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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.[7] 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.[8]

Aldabra giant tortoise
Beauval Zoo, France
CITES Appendix II (CITES)[2]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Cryptodira
Superfamily: Testudinoidea
Family: Testudinidae
Genus: Aldabrachelys
Species:
A. gigantea
Binomial name
Aldabrachelys gigantea
(Schweigger, 1812)
Synonyms[6]
(video) A pair of Aldabra giant tortoises at Tobu Zoo in Saitama, Japan

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.[9] Although some remnant individuals of A. g. hololissa and A. g. arnoldi may remain in captivity,[9] in recent times, these have all been reduced as subspecies of A. gigantea.[6]

Description

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A skeleton of Aldabra giant tortoise found on Cousin Island (Seychelles)

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 122 cm (48 in) in length. Males have an average weight of 250 kg (550 lb).[10]

Females are generally smaller than males, with average specimens measuring 91 cm (36 in) in carapace length and weighing 159 kg (351 lb).[11] Medium-sized specimens in captivity were reported as 70 to 110 kg (150 to 240 lb) in body mass.[12] Another study found body masses of up to 132 kg (291 lb) most commonplace.[13]

Nomenclature and systematics

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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).[14] This also affected the genus name for the species, establishing Aldabrachelys gigantea as nomen protectum.

Four subspecies are currently recognized.[6] A trinomial authority in parentheses indicates that the subspecies was originally described in a genus other than Aldabrachelys:

The subspecific name, daudinii, is in honor of French zoologist François Marie Daudin.[16]

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.[17]

Range and distribution

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An isolated population resides on Changuu island in Zanzibar

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.[18] Smaller populations of A. gigantea in the Seychelles exist on Frégate Island and in the Sainte Anne Marine National Park (e.g. Moyenne Island),[19] where they are a popular tourist attraction.

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.

Ecology

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Habitat

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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 ecology

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A giant tortoise browsing leaves

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. [20][21]

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.[citation needed]

Tortoise turf

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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:[22][23]

Behavior

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Aldabra giant tortoise foot, an efficient digging tool

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.

Lifespan

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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 brought 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).[24]

As of 2022, Jonathan, a Seychelles giant tortoise, is thought to be the oldest living giant tortoise at the age of 192 years.[25] Esmeralda, an Aldabra giant tortoise, is second at the age of 180 years, since the death of Harriet, a Galapagos giant tortoise, at 175. An Aldabra giant tortoise living on Changuu off Zanzibar is reportedly 198 years old.

Breeding

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Mating Aldabra giant tortoises

Mating takes place between February and May, and in July-September[26] 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.[27]

In captivity, oviposition dates vary. Tulsa Zoo[28] 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.[26]


Conservation

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Engraving of an Elephant-Tortoise from “The Royal Natural History” (1896)

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.[29][30]

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.[31]

References

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  1. ^ Tortoise & Freshwater Turtle Specialist Group (1996). "Geochelone gigantea". IUCN Red List of Threatened Species. 1996: e.T9010A12949962. doi:10.2305/IUCN.UK.1996.RLTS.T9010A12949962.en.
  2. ^ "Appendices | CITES". cites.org. Archived from the original on 2017-12-05. Retrieved 2022-01-14.
  3. ^ a b Schweigger, Augustus F. (1812). "Prodromus monographiae Cheloniorum". Königsberger Archiv für Naturwissenschaft und Mathematik 1:271–368, 406–462.
  4. ^ a b Duméril, André Marie Constant, and Bibron, Gabriel. (1835). Erpétologie Générale ou Histoire Naturelle Complète des Reptiles. Tome Second. Paris: Roret.
  5. ^ a b Günther, Albert C.L.G. 1877. The Gigantic Land-Tortoises (Living and Extinct) in the Collection of the British Museum. London: Taylor and Francis, 96 pp.
  6. ^ a b c d Turtle Taxonomy Working Group [van Dijk, P.P., Iverson, J.B., Rhodin, A.G.J., Shaffer, H.B., and Bour, R.]. 2014. Turtles of the world, 7th edition: annotated checklist of taxonomy, synonymy, distribution with maps, and conservation status. In: Rhodin, A.G.J., Pritchard, P.C.H., van Dijk, P.P., Saumure, R.A., Buhlmann, K.A., Iverson, J.B., and Mittermeier, R.A. (Eds.). Conservation Biology of Freshwater Turtles and Tortoises: A Compilation Project of the IUCN/SSC Tortoise and Freshwater Turtle Specialist Group. Chelonian Research Monographs 5(7):000.329–479 Archived 2014-07-15 at the Wayback Machine, doi:10.3854/ crm.5.000.checklist.v7.2014.
  7. ^ Pritchard, Peter C.H. (1979) Encyclopedia of Turtles. Neptune, New Jersey: T.F.H. Publications, Inc. Ltd.
  8. ^ Palkovacs, Eric P.; Gerlach, Justin; Caccone, Adalgisa (2002). "The evolutionary origin of Indian Ocean tortoises (Dipsochelys)" (PDF). Molecular Phylogenetics and Evolution. 24 (2): 216–227. doi:10.1016/s1055-7903(02)00211-7. hdl:10161/6529. PMID 12144758. Archived (PDF) from the original on 2022-11-02. Retrieved 2022-11-02.
  9. ^ a b Palkovacs, Eric P.; Marschner, Monique; Ciofi, Claudio; Gerlach, Justin; Caccone, Adalgisa (2003). "Are the native giant tortoises from the Seychelles really extinct? A genetic perspective based on mtDNA and microsatellite data" (PDF). Molecular Ecology. 12 (6): 1403–1413. Bibcode:2003MolEc..12.1403P. doi:10.1046/j.1365-294X.2003.01834.x. hdl:10161/6530. PMID 12755870. S2CID 10014935. Archived (PDF) from the original on 2022-12-22. Retrieved 2022-11-02.
  10. ^ "ANIMAL BYTES – Aldabra Tortoise". Seaworld.org. Archived from the original on 2018-02-03. Retrieved 2015-11-02.
  11. ^ "ANIMAL BYTES – Aldabra Tortoise". Seaworld.org. Archived from the original on 2018-02-03. Retrieved 2015-11-02.
  12. ^ Hansen, Dennis M.; Kaiser, Christopher N.; Müller, Christine B. (2008). "Seed Dispersal and Establishment of Endangered Plants on Oceanic Islands: The Janzen-Connell Model, and the Use of Ecological Analogues". PLOS ONE. 3 (5): e2111. Bibcode:2008PLoSO...3.2111H. doi:10.1371/journal.pone.0002111. PMC 2358974. PMID 18461169.
  13. ^ Hughes, G. M.; Gaymer, R.; Moore, Margaret; Woakes, A. J. (1971). "Respiratory Exchange and Body Size in the Aldabra Giant Tortoise". Journal of Experimental Biology. 55 (3): 651–665. doi:10.1242/jeb.55.3.651. PMID 5160860.
  14. ^ "Opinion 2316 (Case 3463). Testudo gigantea Schweigger, 1812 (currently Geochelone (Aldabrachelys) gigantea; Reptilia, Testudines): usage of the specific name conserved by maintenance of a designated neotype, and suppression of Testudo dussumieri Gray, 1831 (currently Dipsochelys dussumieri)". The Bulletin of Zoological Nomenclature. 70: 61–65. 2013. doi:10.21805/bzn.v70i1.a13. S2CID 81969212.
  15. ^ 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).
  16. ^ Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). "Dipsochelys daudinii", p. 66 in The Eponym Dictionary of Reptiles. Baltimore: Johns Hopkins University Press. ISBN 978-1-4214-0135-5.
  17. ^ Kehlmaier, Christian; Graciá, Eva; Ali, Jason R.; Campbell, Patrick D.; Chapman, Sandra D.; Deepak, V.; Ihlow, Flora; Jalil, Nour-Eddine; Pierre-Huyet, Laure; Samonds, Karen E.; Vences, Miguel; Fritz, Uwe (2023-01-13). "Ancient DNA elucidates the lost world of western Indian Ocean giant tortoises and reveals a new extinct species from Madagascar". Science Advances. 9 (2): eabq2574. Bibcode:2023SciA....9.2574K. doi:10.1126/sciadv.abq2574. ISSN 2375-2548. PMC 9833658. PMID 36630487.
  18. ^ Payne, Roger (2004-04-05). "Losing Aldabra". Voyage of the Odyssey. PBS. Archived from the original on 2021-05-16. Retrieved 2008-10-21.
  19. ^ "Marine Parks: St. Anne". Seychelles National Parks Authority. Archived from the original on 2016-04-16.
  20. ^ Lambert, Jonathan (2021-08-23). "A giant tortoise was caught stalking, killing and eating a baby bird". Science News. Archived from the original on 2021-08-23. Retrieved 2021-08-23.
  21. ^ Zora, Anna; Gerlach, Justin (23 August 2021). "Giant tortoises hunt and consume birds". Current Biology. 31 (16): R989–R990. Bibcode:2021CBio...31.R989Z. doi:10.1016/j.cub.2021.06.088. PMID 34428417.
  22. ^ Grubb, P. (1971). "The growth, ecology and population structure of giant tortoises on Aldabra". Philosophical Transactions of the Royal Society of London. B, Biological Sciences. 260 (836): 327–372. Bibcode:1971RSPTB.260..327G. doi:10.1098/rstb.1971.0018.
  23. ^ Hnatiuk, R.J. and L.F.H. Merton (1979) "Vegetation of Aldabra, a Reassessment" Archived 2012-03-27 at the Wayback Machine. Atoll Research Bulletin No. 239, The Smithsonian Institution
  24. ^ "'Clive of India's' tortoise dies". BBC News. BBC Online. 2006-03-23. Archived from the original on 2019-05-30. Retrieved 2014-01-23.
  25. ^ ninemsn Pickle. - Australia (2016-03-25). "Tortoise has his first bath in 184 years". Pickle. ninemsn. Archived from the original on 2016-03-27. Retrieved 2016-03-25.
  26. ^ a b Bourn, David (1977). "Reproductive study of Giant tortoises on Aldabra". Journal of Zoology. 182: 27–38. doi:10.1111/j.1469-7998.1977.tb04138.x.
  27. ^ Stearns, Brett C. (2007). "Captive husbandry and propagation of the Aldabra giant tortoise Geochelone gigantea: At the institute for Herpetological Research". International Zoo Yearbook. 27: 98–103. doi:10.1111/j.1748-1090.1988.tb03201.x.
  28. ^ Collins, Dave. (1984) "Captive Breeding and Management of the Aldabra Tortoise". Presented to 8th International Herpetological Symposium, Jacksonville Zoo, Jacksonville, Fl.
  29. ^ Spratt, David M.J. (1989). "Operation Curiesue: A Conservation Programme for the Aldabra Giant Tortoise in the Republic of Seychelles". Int. Zoo Yearb. 28: 66–69. doi:10.1111/j.1748-1090.1989.tb03256.x.
  30. ^ Swingland, Ian R. Aldabran Giant Tortoise. The Conservation Biology of Tortoises, Occasional Papers of the IUCN Species Survival Commission (SSC), No. 5, 1989.
  31. ^ Çilingir, F Gözde; A'Bear, Luke; Hansen, Dennis; Davis, Leyla R; Bunbury, Nancy; Ozgul, Arpat; Croll, Daniel; Grossen, Christine (2022). "Chromosome-level genome assembly for the Aldabra giant tortoise enables insights into the genetic health of a threatened population". GigaScience. 11: giac090. doi:10.1093/gigascience/giac090. PMC 9553416. PMID 36251273.
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