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.

Thanks to their ability to produce milk, female mammals are able to feed their offspring for weeks, months, or even years after they are born. This sets the stage for the extended childhoods of many mammal offspring. With food and shelter taken care of (by the mother in most species), a remarkable amount of learning occurs for mammal offspring before they become independent. Elephants tend to their young for as long as 10 years, and humans for longer still. Young mammals may learn about food, shelter, migration routes, social structures, or other keys to survival from their parents. While extended childhoods support learning, they are costly, especially for the female. Lactating females must consume enough calories to feed their offspring and maintain themselves. Offspring that stay around after weaning from their mother's milk may still require protection and food. Human offspring are the most extreme case of investment, with parents usually providing intensive support until age 18 and continuing support beyond.