The Galápagos tortoise complex or Galápagos giant tortoise complex is a species complex of 15 (12 extant and 2-3 extinct) very large tortoise species in the genus Chelonoidis (which also contains three other species from mainland South America). They are the largest living species of tortoise, with some modern Galápagos tortoises weighing up to 417 kg (919 lb).[2] With lifespans in the wild of over 100 years, they are one of the longest-lived vertebrates. Captive Galapagos tortoises can live up to 177 years.[3] 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".[4]
The Galápagos tortoises are native to seven of the Galápagos Islands. 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. 12 species of the original 14-15 survive in the wild; a 13th species (Chelonoidis abingdonii) had only a single known living individual, kept in captivity and nicknamed Lonesome George until his death in June 2012. Two other species, C. niger (the type species of the complex) from Floreana Island and an undescribed species 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 species is estimated to have exceeded 19,000 at the start of the 21st century. Despite this rebound, all surviving species are classified as "threatened" by the International Union for Conservation of Nature.
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.
In May 2021, a genetic test carried out by scientists from the California Academy of Sciences confirmed that a single female tortoise discovered during a 2019 expedition to Fernandina Island is from the species Chelonoidis phantasticus.[5]
