Galápagos tortoise, Charles Darwin Research Station, Santa Cruz Island, Galápagos Islands, Ecuador

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• published: 29 May 2013
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The Galápagos tortoise or Galápagos giant tortoise is the largest living species of tortoise and the 14th-heaviest living reptile. Modern giant tortoises can weigh up to 250 kg; even larger versions, now extinct, roamed every continent except Antarctica and Australia into the Pleistocene. Today, they exist only on two remote archipelagos: the Galápagos 1000 km due west of Ecuador, and Aldabra in the Indian Ocean, 700 km east of Tanzania. The tortoise is native to seven of the Galápagos Islands, a volcanic archipelago about 1,000 km west of the Ecuadorian mainland. With lifespans in the wild of over 100 years, it is one of the longest-lived vertebrates. A captive individual lived at least 170 years. Tortoises are not likely to migrate, as they are too big and have an extremely slow metabolism. Spanish explorers, who discovered the islands in the 16th century, named them after the Spanish galápago, meaning tortoise. Tortoises also live very uncomplicated lives, and can nap up to 16 hours a day. 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. Charles Darwin's observations of these differences on the second voyage of the Beagle in 1835, contributed to the development of his theory of evolution. Darwin was highly impressed by these giants, although he referred to them as “antediluvian” and “giant monsters”, less than affectionate terms. Tortoise numbers declined from over 250,000 in the 16th century to a low of around 3,000 in the 1970s. This decline was caused by overexploitation of the species for meat and oil, habitat clearance for agriculture, and introduction of non-native animals to the islands, such as rats, goats, and pigs. The extinction of most giant tortoise lineages is thought to have also been caused by predation by humans or human ancestors. Tortoise populations on at least three islands have become extinct in historical time due to human activities. Specimens of these extinct taxa exist in several museums and also are being subjected to DNA analysis. Ten subspecies of the original 15 survive in the wild; an 11th subspecies had only a single known living individual, kept in captivity and nicknamed Lonesome George until his death in June 2012. Conservation efforts, beginning in the 20th century, have resulted in thousands of captive-bred juveniles being released onto their ancestral home islands, and the total number of the species is estimated to have exceeded 19,000 at the start of the 21st century. Despite this rebound, the species as a whole is classified as "vulnerable" by the International Union for Conservation of Nature. The discoverer of the Galápagos Islands, Fray Tomás de Berlanga, Bishop of Panama, wrote in 1535 of "such big tortoises that each could carry a man on top of himself." Naturalist Charles Darwin remarked after his trip three centuries later in 1835, "These animals grow to an immense size ... several so large that it required six or eight men to lift them from the ground". The largest recorded individuals have reached weights of over 400 kg and lengths of 1.87 meters. Size overlap is extensive with the Aldabra giant tortoise however taken as a species, the Galápagos tortoise seems to average slightly larger, with weights in excess of 185 kg being slightly more commonplace. Weights in the larger bodied subspecies range from 272 to 317 kg in mature males and from 136 to 181 kg in adult females. However, the size is variable across the islands and subspecies, those from Pinzón Island are relatively small with a maximum known weight of 76 kg and carapace length of approximately 61 cm compared to 75 to 150 cm range in tortoises from Santa Cruz Island. The tortoises' gigantism was probably a preadapted condition 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 and have a smaller surface area/volume ratio, which reduces osmotic water loss. 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. Fossil giant tortoises from mainland South America have been described that support this hypothesis of preadapted gigantism. The tortoises are ectothermic, so bask for 1–2 hours after dawn to absorb the sun's heat through their dark shells before actively foraging for 8–9 hours a day. They travel mostly in the early morning or late afternoon between resting and grazing areas.