 The tortoises are herbivores that consume a diet of cacti , grasses , leaves , lichens , and berries . They have been documented feeding on Hippomane mancinella ( ' poison apple ' ) , the endemic guava Psidium galapageium , the water fern Azolla microphylla , and the bromeliad Tillandsia insularis . Juvenile tortoises eat an average of 16 @.@ 7 % of their own body weight in dry matter per day , with a digestive efficiency roughly equal to that of hindgut @-@ fermenting herbivorous mammals such as horses and rhinos . The tortoises are herbivores that consume a diet of cacti , grasses , leaves , lichens , berries , melons , oranges , and milkweed . 
