|Aldabra giant tortoise|
Giant tortoises are characteristic reptiles of certain tropical islands. Often reaching enormous size—they can weigh as much as 300 kg (660 lbs) and can grow to be 1.3 m (4 ft) long—they live, or lived (some species are recently extinct), in the Seychelles, the Mascarenes and the Galapagos. Today, the world's largest population inhabits Aldabra Atoll in Seychelles, where there are approximately 150,000 individuals. Although appearing similar, the tortoises represent separate branches of evolution. The Seychelles and Mascarenes tortoises derive from nearby Madagascar, while the Galapagos tortoises came from nearby Ecuador.
Although often considered examples of island gigantism, prior to the arrival of Homo sapiens giant tortoises also occurred in non-island locales, as well as on a number of other, more accessible islands. During the Pleistocene, and mostly during the last 50,000 years, tortoises of the mainland of southern Asia (Colossochelys atlas), North and South America, Australia (Meiolania), Indonesia, Madagascar (Dipsochelys), and even the island of Malta became extinct. The giant tortoises formerly of Africa died out somewhat earlier, during the late Pliocene. While the timing of the disappearances of various extinct giant tortoise species seems to correlate with the arrival of humans, direct evidence for human involvement in these extinctions is usually lacking; however, such evidence has been obtained in the case of Meiolania damelipi in Vanuatu. One interesting relic is the shell of an extinct giant tortoise found in a submerged sinkhole in Florida with a wooden spear piercing it, carbon dated to 12,000 years ago.
These animals belong to an ancient group of reptiles, appearing about 250 million years ago. In the Upper Cretaceous, 70 or 80 million years ago some already became gigantic. About 1 million years ago tortoises reached the Galápagos Islands. Since 100,000 years ago most of the gigantic species began to disappear. Only 250 years ago there were at least 20 species and subspecies in islands of the Indian Ocean and 14 or 15 subspecies in the Galápagos Islands.
During the 16th and 17th centuries, the Galápagos were frequented by buccaneers preying on Spanish treasure ships. Filling a ship's hold with tortoises was an easy way to stock up on food, a tradition that was continued by whalers in the centuries that followed: "'whaling skippers were almost lyrical in their praise of tortoise meat, terming it far more delicious than chicken, pork or beef'. They said the meat of the giant tortoise was 'succulent meat and the oil from their bodies as pure as butter, but best of all, the giants could hibernate in a ship’s damp for a year or more.'"
Today, only one of the species of the Indian Ocean survives in the wild, the Aldabra giant tortoise (two more are claimed to exist in captive or re-released populations, but some genetic studies have cast doubt on the validity of these as separate species) and 11 subspecies in the Galápagos.
Giant tortoises are among the world's longest-living animals, with an average lifespan of 100 years or more. The Madagascar radiated tortoise Tu'i Malila was 188 at death in Tonga in 1965. Harriet (initially thought to be one of the three Galápagos tortoises brought back to England from Charles Darwin's Beagle voyage but later shown to be from an island not even visited by Darwin) was reported by the Australia Zoo to be 176 years old when she died in 2006. Also, on 23 March 2006, an Aldabra giant tortoise named Adwaita died at Alipore Zoological Gardens in Kolkata. He was brought to the zoo in the 1870s from the estate of Lord Clive and is thought to have been around 255 years old when he died. Around the time of its discovery, they were caught and killed for food in such large quantities that they became virtually extinct by 1900. Giant tortoises are now under strict conservation laws and are categorised as threatened species.
Species and subspecies
- Aldabrachelys (formerly Dipsochelys)
- †A. abrupta – Madagascar, (extinct)
- A. gigantea – Aldabra giant tortoise Aldabra, granitic islands of Seychelles, four subspecies recognised
- A. g. arnoldi – Arnold's giant tortoise - Seychelles (extinct in the wild)
- †A. g. daudinii – Daudin's giant tortoise, Seychelles, (extinct)
- A. g. gigantia – Aldabra Atoll, Seychelles
- A. g. hololissa – Seychelles giant tortoise (extinct in the wild)
- †A. grandidieri – southwestern Madagascar, (extinct)
- Chelonoidis nigra – Galápagos tortoise
- C. n. abingdonii – Pinta Island Tortoise (Listed as extinct in the wild. The last known of this subspecies, named Lonesome George, died June 24, 2012.)
- C. n. becki – Volcán Wolf tortoise
- C. n. chathamensis – Chatham Island tortoise
- C. n. darwini – James Island tortoise
- C. n. duncanensis – Duncan Island tortoise
- C. n. guentheri – Sierra Negra tortoise
- C. n. hoodensis – Hood Island tortoise
- C. n. microphyes – Volcán Darwin tortoise
- C. n. nigra – Charles Island tortoise
- C. n. porteri – Indefatigable Island tortoise
- C. n. vandenburghi – Volcán Alcedo tortoise
- C. n. vicina – Iguana Cove tortoise
- †Meiolania platyceps - New Caledonia (extinct)
Gallery Galápagos Tortoise Conservation - (Chelonoidis nigra)
Gigantic galapagos tortoise on the island of santa cruz - Chelonoidis nigra
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- IUCN 2006. 2006 IUCN Red List of Threatened Species. <www.iucnredlist.org>. Downloaded on 20 May 2006.
- Gerlach, J. (editor) 2014. Western Indian Ocean Tortoises: Ecology, Diversity, Evolution, Conservation, Palaeontology. Siri Scientific Press, Manchester, 352 pp, 200+ illustrations. ISBN 978-0-9929979-0-8