Galápagos tortoise

The Galápagos tortoise (Chelonoidis niger), also called the Galápagos giant tortoise, is a very large species of tortoise in the genus Chelonoidis (which also contains three smaller species from mainland South America). The species comprises 15 subspecies (12 extant and 3 extinct). It is the largest living species of tortoise, and can weigh up to 417 kg (919 lb). They are also the largest extant terrestrial cold-blooded animals (ectotherms).

With lifespans in the wild of over 100 years, it is one of the longest-lived vertebrates. Captive Galapagos tortoises can live up to 177 years. For example, a captive individual, Harriet, lived for at least 175 years. Spanish explorers, who discovered the islands in the 16th century, named them after the Spanish galápago, meaning "tortoise".

Galápagos tortoises are native to seven of the Galápagos Islands. Shell size and shape vary between subspecies and populations. On islands with humid highlands and abundant low vegetation, the tortoises are larger, with domed shells and short necks; on islands with dry lowlands and less ground-level vegetation, 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 15,000 in the 1970s. This decline was caused by overexploitation of the subspecies 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. 12 subspecies of the original 14–15 survive in the wild; a 13th subspecies (C. n. abingdonii) had only a single known living individual, kept in captivity and nicknamed Lonesome George until his death in June 2012. Two other subspecies, C. n. niger (the type subspecies of Galápagos tortoise) from Floreana Island and an undescribed subspecies from Santa Fe Island are known to have gone extinct in the mid-late 19th century. 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 subspecies is estimated to have exceeded 19,000 at the start of the 21st century. Despite this rebound, all surviving subspecies are classified as Threatened by the International Union for Conservation of Nature.

The Galápagos tortoises are one of two insular radiations of giant tortoises that still survive to the modern day; the other is Aldabrachelys gigantea of Aldabra and the Seychelles in the Indian Ocean, 700 km (430 mi) east of Tanzania. While giant tortoise radiations were common in prehistoric times, humans have wiped out the majority of them worldwide; the only other radiation of tortoises to survive to historic times, Cylindraspis of the Mascarenes, was driven to extinction by the 19th century, and other giant tortoise radiations such as a Centrochelys radiation on the Canary Islands and another Chelonoidis radiation in the Caribbean were driven to extinction prior to that.

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 thought to be the same subspecies. Naturalists thought that sailors had transported the tortoises there. In 1676, the pre-Linnaean authority Claude Perrault referred to both subspecies 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 subspecies, 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 had spread by sunken land bridges. This hypothesis was later disproven by the understanding that the Galápagos, the atolls of Seychelles, and the 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.