 Tortoises acquire most of their moisture from the dew and sap in vegetation ( particularly the Opuntia cactus ) ; therefore , they can survive longer than 6 months without water . They can endure up to a year when deprived of all food and water , surviving by breaking down their body fat to produce water as a byproduct . Tortoises also have very slow metabolisms . When thirsty , they may drink large quantities of water very quickly , storing it in their bladders and the " root of the neck " ( the pericardium ) , both of which served to make them useful water sources on ships . On arid islands , tortoises lick morning dew from boulders , and the repeated action over many generations has formed half @-@ sphere depressions in the rock . 
