Most of the carbon that flows from plants to animals is in the form of carbohydrate. Carbohydrates are formed as simple sugars, and as structural and non-structural polysaccharides. Non-structural polysaccharides such as starch are important forms of carbon storage in plant cells that are readily digested by the endogenous enzymes of animals. Structural polysaccharides in plant cell walls (dietary fiber) are degraded mainly by enzymes produced by microbes inhabiting the foregut and hindgut. The SCFA produced in these fermentations can make substantial contributions to the maintenance energy requirement of animals: up to 80% of the energy required by mammalian foregut fermenters and up to 40% of the energy required by hindgut fermenters. Contributions are smaller in birds (up to 10%), but ectothermic reptiles can derive virtually their entire energy requirement from fermentation.