Herbivores are animals whose primary food source is plant-based. Examples of herbivores include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and caterpillars . These animals have evolved digestive systems capable of digesting large amounts of plant material. The plants are high in fiber and starch, which provide the main energy source in their diet. Since some parts of plant materials, such as cellulose, are hard to digest, the digestive tract of herbivores is adapted so that food may be digested properly. Many large herbivores have symbiotic bacteria within their guts to assist with the breakdown of cellulose. They have long and complex digestive tracts to allow enough space and time for microbial fermentation to occur. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).