The Galápagos tortoise is the longest lived of all vertebrates, averaging around 100 years. They are also the world’s largest tortoises and the 13th heaviest living reptile with some specimens exceeding 1.5 metres in length and weighing 150 kilograms. Larger tortoises are big enough to carry adult humans on their backs; in 1835 Charles Darwin wrote “I frequently got on their backs, and then giving a few raps on the hinder part of their shells, they would rise up and walk away; but I found it very difficult to keep my balance.”.
The giant tortoise population in the Galápagos archipelago was once so large that the islands were named for these huge reptiles; the old Spanish word galapago meant saddle and was used by early explorers for the tortoises due to the shape of their shells.
There is a great amount of variation in size and shape among the Galápagos tortoises, however two main morphological forms exist. The domed carapace, which is most similar to their ancestral form, and the saddle-backed carapace. The domed tortoises tend to be much larger and are generally found on higher and more humid islands, while the smaller saddle-backed tortoises evolved on arid islands as a response to the lack of available food during drought.
These herbivores spend their days grazing on grass, leaves and cactus, basking in the sun and napping for nearly 16 hours per day. A slow metabolism and large internal stores of water mean they can survive up to a year without eating or drinking.