Animals need food to provide energy to existing cells and to provide the raw materials and energy for the construction of new cells. Cells in animal bodies combine oxygen with food to release energy. Even if an animal has reached its adult size, its body must still replace its cells regularly as the old ones eventually sustain damage or die.
Without food, animals eventually die because they are unable to fuel their energy needs or replace damaged cells. The chemical processes involved in the digestion of food and construction of new tissues is called metabolism. Animals engage in two different forms of metabolism. Anabolism is the building of new structures while catabolism is the act of breaking down the bonds of food particles to release the energy contained within them.
Animals have a wide variety of metabolisms; some animals have metabolisms that convert food to energy and new tissue very quickly while others have very slow metabolic rates. Some animals, such as birds and many mammals, have metabolisms that are always running at or near their full capacity while others, including turtles, snakes, insects and other “cold-blooded” creatures, have metabolisms with activity levels that fluctuate greatly. In many cases, the metabolic rate of such animals is temperature dependent.