The Galapagos tortoise or Galapagos giant tortoise are the largest living species of tortoise. Modern Galápagos tortoises can weigh up to 400 kg. Today, giant tortoises exist on only two remote archipelagos: the Galapagos Islands and Aldabra.
The Galápagos tortoises are native to seven of the Galapagos Islands. With lifespans in the wild of over 100 years, it is one of the longest-lived vertebrates.
Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks. On islands with dry lowlands, the tortoises are smaller, with “saddleback” shells and long necks.
The tortoises have a large bony shell of a dull brown or gray colour. The plates of the shell are fused with the ribs in a rigid protective structure that is integral to the skeleton. Lichens can grow on the shells of these slow-moving animals. Tortoises keep a characteristic scute (shell segment) pattern on their shells throughout life, though the annual growth bands are not useful for determining age because the outer layers are worn off with time. A tortoise can withdraw its head, neck, and fore limbs into its shell for protection. The legs are large and stumpy, with dry, scaly skin and hard scales. The front legs have five claws, the back legs four.
The largest recorded individuals have reached weights of over 400 kg and lengths of 1.87 m. Size overlap is extensive with the Aldabra giant tortoise, however taken as a species, the Galápagos tortoise seems to average slightly larger. The tortoises’ gigantism was probably a trait useful on continents that was fortuitously helpful for successful colonisation of these remote oceanic islands rather than an example of evolved insular gigantism. Large tortoises would have a greater chance of surviving the journey over water from the mainland as they can hold their heads a greater height above the water level. Their significant water and fat reserves would allow the tortoises to survive long ocean crossings without food or fresh water, and to endure the drought-prone climate of the islands. A larger size allowed them to better tolerate extremes of temperature due to gigantothermy.
Sexual dimorphism is most pronounced in saddleback populations in which males have more angled and higher front openings, giving a more extreme saddled appearance. Males of all varieties generally have longer tails and shorter, concave plastrons with thickened knobs at the back edge to facilitate mating.
The tortoises are herbivores that consume a diet of cacti, grasses, leaves, lichens, berries, melons, oranges and milkweed. 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.
Tortoises acquire most of their moisture from the dew and sap in vegetation. 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”.
Females journey up to several kilometres in July to November to reach nesting areas of dry, sandy coast. Nest digging is a tiring and elaborate task which may take the female several hours a day over many days to complete. It is carried out blindly using only the hind legs to dig a 30 cm-deep cylindrical hole, in which the tortoise then lays up to 16 spherical, hard-shelled eggs and the size of a billiard ball. Temperature plays a role in the sex of the hatchlings with lower-temperature nests producing more males and higher-temperature nests producing more females.