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Giant tortoise

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Giant tortoises
Aldabra giant tortoise
Aldabra giant tortoise
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Cryptodira
Superfamily: Testudinoidea
Family: Testudinidae
Groups included
Cladistically included but traditionally excluded taxa

Giant tortoises are reptiles that are currently found on two remote groups of tropical islands: the Aldabra Atoll and Fregate Island in Seychelles and the Galápagos Islands in Ecuador (a population in the Mascarene Islands was exterminated by the 1900s). These tortoises can weigh as much as 417 kg (919 lb) and can grow to be 1.3 m (4 ft 3 in) long. Giant tortoises originally made their way to islands from the mainland via oceanic dispersal; for example, the Aldabra Atoll and Mascarenes giant tortoises are related to Madagascan tortoises while the Galapagos giant tortoises are related to South American mainland tortoises. Tortoises are aided in such dispersal by their ability to float with their heads up, and to survive up to six months without food or fresh water.[1]

The phenomenon of animal species evolving to unusually large size on islands (in comparison to continental relatives) is known as island gigantism or insular gigantism. This may occur due to factors such as relaxed predation pressure, competitive release, or as an adaptation to increased environmental fluctuations on islands.[2][3] However, giant tortoises are no longer considered to be classic examples of island gigantism, as similarly massive tortoises are now known to have once been widespread. Giant tortoises were formerly common (prior to the Quaternary extinctions) across the Cenozoic faunas of Eurasia, Africa and the Americas.[4][5]

These animals belong to an ancient group of reptiles, appearing about 250 million years ago. By the Late Cretaceous, 70 or 80 million years ago, some had already become gigantic. About 1 million years ago, tortoises reached the Galápagos Islands. Most of the gigantic species began to disappear about 100,000 years ago. 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.[6][better source needed]

Galápagos tortoise on Santa Cruz Island (Galápagos)

History

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 (Megalochelys atlas),[4] North America[4] and South America,[5] Indonesia,[4] Madagascar (Aldabrachelys),[4] and even the island of Malta[4] became extinct. The giant tortoises formerly of Africa died out somewhat earlier, during the Late Pliocene.[7] 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 the distantly-related giant meiolaniid turtle Meiolania damelipi in Vanuatu.[8][9] 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.[10][better source needed] 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[vague] genetic studies have cast doubt on the validity of these as separate species)[citation needed] and 10 extant species in the Galápagos.

Life expectancy

Giant tortoises are among the world's longest-living animals, with an average lifespan of 100 years or more.[6][better source needed] The Madagascar radiated tortoise Tu'i Malila was 188 at death in Tonga in 1965.[citation needed] 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.[citation needed] 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.[11][better source needed] Around the time of its discovery, they were caught for food in such large numbers that they became virtually extinct by 1900.[citation needed] Giant tortoises are now protected by strict conservation laws and are categorized as threatened species.

Aldabra giant tortoise

The Aldabra giant tortoise lives on the remote Aldabra Atoll, one of the Seychelles group of islands in the Indian Ocean. It is the only Indian Ocean giant tortoise species alive today, others having become extinct soon after the arrival of human settlers (including the Seychelles giant tortoise which is now thought to be extinct in the wild, although the Aldabra giant tortoise and the Seychelles giant tortoise are so similar genetically that they are thought by some to be the same species).[11][12][full citation needed]

Today, the Aldabra giant tortoise is listed as an animal that is vulnerable to extinction in the wild. However, the Aldabra atoll has now been protected from human influence after having been declared a World Heritage Site, and is home to some 152,000 Aldabra giant tortoises, the world's largest population of this species. Another isolated population of the Aldabra giant tortoise resides on the island of Zanzibar, and other captive populations exist in conservation parks in Mauritius and Rodrigues. The captive breeding programmes on these other islands are trying to revive the species, and populations on them today appear to be thriving.[11][better source needed]

Life cycle

Aldabra giant tortoises reproduce and lay up to 25 rubbery eggs between February and May.[citation needed] Mating is a noisy process, with the males bellowing like a bull, and chasing after the females. During copulation, the male's tail guides his penis into position to fertilize the female, with copulation lasting for 10–15 minutes.[13][full citation needed] Eggs are deposited into a dry, shallow nest on the ground, making them particularly vulnerable to being eaten by predators. It is thought[vague] that female Aldabra giant tortoises are able to produce more than one clutch a year. The hatchlings emerge after an incubation period of 8 months. The baby Aldabra giant tortoises tend to all emerge during the same two week period which coincides with the arrival of the rainy season.[11][better source needed] They reach sexual maturity at around the age of 30 years.[13] Although some individuals have been known to live for more than 250 years, their life expectancy is between 80 and 120 years old.

Distribution and habitat

Aldabra giant tortoise cooling down in a freshwater pond on Curieuse, Seychelles

The Aldabra giant tortoise has a dome-shaped shell which acts as protective armor for their soft, vulnerable body that lies underneath. They have incredibly long necks which they use to tear leaves from the branches higher up trees. The males, although not really that much bigger, are also known to weigh nearly 100 kg (220 lb) more than females. They are slow moving animals with thick, short legs and round, almost flat feet that help them when they are walking on the sand.

The Aldabra giant tortoise is primarily found inhabiting grasslands and swamps on the islands of the Aldabra atoll, which forms part of the Seychelles island chain in the Indian Ocean. They once shared these islands with a number of other giant tortoise species, but many of these were hunted to extinction in the 1700s and 1800s.[citation needed] Although they are usually found in areas of dense low-lying vegetation, they are also known to wander into more sparse and rocky regions when food is in short supply. They can also be found resting in the shade or in a very shallow pool of water to cool themselves on hot days.[11][better source needed] Aldabra tortoises tend to spend their lives grazing, but will cover surprising distances in search of food and are also observed on bare rock and thin soil. They can drink from very shallow pools through their nostrils; the name Dipsochelys refers to this remarkable adaptation.[14][full citation needed]

Species and subspecies

Canary Islands giant tortoises

Restoration of the Tenerife giant tortoise Centrochelys burchardi in Museo de la Naturaleza y el Hombre of Santa Cruz de Tenerife

Remains have been found in the Canary Islands of two extinct species of large tortoises: the Tenerife giant tortoise (Centrochelys burchardi) and the Gran Canaria giant tortoise (Centrochelys vulcanica). These species are believed to have died out due to volcanic eruptions.[15][full citation needed]

The Canary Islands tortoises were similar to those currently found in the Galapagos and Seychelles. C. burchardi had a larger shell, with a length of approximately 65 to 94 cm, while G. vulcanica had a 61 cm long shell.[15]

The remains date mostly to the Miocene.[when?] It is believed that these species survived until the Late Pleistocene.[15]

Galapagos giant tortoises

The closest living relative of the Galapagos giant tortoise is the small Chaco tortoise from South America, although it is not a direct ancestor. Scientists believe the first tortoises arrived to Galapagos 2–3 million years ago by drifting 600 miles from the South American coast on vegetation rafts or on their own.[citation needed] They were already large animals before arriving in Galapagos. Colonizing the eastern-most islands of Española and San Cristóbal first, they then dispersed throughout the archipelago, eventually establishing about 16 separate populations on ten of the largest Galapagos Islands. Currently there are only 10 species of Galapagos giant tortoises left of the original 16 species. 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."[16][better source needed] The tortoises also conveniently held water in their neck that could be used as drinking water.

Lonesome George, the last known individual of the Pinta Island tortoise (C. abingdonii)

These buccaneers took giant tortoises not only because of their meat and oil[clarification needed] but because of their incredible adaption that allows these animals to survive one year without food or water.[citation needed] Once buccaneers, whalers and fur sealers discovered that they could have fresh meat for their long voyages by storing live giant tortoises in the holds of their ships, massive exploitation of the species began. Tortoises were also exploited for their oil,[citation needed] which was used to light the lamps of Quito. Two centuries of exploitation resulted in the loss of between 100,000 and 200,000 tortoises. Three species have been extinct for some time, and a fourth species lost its last member, Lonesome George, in June 2012. In February 2019, a tortoise species once thought to have been extinct since 1906, Fernandina giant tortoise, was discovered on its namesake island in the Galapagos.[17] It is estimated that 20,000–25,000 wild tortoises live on the islands today.[18][better source needed]

Distribution and habitat

Galapagos tortoises are herbivorous, feeding primarily on cactus pads, grasses, and native fruit. They drink large quantities of water when available that they can store in their bladders for long periods of time. There are two main types of shell among them, the saddle-backed shell and the domed shell. They both provide special adaption to different environments. The saddle-backed tortoises are the smallest Galapagos tortoises, but present a very long neck and pairs of legs. They live on arid zone and feed on cactus. The domed tortoises are bigger with shorter neck and legs, they are found in the more vegetated islands and feed on grass.[19] They spend an average of 16 hours a day resting. Their activity level is driven by ambient temperature and food availability. In the cool season, they are active at midday, sleeping in in the morning and afternoon. In the hot season, their active period is early morning and late afternoon, while midday finds them resting and trying to keep cool under the shade of a bush or half-submerged in muddy wallows.[citation needed]

Life cycle

Tortoises breed primarily during the hot season from January to May; however, tortoises can be seen mating any month of the year. During the cool season (June to November), female tortoises migrate to nesting zones, which are generally located in low lands of the islands, to lay their eggs. A female can lay from 1–4 nests over a nesting season from June to December. She digs the hole with her hind feet, then lets the eggs drop down into the nest, and finally covers it again with her hind feet. The number of eggs ranges from 2 to 7 for saddle-backed tortoises to sometimes more than 20 to 25 eggs for domed tortoises. The eggs incubate from 110 to 175 days (incubation periods depend on the month the clutch was produced, with eggs laid early in the cool season requiring longer incubation periods than eggs laid at the end of the cool season when the majority of their incubation will occur at the start of the hot season). After hatching, the young hatchlings remain in the nest for a few weeks before emerging out a small hole adjacent to the nest cap. Usually the temperature of the nest influences on the sex of the hatchling. Warm temperatures would yield more females while colder temperatures would yield more males.[citation needed]

Species and subspecies

Main article: List of species of Galápagos tortoise

Mascarenes giant tortoises

The Mascarene Islands of Mauritius, Réunion and Rodrigues once harboured five species of giant tortoise, comprising two species occurring on Mauritius, another two on Rodrigues, and one on Réunion. The tortoises were unique to these islands and had gained a number of special adaptations in the absence of ground predators. They differed from any other giant tortoise species because of their modified jaws, reduced scales on the legs and shells averaging just 1mm thick. The shells of the giant tortoises were open-ended; the name Cylindraspis actually means cylinder-shaped. This was a specific adaptation in response to the lack of predators, where thick, heavily armored shells were no longer necessary.[citation needed]

Around the 16th century, with human arrival and the subsequent introduction of domestic animals, particularly pigs, the tortoises were rapidly[vague][citation needed] hunted to extinction. Unfortunately, the thin shells were of no protection against these new invaders; rats, cats and pigs devoured the eggs and young and thousands were collected alive for provisioning ships. Sometimes they were even hunted for their oil,[citation needed] which was very valuable around that time because it provided a cure for many ailments, including scurvy.

On Mauritius, the giant tortoise disappeared from the mainland by the end of the 17th century and the very last tortoises survived until the 1730s on the islets in the north. Around the late 1800s, large number of tortoises bones were discovered in the Mare aux Songes excavations.[citation needed] These resulted in the description of the two species of giant tortoise endemic to Mauritius, the saddle-backed Mauritius (Cylindraspis inepta) and the domed Mauritius (Cylindraspis triserrata).[23]

Today, the only remains from these five species are a number of fossil bones and shells, a few drawings of live animals, and one stuffed saddle-backed Rodrigues giant tortoise in France's National Museum of Natural History.[24][better source needed]

Species and subspecies

See also

References

  1. ^ Le, M.; Raxworthy, C. J.; McCord, W. P.; Mertz, L. (5 May 2006). "A molecular phylogeny of tortoises (Testudines: Testudinidae) based on mitochondrial and nuclear genes". Molecular Phylogenetics and Evolution. 40 (2): 517–531. doi:10.1016/j.ympev.2006.03.003. PMID 16678445.
  2. ^ Pritchard, P. C. H. (1996). The Galápagos Tortoises: Nomenclatural and Survival Status. Lunenburg, MA: Chelonian Research Foundation. ISBN 978-0965354004.
  3. ^ Jaffe, A. L.; Slater, G. J.; Alfaro, M. E. (2011). "The evolution of island gigantism and body size variation in tortoises and turtles". Biology Letters. 7 (4): 558–561. doi:10.1098/rsbl.2010.1084. PMC 3130210. PMID 21270022.
  4. ^ a b c d e f Hansen, D. M.; Donlan, C. J.; Griffiths, C. J.; Campbell, K. J. (April 2010). "Ecological history and latent conservation potential: large and giant tortoises as a model for taxon substitutions" (PDF). Ecography. 33 (2): 272–284. doi:10.1111/j.1600-0587.2010.06305.x. Retrieved 6 May 2019.
  5. ^ a b Cione, A. L.; Tonni, E. P.; Soibelzon, L. (2003). "The Broken Zig-Zag: Late Cenozoic large mammal and tortoise extinction in South America" (PDF). Rev. Mus. Argentino Cienc. Nat., N.s. 5 (1): 1–19. doi:10.22179/REVMACN.5.26. ISSN 1514-5158. Archived from the original (PDF) on 6 July 2011. Retrieved 2019-05-06.
  6. ^ a b "Galápagos Tortoise | National Geographic". 10 September 2010. Retrieved 26 December 2017.
  7. ^ Harrison, T. (2011). "Tortoises (Chelonii, Testudinidae)". Paleontology and Geology of Laetoli: Human Evolution in Context, Vol. 2: Fossil Hominins and the Associated Fauna. Vertebrate Paleobiology and Paleoanthropology. Springer Science+Business Media. pp. 479–503. doi:10.1007/978-90-481-9962-4_17. ISBN 978-90-481-9961-7. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  8. ^ White, A. W.; Worthy, T. H.; Hawkins, S.; Bedford, S.; Spriggs, M. (16 August 2010). "Megafaunal meiolaniid horned turtles survived until early human settlement in Vanuatu, Southwest Pacific". Proc. Natl. Acad. Sci. USA. 107 (35): 15512–15516. Bibcode:2010PNAS..10715512W. doi:10.1073/pnas.1005780107. PMC 2932593. PMID 20713711.
  9. ^ Keim, Brandon (17 August 2010). "Extinct, King Koopa-Style Giant Turtle Found on Pacific Island". Wired. (Popular presentation of some material from the PNAS article)
  10. ^ "Matter of Time". www6.miami.edu. Retrieved 26 December 2017.
  11. ^ a b c d e A-Z-Animals.com. "Aldabra Giant Tortoise". Retrieved 26 December 2017.
  12. ^ "Taxonomy of Dipsochelys (Aldabra giant tortoise)". nhm.ac.uk. National History Museum. Retrieved 31 December 2014.
  13. ^ a b "Biology of Dipsochelys (Aldabra giant tortoise)". nhm.ac.uk. National History Museum. Archived from the original on 13 December 2014. Retrieved 31 December 2014.
  14. ^ "Habitat of Dipsochelys (Aldabra giant tortoise)". nhm.ac.uk. National History Museum. Retrieved 31 December 2014.
  15. ^ a b c La Paleontología de vertebrados en Canarias
  16. ^ "Floreana History – Pre 1900's". Diving The Galapagos blog. Diving The Galapagos Blog. 28 July 2009. Retrieved 26 February 2011.
  17. ^ Langlois, Jill (22 February 2019). "How an 'extinct' tortoise was rediscovered after a century's absence". Animals. Retrieved 27 September 2019.{{cite web}}: CS1 maint: url-status (link)
  18. ^ "Giant Tortoises".
  19. ^ Fitter, Julia; Fitter, Daniel; Hosking, David (2007). Wildlife of Galapagos (2 ed.). UK: collins. p. 256. ISBN 978-0-00-724818-6.
  20. ^ Raferty, Isolde. "Lonesome George, last-of-its-kind Galápagos Tortoise, dies". MSNBC.com. Archived from the original on 26 June 2012. Retrieved 2012-06-24.
  21. ^ Nuwer, Rachel. "New Species of Galapagos Tortoise Found on Santa Cruz Island". Smithsonian. Retrieved 26 December 2017.
  22. ^ "New Species of Giant Tortoise Found in the Galapagos". Live Science. Retrieved 26 December 2017.
  23. ^ Hume, Julian Pender (September 2010). "Mascarene Giant Tortoises – Naturalis Biodiversity Center". www.naturalis.nl. Naturalis. Archived from the original on 13 December 2014. Retrieved 31 December 2014.
  24. ^ "Recently Extinct Animals – Species Info – Saddle-backed Rodrigues Giant Tortoise". The Extinction Website. PeterMaas. August 2009. Archived from the original on 3 March 2016. Retrieved 31 December 2014.

Bibliography and further reading

  • 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
  • Pritchard P.C.H. (1996). The Galapagos tortoises: nomenclatural and survival status. Lunenburg, MA: Chelonian Research Foundation in association with Conservation International and Chelonia Institute.
  • Fitter, Julia; Fitter, Daniel; Hosking, David (2007). Wildlife of Galapagos (2 ed.). UK: collins. p. 256. ISBN 978-0-00-724818-6.
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