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 named for their special milk-producing structures called mammary glands. A female mammal may have two (like humans) or as many as 18 mammary glands. Each gland is full of small spaces lined with milk-secreting cells that all drain into a nipple. Newborn mammals have an innate sucking reflex that stimulates the mother's mammary glands to release milk. Milk is a superfood for young mammals, containing a nourishing mix of fats, proteins, vitamins, minerals, and even immune-system chemicals. The first milk (colostrum) contains antibodies from the mother to protect against disease. As the offspring grows, milk changes to maintain the right balance of nutrients. Production of milk (lactation) makes most female mammals temporarily unable to get pregnant, resulting in natural gaps between offspring. The spacing of births benefits both the offspring and the mother in terms of survival. Mother mammals stop nursing (wean) their babies after several days or years, depending on the species and environmental conditions.