Organisms that live on Earth are either autotrophs or heterotrophs. Autotrophs obtain energy directly from their physical environment through photosynthesis or chemosynthesis. Heterotrophs must obtain their energy by eating autotrophs or by eating other heterotrophs. Plants are autotrophs, but all animals are heterotrophs. Animals that feed on living organisms or parts of living organisms can be classified into three general groups based on feeding behavior: herbivores, carnivores, and omnivores.

A herbivore is an animal that eats the tissues of green plants and only plants. Herbivores consume many different parts of plants. This group of animals can be further subdivided into such categories as folivores (eaters of leaves), frugivores (eaters of fruit), and granivores (seed eaters). Herbivores have special adaptations that allow them to extract sufficient energy and nutrients from plants. For example, herbivores have large stomachs, allowing them to eat a large amount of material. Some herbivores have...