Animals need energy to carry out all the body processes required to maintain life. Without energy, an animal is unable to move, to digest its food, to reproduce, to grow or even to breathe.
Energy is defined as the "ability to do work." Animals must work to find food, to establish shelter and to protect themselves from harm. Even more basic is an animal's need for energy to carry out cellular functions. Activity is a constant in animal life: even while the animal sleeps, body functions requiring energy continue. Life depends upon a constant supply of energy in the form of food or it ceases to exist.