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The African ancestors of the Aldabran giant tortoises were probably of a normal size which means that the Aldabrans have become giants because they live on an isolated island. This isolation may also have had another effect. A baby giant tortoise can withdraw its head and legs into its shell just as other tortoises do. But an adult Aldabran cannot do this, because its limbs and head are too big. If there were predators like hyenas on the island, the hyenas would probably be able to kill it, but there are no predators on the island so the Aldabran giant tortoises are safe. It is not known why the tortoises reach giant size. Perhaps being large and having large reserves of fat means that they are better able to survive hard times. Or perhaps without predators to kill them, they just keep on growing.