Galapagos tortoise

The Galápagos tortoise complex or Galápagos giant tortoise complex (Chelonoidis nigra and related species) are the largest living species of tortoise. Modern Galápagos tortoises can weigh up to 417 kg (919 lb). Today, giant tortoises exist on only two remote archipelagos: the Galápagos Islands 1000 km due west of mainland Ecuador; and Aldabrachelys gigantea of Aldabra in the Indian Ocean, 700 km east of Tanzania. The Galápagos tortoises are native to seven of the Galápagos Islands, a volcanic archipelago about 1,000 km (620 mi) 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. Spanish explorers, who discovered the islands in the 16th century, named them after the Spanish galápago, meaning 'tortoise'. 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. 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, as the tortoises themselves have no natural predators. Tortoise populations on at least three islands have become extinct in historical times due to human activities. Specimens of these extinct taxa exist in several museums and also are being subjected to DNA analysis. 10 species of the original 15 survive in the wild; an 11th species (Chelonoidis abingdonii) 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 Galápagos Islands were discovered in 1535, but first appeared on the maps, of Gerardus Mercator and Abraham Ortelius, around 1570. The islands were named 'Insulae de los Galopegos' (Islands of the Tortoises) in reference to the giant tortoises found there. Initially, the giant tortoises of the Indian Ocean and those from the Galápagos were considered to be the same species. Naturalists thought that sailors had transported the tortoises there. In 1676, the pre-Linnaean authority Claude Perrault referred to both species as Tortue des Indes. In 1783, Johann Gottlob Schneider classified all giant tortoises as Testudo indica ('Indian tortoise'). In 1812, August Friedrich Schweigger named them Testudo gigantea ('gigantic tortoise'). In 1834, André Marie Constant Duméril and Gabriel Bibron classified the Galápagos tortoises as a separate species, which they named Testudo nigrita ('black tortoise'). The first systematic survey of giant tortoises was by the zoologist Albert Günther of the British Museum, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles, and four from the Mascarenes. Günther hypothesized that all the giant tortoises descended from a single ancestral population which spread by sunken land bridges. This hypothesis was later disproven by the understanding that the Galápagos, Seychelles, and Mascarene islands are all of recent volcanic origin and have never been linked to a continent by land bridges. Galápagos tortoises are now thought to have descended from a South American ancestor, while the Indian Ocean tortoises derived from ancestral populations on Madagascar. At the end of the 19th century, Georg Baur and Walter Rothschild recognised five more populations of Galápagos tortoise. In 1905–06, an expedition by the California Academy of Sciences, with Joseph R. Slevin in charge of reptiles, collected specimens which were studied by Academy herpetologist John Van Denburgh. He identified four additional populations, and proposed the existence of 15 species. Van Denburgh's list still guides the taxonomy of the Galápagos tortoise, though now 10 populations are thought to have existed.