Aldabra giant tortoise
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The Aldabra giant tortoise (Aldabrachelys gigantea) is a species of tortoise in the family Testudinidae and genus Aldabrachelys. The species is endemic to the Seychelles, with the nominate subspecies, A. g. gigantea native to Aldabra atoll. It is one of the largest tortoises in the world.<ref name=pritchard>Pritchard, Peter C.H. (1979) Encyclopedia of Turtles. Neptune, New Jersey: T.F.H. Publications, Inc. Ltd.</ref> Historically, giant tortoises were found on many of the western Indian Ocean islands, as well as Madagascar, and the fossil record indicates giant tortoises once occurred on every continent and many islands with the exception of Australia and Antarctica.<ref name=palk2002>Template:Cite journal</ref>
Many of the Indian Ocean species were thought to be driven to extinction by over-exploitation by European sailors, and they were all seemingly extinct by 1840 with the exception of the Aldabran giant tortoise on the island atoll of Aldabra.<ref name=palk2003>Template:Cite journal</ref> Although some remnant individuals of A. g. hololissa and A. g. arnoldi may remain in captivity,<ref name=palk2003 /> in recent times, these have all been reduced as subspecies of A. gigantea.<ref name="ttwg2014" />
DescriptionEdit
The carapace of A. gigantea is a brown or tan in color with a high, domed shape. The species has stocky, heavily scaled legs to support its heavy body. The neck of the Aldabra giant tortoise is very long, even for its great size, which helps the animal to exploit tree branches up to a meter from the ground as a food source. Similar in size to the famous Galápagos giant tortoise, its carapace averages Template:Convert in length. Males have an average weight of Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Females are generally smaller than males, with average specimens measuring Template:Convert in carapace length and weighing Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Medium-sized specimens in captivity were reported as Template:Convert in body mass.<ref>Template:Cite journal</ref> Another study found body masses of up to Template:Convert most commonplace.<ref>Template:Cite journal</ref>
Nomenclature and systematicsEdit
This species is widely referred to as Aldabrachelys gigantea, but in recent times, attempts were made to use the name Dipsochelys as Dipsochelys dussumieri. After a debate that lasted two years with many submissions, the ICZN eventually decided to conserve the name Testudo gigantea over this recently used name (ICZN 2013).<ref name=iucn13>Template:Cite journal</ref> This also affected the genus name for the species, establishing Aldabrachelys gigantea as nomen protectum.
Four subspecies are currently recognized.<ref name="ttwg2014" /> A trinomial authority in parentheses indicates that the subspecies was originally described in a genus other than Aldabrachelys:
- A. g. gigantea Template:Small,<ref name=schweig12 /> Aldabra giant tortoise from the Seychelles island of Aldabra
- A. g. arnoldi Template:Small,<ref name=bour82>Bour, Roger (1982). "Contribution à la connaisance des tortues terrestres des Seychelles: définition du genre endémique et description d'une espéce nouvelle probablement originaire des îles grantiques & bord de l'extinction". Comptes Rendus de l'Académie des Sciences 295: 117–122. (Dipsochelys arnoldi, new species, p. 118). (in French).</ref> Arnold's giant tortoise from the Seychelles island of Mahé
- A. g. daudinii † Template:Small,<ref name="d&b1835" /> Daudin's giant tortoise, from the Seychelles island of Mahé (extinct 1850)<ref name="ttwg2014" />
- A. g. hololissa Template:Small,<ref name=gunther77 /> Seychelles giant tortoise, from the Seychelles islands of Cerf, Cousine, Frégate, Mahé, Praslin, Round, and Silhouette
The subspecific name, daudinii, is in honor of French zoologist François Marie Daudin.<ref>Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). "Dipsochelys daudinii", p. 66 in The Eponym Dictionary of Reptiles. Baltimore: Johns Hopkins University Press. Template:ISBN.</ref>
Genetic evidence suggests that A. gigantea is most closely related to the extinct giant tortoise Aldabrachelys abrupta from Madagascar, from which it is estimaged to have diverged from approximately 4.5 million years ago.<ref>Template:Cite journal</ref>
Range and distributionEdit
The main population of the Aldabra giant tortoise resides on the islands of the Aldabra Atoll in the Seychelles. The atoll has been protected from human influence and is home to some 100,000 giant tortoises, the world's largest population of the animal.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Smaller populations of A. gigantea in the Seychelles exist on Frégate Island and in the Sainte Anne Marine National Park (e.g. Moyenne Island),<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> where they are a popular tourist attraction. Conservationists have also reintroduced 50 small individuals to Aride Island, joining 10 larger tortoises previously rehomed there.<ref>http://www.seychellesnewsagency.com/articles/20753/Seychelles'+conservationists+re-home++Aldabra+giant+tortoises+on+Aride+Island</ref>
Another isolated population of the species resides on the island of Changuu, near Zanzibar. Other captive populations exist in conservation parks in Mauritius and Rodrigues. The tortoises exploit many different kinds of habitat, including grasslands, low scrub, mangrove swamps, and coastal dunes.
EcologyEdit
HabitatEdit
A peculiar habitat has coevolved due to the grazing pressures of the tortoises: "tortoise turf", a comingling of 20+ species of grasses and herbs. Many of these distinct plants are naturally dwarfed and grow their seeds not from the tops of the plants, but closer to the ground to avoid the tortoises' close-cropping jaws.
As the largest animal in its environment, the Aldabra tortoise performs a role similar to that of the elephant. Their vigorous search for food fells trees and creates pathways used by other animals.
Feeding ecologyEdit
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Primarily herbivores, Aldabra giant tortoises eat grasses, leaves, woody plant stems, and fruit. They occasionally indulge in small invertebrates and carrion, even eating the bodies of other dead tortoises. In captivity, Aldabra giant tortoises are known to consume fruits such as apples and bananas, as well as compressed vegetable pellets. In 2020, a female Aldabra giant tortoise on Fregate Island was observed hunting and eating a juvenile lesser noddy, indicating that the species was in the process of learning to catch birds. <ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>
Little fresh water is available for drinking in the tortoises' natural habitat, so they obtain most of their moisture from their food.
The Aldabra giant tortoise has two main varieties of shells, related to their habitat. Specimens living in habitats with food available primarily on the ground have more dome-shaped shells with the front extending downward over the neck. Those living in an environment with food available higher above the ground have more flattened top shells with the front raised to allow the neck to extend upward freely.Template:Cn
Tortoise turfEdit
As the Aldabra giant tortoise is primarily herbivorous it spends much of its time browsing for food in its surrounding well-vegetated environment. The Aldabra giant tortoise is known to be found in places that are commonly known as "tortoise turf". Tortoise turf is composed of:<ref name=grubb>Template:Cite journal</ref><ref name=Hnat>Hnatiuk, R.J. and L.F.H. Merton (1979) "Vegetation of Aldabra, a Reassessment" Template:Webarchive. Atoll Research Bulletin No. 239, The Smithsonian Institution</ref>
BehaviorEdit
Aldabra tortoises are found both individually and in herds, which tend to gather mostly on open grasslands. They are most active in the mornings, when they spend time grazing and browsing for food. They dig wallows, hide under shade trees or in small caves, as well as submerge themselves in pools to keep cool during the heat of the day.
LifespanEdit
Large tortoises are among the longest-lived animals. Some individual Aldabra giant tortoises are thought to be over 200 years of age, but this is difficult to verify because they tend to outlive their human observers. Adwaita was reputedly one of four captured by British seamen from the Seychelles Islands as gifts to Robert Clive of the British East India Company in the 18th century, and came to Calcutta Zoo in 1875. At his death in March 2006 at the Kolkata (formerly Calcutta) Zoo in India, Adwaita is reputed to have reached the longest ever measured lifespan of 255 years (birth year 1750).<ref name=BBC4837988>Template:Cite news</ref>
As of 2022, Jonathan, a Seychelles giant tortoise, is thought to be the oldest living giant tortoise at the age of Template:Years ago years.<ref>Template:Cite news</ref> Esmeralda, an Aldabra giant tortoise, is second at the age of Template:Years ago years, since the death of Harriet, a Galapagos giant tortoise, at 175. An Aldabra giant tortoise living on Changuu off Zanzibar is reportedly Template:Years ago years old.
BreedingEdit
Mating takes place between February and May, and in July-September<ref name=bourn>Template:Cite journal</ref> females lay between 9 and 25 hard-shelled eggs in a 30 cm deep nest. Usually, less than half of the eggs are fertile. Females can produce multiple clutches of eggs in a year. After incubating for about eight months, the tiny, independent young hatch between October and December.<ref name=stearns>Template:Cite journal</ref>
In captivity, oviposition dates vary. Tulsa Zoo<ref name=collins>Collins, Dave. (1984) "Captive Breeding and Management of the Aldabra Tortoise". Presented to 8th International Herpetological Symposium, Jacksonville Zoo, Jacksonville, Fl.</ref> maintains a small herd of Aldabra tortoises and they have reproduced several times since 1999. One female typically lays eggs in November and again in January, providing the weather is warm enough to go outside for laying. The zoo also incubates their eggs artificially, keeping two separate incubators at 27 °C (81 °F) and 30 °C (86 °F). On average, the eggs kept at the latter temperature hatch in 107 days.<ref name=bourn/>
ConservationEdit
The Aldabra giant tortoise has an unusually long history of organized conservation. Albert Günther of the British Museum, who later moved to the Natural History Museum of London, enlisting Charles Darwin and other famous scientists to help him, worked with the government of Mauritius to establish a preserve at the end of the 19th century. The related, but distinct, species of giant tortoise from the Seychelles islands, Seychelles giant tortoise A. g. hololissa and Arnold's giant tortoise A. g. arnoldi, were the subject of a captive-breeding and reintroduction program by the Nature Protection Trust of Seychelles.<ref name=spratt>Template:Cite journal</ref><ref name=swingland>Swingland, Ian R. Aldabran Giant Tortoise. The Conservation Biology of Tortoises, Occasional Papers of the IUCN Species Survival Commission (SSC), No. 5, 1989.</ref>
A reference genome and low-coverage sequencing analyses has looked at revealing within- and among-island genetic differentiation within the Aldabra population, as well as assigning likely origins for zoo-housed individuals. This has managed to differentiate between individuals sampled on Malabar and Grande Terre and resolve the exact origin of zoo-housed individuals.<ref>Template:Cite journal</ref>
ReferencesEdit
External linksEdit
- Sea World Animal Bytes
- Aldabra giant tortoise at EMY System and World Turtle Database
- Seychelles Giant Tortoise Conservation Project
- Aldabra giant tortoise in the Encyclopedia of Life
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