Because of the way they metabolize fat stored in their tissues, the giant tortoises can survive long periods without food or water. They were thus a convenient source of fresh meat for early explorers who caught them on land and stored them upside-down in ship holds for up to a year. In fact, Charles Darwin's first encounter with the Galapagos tortoises, one variety of which bears his name, was during a stop at the islands to collect tortoises for food. Darwin recorded that the vice-governor of the islands claimed he could tell which island any particular tortoise came from by its shell. Darwin's writings indicate that such diversity of form was one factor that led to his ideas on evolution.
All giant tortoises are indeed giant.
There are two main carapace designs. Tortoises that occupy islands with fairly lush vegetation are large with a domed carapace. This carapace shape along with long necks and limbs helps them reach their food. Intermediate forms occupy other islands. Tortoises are vegetarian, eating large quantities of over 50 native plant species including prickly pear cactus and fruits, bromeliads, water ferns, and the so-called poison apple.