their preferred body temperatures in the face of these extremes. Mammals also regulate body temperature with behavior: elephants flapping ears to cool off, humans wearing coats to warm up, dogs panting to cool down. The hairs making up mammal fur are raised or lowered to provide more or less insulation as needed.