Folivore

In zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds. For this reason folivorous animals tend to have long digestive tracts and slow metabolisms. Many enlist the help of symbiotic bacteria to release the nutrients in their diet. Additionally, as has been observed in folivorous primates, they exhibit a strong preference towards immature leaves, which tend to be easier to masticate, tend to be higher in energy, protein, lower in fibre and poisons than more mature fibrous leaves.

Evolution

Herbivory has evolved several times among different groups of animals. The first vertebrates were piscivores, then insectivores, carnivores and finally herbivores. Since a complex set of adaptations was necessary for feeding on highly fibrous plant materials (structural modifications to the teeth, jaws, and digestive tract) and only a small proportion of extant tetrapods are obligate herbivores, it could be that early tetrapods made the transition to fully fledged herbivory by way of omnivory.