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, animals may exhibit a strong preference for immature leaves, which tend to be higher in energy and protein and lower in fibre and poisons than more mature leaves.