View source for Galápagos tortoise

{{Taxobox
| name = Galápagos tortoise
| status = VU
| status_system = iucn2.3
| status_ref = &nbsp;<ref name='iucnredlist.org'>{{IUCN2009.2|assessors=Tortoise & Freshwater Turtle Specialist Group |year=1996 |id=9011 |title= Chelonoidis nigra |downloaded=2010-09-04}}</ref>
| image = Galapagos giant tortoise Geochelone elephantopus.jpg
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Reptile|Sauropsid]]a
| ordo = [[Testudines]]
| familia = [[Testudinidae]]
| genus = ''[[Geochelone]]''
| species = '''''G. nigra'''''
| binomial = ''Geochelone nigra'' 
| binomial_authority = ([[Jean René Constant Quoy|Quoy]] & [[Joseph Paul Gaimard|Gaimard]], 1824b<ref name='Quoy b'>Quoy and Gaimard 1824b. ''Sous-genre Tortue de Terre – Testudo. Brongn. Tortue Noire – Testudo nigra. N. In: M. L. de Freycinet (ed.), Voyage autour du Monde executé sur l'Uranie et la Physicienne pendent les années 1817–1820'', pp. 174–175.</ref>)
| synonyms =
*''Geochelone elephantopus'' ([[Richard Harlan|Harlan]], 1827<ref name='Harlan'>Harlan 1827. ''Description of a land tortoise, from the Galápagos Islands, commonly known as the "Elephant Tortoise."'' Journal of the Academy of Natural Sciences of Philadelphia 5: 284–292.</ref>)
| subdivision_ranks = [[Subspecies]]
| subdivision = 12, see [[Subspecies of Geochelone nigra]]
}}

The '''Galápagos tortoise''' or '''Galápagos giant tortoise''' (''Geochelone nigra'') is the largest living species of [[tortoise]],  reaching weights of over {{kg to lb|400}} and lengths of {{m to ft|1.8}}. It is among the [[List of long-living organisms|longest lived]] of all vertebrates. [[Life expectancy]] in the wild is over 100 years, and the oldest known individual is estimated to have reached at least 170 years of age.  The tortoise is [[Endemism|endemic]] to seven volcanic [[island]]s of the [[Galápagos Islands|Galápagos archipelago]], lying west of [[Ecuador]]. Spanish explorers who discovered the islands in the 16th century were purportedly struck by  the resemblance of the local tortoises' shells to the shape of a saddle known as a 'galápago', and this is popularly said to be the origin of the name of the archipelago<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p14</ref><ref name='NatGeo'>National Geographic. [http://animals.nationalgeographic.com/animals/reptiles/galapagos-tortoise/ Galápagos Tortoise ''Geochelone elephantophus'']. Retrieved 2010-09-04</ref><ref name='amnh.org'>American Museum of Natural History [http://www.amnh.org/exhibitions/darwin/cam/about.php – Giant tortoises of the Galápagos]. Retrieved 2010-09-04</ref>. The size and shell shape of the tortoise differs between populations on different islands according to habitat. This variation in form played a role in the [[Inception of Darwin's theory|inception of Charles Darwin's theory]] of [[evolution]].

Tortoise numbers have fallen from over 250,000 in the 16th century to 20,000 now due to human disturbance . The decline was brought about by the hunting for tortoise meat and oil, [[Habitat destruction|habitat clearance]] for agriculture, and the introduction of [[vermin]] and destructive [[Grazing|grazers]] to the once-isolated islands. Ten [[subspecies]] of the original twelve exist in the wild, and one other subspecies (''G. n. abingdoni'') is represented by a single remaining male nicknamed [[Lonesome George]], said to be the world's 'rarest living creature'. However, conservation measures including the establishment of the [[Galapagos National Park|Galápagos National Park]] and the [[Charles Darwin Foundation]] have had successes, and thousands of [[Captive breeding|captive-bred]] juveniles have been released onto their home islands. The Galápagos giant tortoise has become the [[flagship species]] for conservation efforts throughout the Galápagos.

==Taxonomy==
The current species designation of ''nigra'' ("black"- [[Jean René Constant Quoy|Quoy]] & [[Joseph Paul Gaimard|Gaimard]], 1824b<ref name='Quoy b'/>) was resurrected in 1984<ref>Pritchard 1984. ''Further thoughts on Lonesome George.'' Noticias de Galápagos 39: 20–23</ref>, after it was discovered to be the senior synonym for the commonly used ''elephantopus'' ("elephant-footed"- [[Richard Harlan|Harlan]], 1827<ref name='Harlan'/>). The even more senior synonym of ''californiana'' ("californian" [[Jean René Constant Quoy|Quoy]] & [[Joseph Paul Gaimard|Gaimard]], 1824a<ref>Quoy and Gaimard 1824a. ''Description d'une nouvelle espèce de tortue et de trois espèces nouvelles de scinques.'' Bull. Sci. Nat. Geol., Paris: Zoologie, pp 90–91</ref>) is considered a ''[[nomen oblitum]]'' ("forgotten name")<ref name='Pritchard 1996'/>.

Before a sufficient number of specimens became available for scientific scrutiny and comparison, the [[giant tortoise]]s of the Indian Ocean and those from the Galápagos were considered to be the same species ''Testudo gigantea'' ("gigantic tortoise"-[[August Friedrich Schweigger|Schweigger]], (1812<ref>Schweigger 1812. ''Prodromi monographiae Chelonorum sectio prima'', Köningsberg (Germany)</ref>), then ''Testudo indica'' ("indian tortoise") by [[John Edward Gray|Gray]] (1831<ref>Gray 1831. ''Synopsis Reptilium, Part One: Cataphracta. Treuttel'', Wurtz (London)</ref>), who would later become Keeper of Zoology at the [[British Museum]]. It was postulated that animals from the Indian Ocean had been transported by sailors to the Galápagos<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p27</ref>. Later, the Galápagos tortoise was considered by [[André Marie Constant Duméril|Duméril]] and [[Gabriel Bibron|Bibron]] (1834<ref>Duméril and Bibron 1834. ''Erpétologie Générale ou Histoire Naturelle Complète des Reptiles'', Librarie Encyclopédique de Roret (France)</ref>) as a different species, ''Testudo nigrita'' ("black tortoise"), from the Indian Ocean's ''elephantina'' (Aldabra), ''daudinii'' (Seychelles) and ''peltastes'' (Mascarene). 

The first truly systematic comparison of giant tortoises which recognised their diversity and endemism was by [[Albert Günther]], Gray's successor as Keeper of Zoology, in 1875<ref>[http://rstl.royalsocietypublishing.org/content/165/251.full.pdf Gunther 1875. ''Description of the Living and Extinct Races of Gigantic Land-Tortoises. Parts I. and II. Introduction, and the Tortoises of the Galapagos Islands''], Philosophical Transactions of the Royal Society of London, Biological Sciences, 165, pp.251–84</ref>. He identified at least five distinct populations from the Galápagos, and three in the Indian Ocean. This list was expanded by Gunther in 1877 to six from the Galápagos, four from the [[Seychelles]], and four from the [[Mascarene Islands|Mascarenes]]. He theorised that the tortoises of the Galápagos and the Indian Ocean were derived from a single ancestral population which spread to the isolated islands by sunken land bridges<ref name='Gunther 1877'>Günther 1877. ''The gigantic land tortoises (living and extinct) in the collection of the British Museum'', British Museum (Nat. Hist.), London</ref>. This theory was later definitively disproven by the understanding that the Galápagos, Aldabra and Mascarene islands are all of volcanic origin and have risen from the sea in recent geological history; thus, they could not have been linked by land bridges. It is now thought that the Galápagos tortoises descended from a common ancestor from the South American mainland<ref name='Caccone 1999'/>, whilst those of the Indian Ocean derived from two colonisations from nearby Madagascar – one to the Seychelles, and another to the Mascarenes<ref>[http://rspb.royalsocietypublishing.org/content/268/1485/2515.full.pdf+html Austin and Arnold 2001. ''Ancient mitochondrial DNA and morphology elucidate an extinct island radiation of Indian Ocean giant tortoises (Cylindraspis)''] Philosophical Transactions of the Royal Society of London, Series B, 268, pp.2515–23</ref><ref>[doi/10.1046/j.1365-294X.2003.01842.x/pdf Austin and Arnold 2003. ''Was there a second adaptive radiation of giant tortoises in the Indian Ocean? Using mitochondrial DNA to investigate speciation and biogeography of Aldabrachelys''] Molecular Ecology, 12, pp. 1415–24</ref>.

[[File:Rollo Beck.jpg|thumb|[[Rollo Beck]], an American explorer who visited the Galápagos on collecting expeditions organised by [[Walter Rothschild, 2nd Baron Rothschild|Rothschild]] and the [[California Academy of Sciences|Academy of Sciences]]. He is pictured mounting a Galápagos tortoise.]]

Five more populations of Galápagos tortoise were first recognised by [[Georg Baur (paleontologist)|Baur]]<ref name='Baur'>ref>[http://links.jstor.org/stable/2451262 Baur 1889. ''The gigantic land tortoises of the Galapagos Islands.''] The American Naturalist 23:1039–1057</ref> and [[Walter Rothschild, 2nd Baron Rothschild|Rothschild]]<ref>Rothschild 1901. ''On a new land-tortoise from the Galapagos Islands'' Novitates Zoologicae</ref><ref>Rothschild 1902. ''Description of a new species of gigantic land tortoise from the Galápagos Islands.'' Novitates Zoologicae 9: 619.</ref>
<ref>Rothschild 1903. ''Description of a new species of gigantic land tortoise from Indefatigable Island.'' Novitates Zool. 10: 119.</ref>. Specimens collected by an [[California Academy of Sciences|Academy of Sciences]] expedition in 1906 were studied by [[John Van Denburgh|Van Denburgh]], who identified four additional new populations<ref>Van Denburgh 1907. ''Preliminary descriptions of four new races of gigantic land tortoises from the Galapagos Islands'' Proceedings of the California Academy of Sciences, 4th ser., v. 1.</ref>, and proposed the existence of 15 species<ref name='Van Denburgh 1914'/>. This definitive list still guides the taxonomy of the Galápagos tortoise in the modern day, though with some modifications.

===Elevation of genus ''Chelonoidis''===
There are taxonomic justifications for elevating ''Chelonoidis'' (Fitzinger 1835) from subgenus to genus status based on molecular phylogenetic analyses of testudinids<ref>[http://dx.doi.org/10.1016/j.ympev.2006.03.003 Le 2006. ''A molecular phylogeny of tortoises (Testudines:Testudinidae) based on mitochondrial and nuclear genes.''] Molecular Phylogenetics and Evolution. 40(2):517–531</ref>. If the subgenus is elevated, South American tortoises including the Galápagos tortoise would belong in the genus ''Chelonoidis'' rather than ''Geochelone'', a convention which is being adopted by some authorities<ref name='iucnredlist.org'/><ref>[http://www.vertebrate-zoology.de/vz57-2/57-2_Fritz_149-368.pdf Fritz and Havaš 2007. ''Checklist of Chelonians of the World.''] Vertebrate Zoology. 57(2):149–368</ref><ref name='CITES Appendices I, II and III 2010'>[http://www.cites.org/eng/app/appendices.shtml CITES Appendices I, II and III 2010]</ref>.

==Phylogeny==
===External phylogeny===
All subspecies of Galápagos tortoise evolved from a common ancestor that arrived from mainland South America by overwater [[biological dispersal|dispersal]] of a pregnant female or a breeding pair<ref name='Caccone 1999'/>. Survival on the 1000&nbsp;km oceanic journey is accounted for by the fact that the tortoises are buoyant and can breathe by extending their necks above the water, able to survive months without food or fresh water. As they are poor swimmers, the journey was probably a passive one facilitated by the [[Humboldt Current]], which diverts westwards towards the Galápagos Islands from the mainland<ref name='ReferenceA'/>.

The closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Chaco tortoise (''[[Argentine Tortoise|Geochelone chilensis]]''), a much smaller species from South America. The divergence between ''G.&nbsp;chilensis'' and ''G.&nbsp;nigra'' probably occurred 6–12 million years ago<ref name='Caccone 1999'/>, an evolutionary event preceding the volcanic formation of the oldest modern Galápagos islands 5 million years ago<ref>White 1993. ''Petrology and geochemistry of the Galápagos Islands: portrait of a pathological mantle plume.'' Journal of Geophysical Research. 98(B11):19,533–19,564.</ref>. [[Mitochondrial DNA]] analysis indicates that the oldest existing islands (Española and San Cristóbal) were colonised first, and that these populations seeded the younger islands formed by subsequent volcanism via dispersal events on local currents in a 'stepping stone' fashion<ref>Nicholls 2006. ''Lonesome George: The Life and Loves of a Conservation Icon''. Palgrave Macmillan. ISBN 1403945764. p68</ref><ref>[http://www.pnas.org/content/101/17/6514.full Beheregaray 2004. ''Giant tortoises are not so slow: Rapid diversification and biogeographic consensus in the Galápagos.''] Proceedings of the National Academy of Sciences. 101(17):6514–6519</ref>. Restricted gene flow between isolated populations then resulted in the radiation into the divergent forms observed in the modern subspecies. The species phylogeny thus echoes the paleogeography of the islands.

===Internal phylogeny===
[[File:Galapagos tortoise distribution Line diagram.png|thumb|right|500px|[[Galápagos Islands|Galápagos archipelago]] annotated with ranges of currently recognised [[Subspecies of Geochelone nigra|subspecies of giant tortoise]]. Islands with extant subspecies are shaded.]]{{Main|Subspecies of Geochelone nigra}}
There were probably 12 [[subspecies]] of ''Geochelone nigra'' in the Galápagos Islands, although some recognise up to 15 subspecies<ref name='ReferenceA'>[http://www.jstor.org/stable/3094648 Caccone 2002. ''Phylogeography and history of the Giant Galapagos Tortoises.''] Evolution. 56(10):2052–2066</ref>. Only ten subspecies now exist in the wild, one on each of the islands of [[Santiago Island (Galápagos)|Santiago]], [[Santa Cruz Island (Galápagos)|Santa Cruz]], [[San Cristóbal Island|San Cristóbal]], [[Pinzón Island|Pinzón]], and [[Española Island|Española]], and one on each of the five main volcanoes of the largest island [[Isabela Island (Ecuador)|Isabela]] ([[Volcán Wolf|Wolf]], Darwin, [[Alcedo Volcano|Alcedo]], [[Sierra Negra (Galápagos)|Sierra Negra]], and [[Cerro Azul (Ecuador volcano)|Cerro Azul]]). The eleventh extant subspecies, ''[[Geochelone nigra abingdoni|abingdoni]]'' from Pinta Island, is considered [[extinct in the wild]] and is represented by a single living specimen, 'Lonesome George'.

Prior to widespread knowledge of the differences between the populations from different islands and volcanoes (sometimes called 'races'), captive collections in zoos were indiscriminately mixed. Fertile offspring resulted from pairings of animals from different races, confirming that they are subspecies and not distinct species. However captive crosses between tortoises from different races have lower fertility and higher mortality than those between tortoises of the same race<ref>[http://dx.doi.org/10.1016/0006-3207(74)90068-8 MacFarland 1974b. ''The Galápagos giant tortoises (Geochelone elephantopus). Part II: Conservation methods.''] Biological Conservation. 6: 198–212.</ref><ref name='ReferenceB'>[http://www.ciens.ula.ve/~cires/recol-v3n1a03.pdf Márquez 1988. ''Comportamiento de Apareamiento al Azar en tortugas gigantes. Juveniles en cautiverio el las Islas Galápagos.''] Rept. Charles Darwin Research Station Library. 1988:2–15</ref> and captives in mixed herds normally direct subadult courtship only toward members of the same race<ref name='ReferenceB'/>. The taxonomic status of the various races is not fully resolved<ref>Zug 1997. ''Galapagos tortoise nomenclature: still unresolved.'' Chelonian Conservation and Biology. 2:618–619</ref>, and the valid scientific names of each the individual populations<ref name='Pritchard 1996'/><ref name='Swingland 1989'/><ref name='Fritts 1984'/><ref name='R. Perry pp. 145'/>, whilst some researchers consider the subspecies to be full species<ref>Ernst and Barbour 1989. ''Turtles of the World.'' Washington, D.C.: Smithsonian Institution Press. ISBN 9780874744149.</ref><ref name='Poulakakis 2008'>[http://www.pnas.org/content/105/40/15464.full Poulakakis 2008. ''Historical DNA analysis reveals living descendants of an extinct species of Galapagos tortoise.''] Proceedings of the National Academy of Sciences. 105:15464-15469</ref>.

Modern [[Molecular biology|molecular methods]] have revealed new information on the relationships between the subspecies. The Pinta Island subspecies (''abingdoni'') is most closely related to the subspecies on the islands of San Cristóbal (''chathamensis'') and Española (''hoodensis'') which lie over 300&nbsp;km away<ref name='Caccone 1999'>[http://www.pnas.org/content/96/23/13223.full Caccone 1999. ''Origin and evolutionary relationships of giant Galapagos tortoises.''] Proceedings of the National Academy of Science. 96(23):13223-13228</ref>, rather than neighbouring Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta<ref>[http://www.agu.org/pubs/crossref/1973/JC078i033p07845.shtml Pak and Zaneveld 1973. ''The Cromwell Current on the east side of the Galapagos Islands.''] Journal of Geophysical Research. 78(33):7845–7859</ref>. The discovery informed further attempts for the preservation of the ''abingdoni'' lineage and the search for an appropriate mate for Lonesome George, who had been penned with females from Isabela<ref>Nicholls 2006. ''Lonesome George: The Life and Loves of a Conservation Icon''. Palgrave Macmillan. ISBN 1403945764. p161</ref>. This hope was bolstered by the discovery of an ''abingdoni'' hybrid male in the Volcán Wolf population on northern Isabela, raising the possibility that there are more living undiscovered Pinta descendants<ref name='Russello 2007'>[http://dx.doi.org/10.1016/j.cub.2007.03.002 Russello 2007. ''Lonesome George is not alone among Galapagos tortoises.''] Current Biology. 17:R317-R318</ref>.

Mitochondrial DNA studies of tortoises on Santa Cruz show up to three genetically distinct lineages found in non-overlapping population distributions around Cerro Monturra, Cerro Fatal and La Caseta<ref>[http://rsbl.royalsocietypublishing.org/content/1/3/287.full Russello 2005. ''A cryptic taxon of Galapagos tortoise in conservation peril.''] Biological Letters. 1:287–290</ref>. Although currently united in a single subspecies (''porteri''), the lineages are all more closely related to tortoises on other islands than to each other<ref name='Chiari 2009'>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2707613/ Chiari 2009. ''Morphometrics parallel genetics in a newly discovered and endangered taxon of Galapagos Tortoise.''] PLoS One. 4(7):e6272. {{doi|10.1371/journal.pone.0006272}}</ref>; Cerro Monturra tortoises are most closely related to ''duncanensis'' from Pinzón, Cerro Fatal to ''chathamensis'' from San Cristóbal, and La Caseta to the four southern races of Isabela<ref>[http://onlinelibrary.wiley.com/doi/10.1046/j.1365-294X.2002.01617.x/pdf Ciofi 2002. ''Microsatellite analysis of genetic divergence among populations of giant Galápagos tortoises.''] Molecular Ecology. 11(11):2265–2283</ref>.

The four southern subspecies on Isabela, though separated from each other by barren stretches of lava between volcanoes, are likely to be a single genetic unit derived from colonists from Santa Cruz. The genetically distinct Volcán Wolf subspecies in northern Isabela (''becki'') is probably the result of a separate colonisation event from Santiago<ref name='Caccone 1999'/>. Tortoises from Sierra Negra in southern Isabela (''guentheri'') are putatively the ancestral source of dispersal to the volcanoes Darwin (''microphyes''), Alcedo (''vandenburghi'') and Cerro Azul (''vicina'')<ref>[http://www.genetics.org/cgi/content/full/172/3/1727 Ciofi 2006. ''Phylogeographic History and Gene Flow Among Giant Galápagos Tortoises on Southern Isabela Island.''] Genetics. 172:1727–1744</ref>. On this basis it has been suggested that the southern populations on Isabela may be considered as a single subspecies ''vicina'', with morphological differences attributable to geographical variation<ref name='Pritchard 1996'>Pritchard 1996. ''The Galapagos tortoises: Nomenclatural and survival status.'' Chelonian Research Monographs. (1):1–85</ref>.

Phylogenetic analysis may also help to 'resurrect' the subspecies of Floreana (''nigra''), which became extinct from human exploitation by 1850<ref>Broom 1929. ''On the extinct Galápagos tortoise that inhabited Charles Island.'' Zoologica 9:313–320.</ref>, just years after [[Charles Darwin]]'s landmark visit in 1835<ref name='Pritchard 1996'/>. The subspecies was only known from [[subfossil]] remains<ref>[http://si-pddr.si.edu/dspace/bitstream/10088/5333/2/SCtZ-0413-Lo_res.pdf Steadman 1986. ''Holocene vertebrate fossils from Isla Floreana, Galápagos.''] Smith Contrib Zool 413:1–103</ref>. Some tortoises from Isabela were found to be a partial match for the genetic profile of Floreana specimens from museum collections, possibly indicating the presence of hybrids from a population transposed by humans from Floreana to Isabela<ref name='Poulakakis 2008'/> either by deliberate moving between the islands<ref name='Pritchard 1996'/> or from individuals thrown overboard ships to lighten loads<ref name='Baur'/>. Nine Floreana descendants have also been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz<ref name='Russello 2010'>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2800188/ Russello 2010. ''DNA from the past informs ex situ conservation for the future: an 'extinct' species of Galapagos tortoise identified in captivity.''] PLoS One. 5(1):e8683</ref>. This permits the possibility of re-establishing a [[Breeding back|reconstructed subspecies]] from selective breeding of the hybrid animals.

====Subspecies of doubtful existence====
Subspecies were described from three other islands, but their existence is based on scant evidence. The purported Rabida island subspecies (''wallacei'') was described from a single specimen removed by the [[United States National Academy of Sciences|Academy of Sciences]] in 1906<ref name='Van Denburgh 1914'/>, which has since been lost. This individual was probably an artificial introduction from another island that was originally corralled on Rabida next to a good anchorage, as no contemporary whaling or sealing logs mention removing tortoises from this island<ref name='Pritchard 1996'/><ref name='MacFarland 1974'>[http://dx.doi.org/10.1016/0006-3207(74)90024-X MacFarland 1974a. ''The galapagos giant tortoises (Geochelone elephantopus). I. Status of the surviving populations.''] Biological Conservation. 6(2):118–133</ref>. The ''phantastica'' subspecies from Fernandina is also known from a single specimen from the voyage of 1906, an old male<ref name='Van Denburgh 1914'/>. No other tortoises or remains have been found on the island, suggesting the specimen was an artificial introduction from elsewhere<ref name='Pritchard 1996'/><ref name='Swingland 1989'/><ref name='MacFarland 1974'/>. Fernandina has neither human settlements nor feral mammals, so if this subspecies ever did exist its extinction must have been by natural means, such as volcanic activity<ref name='Swingland 1989'/>. The Santa Fe subspecies has no binomial name, having been described from the limited evidence of bone fragments (but no shells) of 14 individuals, old eggs and old dung found on the island in 1906<ref name='Van Denburgh 1914'/>. These are also considered artificial introductions<ref name='Swingland 1989'/>.

==Description==
{{multiple image
| align     = left
| direction = vertical
| header    = Galápagos tortoise shell varieties
| width     = 200
| image1    = Lonseome george.jpg
| alt1      = ''G. n. abingdoni''
| caption1  = Saddleback (''abingdoni'')
| image2    = Chathamensis.jpg
| alt2      = ''G. n. chathamensis''
| caption2  = Intermediate (''chathamensis'')
| image3    = GNigrita.jpg
| alt3      = ''G. n. cporteri''
| caption3  = Domed (''porteri'')
}}

The discoverer of the Galápagos islands, Fray Tomás de Berlanga, Bishop of Panama, wrote of 'such big tortoises that each could carry a man on top of himself<ref>Berlanga 1535. ''Letter to His Majesty ... describing his voyage from Panamá to Puerto Viejo.'' In ''Coleccion de Documentos Ineditos relativos al Descubrimiento, Conquista y Organizacion de las Antiguas Posesiones Españolas de América y Oceania'', Tomo XLI, Cuaderno II. Madrid. Imprenta de Manuel G. Hernandez (1884, pp. 538–44)</ref>. Later, naturalist [[Charles Darwin]] remarked 'These animals grow to an immense size ... several so large that it required six or eight men to lift them from the ground'<ref name='Darwin 1839 pp462-466'>Darwin 1839. ''Narrative of the Surveying Voyages of His Majesty's Ships Adventure and Beagle'' pp.[http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F10.3&pageseq=481 462–466].</ref>. The largest recorded individuals have reached weights of over 400&nbsp;kg<ref>Ebersbach 2001. ''[http://elib.tiho-hannover.de/dissertations/ebersbachk_2001.pdf Zur Biologie und Haltung der Aldabra-Riesenschildkröte (Geochelone gigantea) und der Galapagos-Riesenschildkröte (Geochelone elephantopus) in menschlicher Obhut unter besonderer Berücksichtigung der Fortpflanzung]''. PhD Dissertation, Tierärztliche Hochschule Hannover</ref> and lengths of 1.87&nbsp;m<ref name='I. Bowman, M. Berson pp. 107'>Fritts 1983. ''Morphometrics of Galapagos tortoises: Evolutionary implications.'' In: R. I. Bowman, M. Berson, and A. E. Leviton eds. Patterns of Evolution in Galapagos Organisms, pp. 107–122. American Association for the Advancement of Science. ISBN 0934394059.</ref>. The tortoises' gigantism was probably a preadapted condition (allowing significant water and fat storage) for successful colonisation of these remote oceanic islands whose annual rainfall is extremely variable and prone to drought, rather than an [[Insular gigantism|evolutionary adaptation to the island environment]]<ref name='Pritchard 1996'/>. Indeed, fossil giant tortoises from mainland South America have been described which are consistent with current phylogenetic groupings<ref>[http://www.jstor.org/stable/1441604 Auffenberg 1971. ''A new fossil tortoise, with remarks on the origin of South American testudinines.''] Copeia. 1971(1)106</ref>.

The tortoises have a large bony [[Animal shell|shell]] ([[carapace]]) of a dull brown colour. The plates of the carapace are fused with the ribs in a rigid protective structure that is integral to the [[skeleton]]. [[Lichen]] can grow on the shells of the slow-moving animals<ref name='MacFarland 1972'>MacFarland 1972. ''Giant tortoises, Goliaths of the Galapagos''. National Geographic Magazine, May:632–649</ref>. Tortoises keep a characteristic [[scute]] pattern on their shell throughout life, though the annual growth bands are not useful for aging as the outer layers are worn off with time. A tortoise can withdraw its head, neck and forelimbs into its shell for protection. The legs are large and stumpy, with dry scaly skin and hard [[Scale (zoology)|scales]]. The front legs are five-clawed, and the back legs are four-clawed<ref name='Van Denburgh 1914'>Van Denburgh 1914. ''The gigantic land tortoises of the Galapagos archipelago.'' Proceedings of the California Academy of Sciences. Series 4, 2(1):203–374. ISBN 1120885140.</ref>.

Galapágos tortoises are the only lineage of giant tortoise exhibiting different types of shell shape<ref name='Chiari 2009'/>. They exhibit a spectrum of carapace morphology from 'saddleback' (denoting upward arching of the front edge of the shell resembling a saddle) to 'domed' (denoting a rounded convex surface resembling a dome). When saddleback tortoises withdraw their head and forelimbs into their shells a large unprotected gap remains over their neck, evidence of the lack of predation during the evolution of this structure. There is no saddleback/domed dualism, as tortoises can be of intermediate type with characteristics of both. Larger islands with humid ([[Mesic habitat|mesic]]) [[Highland (geography)|highlands]] over 800&nbsp;m in elevation, such as [[Santa Cruz Island (Galápagos)|Santa Cruz]] have abundant vegetation near the ground<ref name='R. Perry pp. 145'>[http://www.biologia.puce.edu.ec/imagesFTP/10618.The_Giant_Tortoises_a_natural_history_disturbed_by_man_low.pdf DeVries 1984. ''The giant tortoises: a natural history disturbed by man.''] In: R. Perry (Ed.) Key Environments. Galapagos, pp. 145–156. Oxford: Pergamon Press. ISBN 0080279961.</ref>. Native tortoises in these environments tend to have domed shells and are larger, with shorter necks and limbs. Saddleback tortoises originate from small islands less than 500&nbsp;m in elevation with [[Deserts and xeric shrublands|xeric]] habitats (e.g. Española and Pinzón) that are more limited in food and other resources<ref name='ReferenceA'/>.

In combination with proportionally longer necks and limbs<ref name='Van Denburgh 1914'/>, the unusual saddleback carapace structure is thought to be an adaptation to increase vertical reach, which enables the tortoise to browse tall vegetation such as the arboreal ''[[Opuntia]]'' (prickly pear) cactus which grows in arid environments<ref>Dawson 1966. ''Cacti in the Galapagos Islands, with special reference to their relations with tortoises.'' pp 209–14. in Bowman, R. I., editor. (ed). The Galapagos University of California Press. Berkeley.</ref>. Saddlebacks are more territorial<ref name='I. Bowman, M. Berson pp. 107'/><ref>[http://www.jstor.org/stable/1445152 de Neira and Roe 1984. ''Emergence success of tortoise nests and the effect of feral burros on nest success on Volcán Alcedo, Galapagos.''] Copeia. 1984(3):702–707</ref> and smaller than domed varieties, possibly adaptations to their limited resources. Alternatively, larger tortoises may be better suited to high elevations, because they can resist the cooler temperatures when there is cloud cover or fog<ref name='Fritts 1984'/>.

Rather than principally a feeding adaptation, the distinctive saddle shape and longer extremities might have been an adaptive change for infraspecific competition over limited food, water and shade on the dry islands, which is settled by dominance displays on the basis of vertical neck height rather than body size<ref name='Fritts 1984'/> ([[#Reproduction|see below]]). This correlates with the observation that saddleback males are more aggressive than domed males<ref name='Schafer and Krekorian 1983'>[http://www.jstor.org/stable/3892541 Schafer and Krekorian 1983. ''Agonistic behavior of the Galapagos tortoise, Geochelone elephantopus, with emphasis on its relationship to saddle-backed shell shape.''] Herpetologica. 39(4):448–456</ref>. The shell distortion and elongation of the limbs and neck in saddlebacks is probably the evolutionary compromise between the need for a small body size in xeric conditions and a high vertical reach for [[agonistic]] encounters<ref name='Fritts 1984'/>.

[[Sexual dimorphism]] is most pronounced in saddleback populations, in which males have more angled and higher front openings, giving a more extreme saddled appearance<ref name='Schafer and Krekorian 1983'/>. Males of all varieties generally have a longer tail and a shorter, concave undershell with thickened knobs at the back edge to facilitate mating. Males are also larger than females: adult males weigh around {{convert|272|-|317|kg}} whilst females are {{convert|136|-|181|kg}}<ref name='Fritts 1984'>[http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1984.tb02059.x/pdf Fritts 1984. ''Evolutionary divergence of giant tortoises in Galapagos.''] Biological Journal of the Linnean Society. 21:165–176</ref>.

The saddleback carapace probably evolved independently several times in the context of xeric biomes<ref name='I. Bowman, M. Berson pp. 107'/>, since genetic similarity between populations does not correspond to carapace shape<ref>[http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1981.tb03474.x/pdf Marlow and Patton 1981. ''Biochemical relationships of the Galápagos Giant tortoises (Geochelone elephantopus).''] Journal of Zoology. 195(3):413–422</ref>. Saddleback tortoises are therefore not necessarily more closely related to each other than to domed counterparts, as the shape reflects not evolutionary proximity but ecological parity<ref name='Fritts 1984'/>. However, the extinction of crucial populations by human activities confounds studies investigating whether domed and saddleback populations are definitively mono- or polyphyletic<ref name='ReferenceA'/>.

===Role in the inception of the theory of evolution===
Charles Darwin visited the Galápagos for five weeks on the [[second voyage of HMS Beagle|second voyage of HMS ''Beagle'']] in 1835. In the first edition of his published account of the voyage he wrote: 'It was confidently asserted, that the tortoises coming from different islands in the archipelago were slightly different in form; and that in certain islands they attained a larger average size than in others. Mr. Lawson [the Vice-Governor] maintained that he could at once tell from which island any one was brought.<ref name='Darwin 1839 pp462-466'/>' He noted in the addenda of the first edition that 'there is every reason for believing that several of the islands possess their own peculiar varieties or species of tortoise<ref name='Darwin 1839 p628'>Darwin 1839. ''Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle'' p.[http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F10.3&pageseq=647 628].</ref>'. He expanded on these thoughts in the second edition, writing, 'I have not as yet noticed by far the most remarkable feature in the natural history of this archipelago; it is, that the different islands to a considerable extent are inhabited by a different set of beings. My attention was first called to this fact by the Vice-Governor, Mr. Lawson, declaring that the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought.<ref name='Darwin p393'>Darwin 1845. ''Journal of Researches'' pp.[http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F14&pageseq=406 393-4]</ref>'. The significance of the differences in tortoises between islands did not strike him as important until it was too late, as he continued, 'I did not for some time pay sufficient attention to this statement, and I had already partially mingled together the collections from two of the islands. I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted<ref name='Darwin p393'/>'.

Although the ''Beagle'' departed from the Galápagos with over 30 adult tortoises on deck, these were not for scientific study but a source of fresh meat for their Pacific crossing. Their shells and bones were thrown overboard, leaving no remains with which to test any hypotheses<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p51</ref>. It has been suggested<ref>Nicholls 2006. ''Lonesome George: The Life and Loves of a Conservation Icon''. Palgrave Macmillan. ISBN 1403945764. p62</ref> that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal<ref name='1839 p456'>Darwin 1839. ''Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle'' p.[http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F10.3&pageseq=475 456]</ref> (''chathamensis'') and Santiago<ref>Keynes 2001. ''Charles Darwin's Beagle Diary''. Cambridge University Press. ISBN 0521003172. p [http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F1925&pageseq=394 362]</ref> (''darwini''), both of which have an intermediate type of shell shape and are not particularly morphologically distinct from each other. Though he did visit Floreana, the ''nigra'' subspecies found there was already nearly extinct and he was unlikely to have seen any mature animals<ref name='Pritchard 1996'/>.

Darwin did however have four live juvenile specimens to compare from different islands. These were pet tortoises taken by himself (from San Salvador), his captain Fitzroy (two from Española) and his servant Syms Covington (from Floreana)<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p41</ref>. Unfortunately they could not help to determine whether each island had its own variety because the specimens were not mature enough to exhibit morphological differences<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p52</ref>. Although the British Museum had a few specimens, their provenance within the Galápagos was unknown<ref name='Gunther 1877'/>. However, conversations with the naturalist Gabriel Bibron, who had seen the mature tortoises of the Paris Natural History Museum confirmed to Darwin that there were distinct varieties<ref>Darwin 1827; 1828–1829; 1837–1839. ''<nowiki>[Edinburgh notebook]</nowiki>''.  CUL-DAR118 p.[http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=CUL-DAR118.-&pageseq=41 99v]</ref>.

He later compared the different tortoise forms with those of mockingbirds, in the first<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p47</ref> tentative statement linking his observations from the Galapagos with the possibility of species transmuting: 'When I recollect the fact that [from] the form of the body, shape of scales and general size, the Spaniards can at once pronounce from which island any tortoise may have been brought; when I see these islands in sight of each other and possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure and filling the same place in nature; I must suspect they are only varieties ... If there is the slightest foundation for these remarks, the zoology of archipelagoes will be well worth examining; for such facts would undermine the stability of species.<ref>Barlow ed. 1963. ''Darwin's Ornithological Notes'' Bulletin of the British Museum (Natural History). Historical Series Vol. 2, No. 7. p [http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F1577&pageseq=64 262]'</ref>'. His views on the mutability of species were restated in his notebooks: 'animals on separate islands ought to become different if kept long enough apart with slightly differing circumstances. – Now Galapagos Tortoises, Mocking birds, Falkland Fox, Chiloe fox, – Inglish and Irish Hare<ref>Darwin 1837–1838. ''Notebook B: <nowiki>[Transmutation of species]''</nowiki>.  p [http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=CUL-DAR121.-&pageseq=9 7]</ref>. These observations served as coutnerexamples to the prevailing contemporary view that species were individually created.

Darwin also found these 'antediluvian animals<ref name='1839 p456'/>' to be a source of diversion: 'I frequently got on their backs, and then giving a few raps on the hinder part of their shells, they would rise up and walk away;—but I found it very difficult to keep my balance<ref name='Darwin 1839 pp462-466'/>'.

==Biology and ecology==
===Behaviour===
The tortoises are [[Ectotherm|cold-blooded]] and therefore bask for 1–2 hours after dawn to absorb the sun's heat through their dark shells before actively foraging for 8–9 hours a day<ref name='Swingland 1989'>Swingland 1989. ''Geochelone elephantopus. Galapagos giant tortoises.'' In: I. R. Swingland and M. W. Klemens, (Eds.) The Conservation Biology of Tortoises. Occasional Papers of the IUCN Species Survival Commission (SSC), No. 5. pp. 24–28. Gland, Switzerland: IUCN. ISBN 2880329868.</ref>.  They travel mostly in the early morning or late afternoon. They have been observed to walk at a speed of {{convert|0.3|km/h}}<ref name='Darwin 1839 pp462-466'/>.
On the larger and more humid islands, the tortoises seasonally migrate down between low elevations, which become grassy plains in the wet season, to meadowed areas of higher elevation (up to 2,000&nbsp;ft<ref name='Van Denburgh 1914'/>) in the dry season. The same routes have been used for many generations, creating well-defined paths through the undergrowth known as 'tortoise highways'<ref name='R. Perry pp. 145'/>. On these wetter islands, the domed tortoises are [[gregarious]] and often found in large herds, in contrast to the more solitary and territorial disposition of the saddleback tortoises.
[[File:Porteribathing.jpg|thumb|left|Several tortoises bathing in a pool on Santa Cruz]]

Tortoises sometimes rest in mud wallows or rain-formed pools, which may be both a [[Thermoregulation|thermoregulatory]] response during cool nights, and a protection from [[parasites]] such as mosquitoes and ticks<ref name='R. Perry pp. 145'/>. Parasites are also countered by taking dust baths in loose soil. Tortoises have been noted to shelter at night under overhanging rocks<ref name='Carpenter 1966'>Carpenter 1966. ''Notes on the behavior and ecology of the Galapagos tortoise on Santa Cruz Island.'' Proceedings of the Oklahoma Academy of Science. 46:28–32</ref>. Other tortoises are observed to sleep in a snug depression in the earth or brush called a 'pallet'. Local tortoises using the same pallet sites, such as on Volcán Alcedo, results in the formation of small sandy pits.<ref>{{cite book|title=Galápagos, a natural history|first=Michael Hume|last=Jackson|publisher=University of Calgary Press|year=1993|page=107|isbn=9781895176070}}</ref>

Regarding their senses, Charles Darwin observed that: 'The inhabitants believe that these animals are absolutely deaf; certainly they do not overhear a person walking near behind them. I was always amused, when overtaking one of these great monsters as it was quietly pacing along, to see how suddenly, the instant I passed, it would draw in its head and legs, and uttering a deep hiss fall to the ground with a heavy sound, as if struck dead<ref name='Darwin 1839 pp462-466'/>'. Although they are not deaf<ref name='Van Denburgh 1914'/>, tortoises depend far more on vision and smell as stimuli<ref name='R. Perry pp. 145'/>. In addition to hissing as they withdraw into their shells due to the forceful expulsion of air<ref>[http://www.jstor.org/stable/3892193 Hayes 1988. ''Defensive behavior in the Galapagos tortoise (Geochelone elephantopus), with comments on the evolution of insular gigantism.''] Herpetologica. 44(1):11–17</ref>, the tortoises also vocalise when struggling to right themselves and during aggressive encounters. Males emit 'rhythmic groans' during sexual congress<ref name='R. Perry pp. 145'/>.

===Diet===
[[File:Galapagos giant tortoise feeding.jpg|thumb|A tortoise feeding.]]
The tortoises are [[herbivore]]s that consume a diet of cactus, grasses, leaves, lichen, and berries. They have been documented to feed on ''[[Manchineel tree|Hippomane mancinella]]'' ('poison apple'), the endemic [[guava]] ''Psidium galapageium'', the [[Mosquito fern|water fern]] ''Azolla microphylla'', and the [[Tillandsia|bromeliad]] ''Tillandsia insularis''.<ref>Cayot 1987. ''Ecology of giant tortoises (''Geochelone elephantopus'') in the Galápagos Islands.'' PhD Dissertation, Syracuse University, Syracuse, New York.</ref>. A tortoise eats an average of {{convert|70|-|80|lb}} per day, though inefficient digestion means that much of this passes through without nutritional extraction<ref>[http://onlinelibrary.wiley.com/doi/10.1002/zoo.20039/pdf Hatt 2005. ''Fiber digestibility in juvenile Galapagos tortoises (Geochelone nigra) and implications for the development of captive animals.''] Zoo Biology. 24(2):185–191</ref>.

Tortoises acquire most of their moisture from the dew and sap in vegetation (particularly the ''Opuntia'' cactus); therefore, they can spend long periods without drinking water. They can also endure 18 months when deprived of all food and water<ref name='Porter 1815'>[http://books.google.co.uk/books?id=IiYFAAAAMAAJ&vq=151&dq=journal%20of%20a%20cruise%20made%20to%20the%20pacific%20ocean&pg=PA151#v=onepage&q&f=false Porter 1815. ''Journal of the Cruise Made to the Pacific Ocean by Captain Porter in the United States Frigate ''Essex'' in the years ''1812'', ''1813'', ''1814''.''] 2 vols. Philadelphia. Second Edition, New York, 1822. p151</ref>, surviving by breaking down their body fat to produce water as a by-product. When thirsty they may drink large quantities of water very quickly, storing it in their bladders and the 'root of the neck' (the [[pericardium]]<ref name='Swingland 1989'/>), both of which used to make them useful water sources on ships<ref name='Porter 1815'/>. On arid islands, tortoises lick morning dew from boulders, and the repeated action over many generations has formed half-sphere depressions in the rock<ref name='Swingland 1989'/>.

===Mutualism===
[[File:Galapagos Tortoise and finch symbiosis.jpg|thumb|left|A finch and a tortoise exhibiting mutualistic behaviour.]]
Tortoises share a [[Mutualism (biology)|mutualistic]] relationship with some species of [[Darwin's finches|Galápagos finch]] and mockingbirds. Small groups of finches initiate the process by hopping on the ground in an exaggerated fashion facing the tortoise. The tortoise signals it is ready by rising up and extending its neck and legs, enabling the birds to reach otherwise inaccessible spots on the tortoise's body such as the neck, rear legs, cloacal opening, and skin between plastron and carapace. The birds benefit from the food source and the tortoises get rid of irritating ecto-parasites<ref>[http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0310.1974.tb01816.x/pdf MacFarland and Reeder 1974. ''Cleaning symbiosis involving Galápagos tortoises and two species of Darwin's finches.''] Zeitschrift für Tierpsychologie. 34:464–483</ref>.

Some tortoises have been observed to insidiously exploit this mutualistic relationship. After rising and extending its limbs, the bird may go beneath the tortoise to investigate, whereupon suddenly the tortoise withdraws its limbs to drop flat and kill the bird. It then steps back to eat the bird, presumably to supplement its diet with protein<ref>Bonin 2006. ''Turtles of the World''. Peter C. H. Pritchard, (translator). Johns Hopkins University Press. ISBN 0801884969.</ref>.

===Reproduction and life cycle===
[[File:Galapagos tortoise dominance display.jpg|thumb|right|A pair of tortoises engaging in a dominance display.]]
[[Mating]] occurs at any time of the year, although it does have seasonal peaks between February and June in the humid uplands during the rainy season<ref name='R. Perry pp. 145'/>. When mature males meet in the mating season they will face each other in a ritualised dominance display, rise up on their legs and stretch up their necks with their mouths gaping open. Occasionally, head-biting occurs, but usually the shorter tortoise will back off, conceding mating rights to the victor. The behaviour is most pronounced in saddleback subspecies, which are more aggressive and have longer necks<ref name='Schafer and Krekorian 1983'/>.

The prelude to mating can be very aggressive, as the male forcefully rams the female's shell with his own and nips her legs<ref>[http://links.jstor.org/stable/1437060?seq=1 DeSola 1930. ''The Liebespiel of Testudo vandenburghi, a new name for the mid-Albemarle Island Galápagos tortoise.''] Copeia. 1930:79–80</ref>. Mounting is an awkward process and the male must stretch and tense to maintain equilibrium in a slanting position. The concave underside of the male's shell helps him to balance when straddled over the female's shell, and brings his cloacal vent (which houses the penis) closer to the female's dilated [[cloaca]]. The male vocalises with hoarse bellows and grunts during mating<ref name='Carpenter 1966'/>.

Females then journey up to several kilometres in July to November to reach nesting areas of dry, sandy coast. Nest digging is a tiring and elaborate task which may take the female several hours a day over many days to complete<ref name='R. Perry pp. 145'/>. It is carried out blindly using only the hind legs to dig a {{convert|30|cm|abbr=on}} deep cylindrical hole, into which she lays up to sixteen spherical, hard-shelled eggs ranging from 82 to 157&nbsp;g in mass<ref name='Swingland 1989'/>, and the size of a billiard ball<ref name='MacFarland 1972'/>. Some observations suggest that the average clutch size for domed populations (9.6 per clutch for ''porteri'' on Santa Cruz) is larger than that of saddlebacks (4.6 per clutch for ''duncanensis'' on Pinzón)<ref name='MacFarland 1974'/>. The female makes a muddy plug for the nest hole out of soil mixed with urine and seals the nest by pressing down firmly with her plastron. Females may lay 1–4 clutches per season, and leaves them to be incubated by the sun. [[Temperature-dependent sex determination|Temperature plays a role in the sex of the hatchling]], with lower temperature nests producing more males and higher temperature nests producing more females. This is related closely to incubation time, since clutches laid early will incubate during the cool season and have longer incubation periods (producing more males), while nests that are laid later incubate for a shorter period in the hot season (producing more females)<ref>[http://dx.doi.org/10.1016/S0168-1591(96)01108-2 Cioﬁ and Swingland 1997. ''Environmental sex determination in reptiles.''] Applied Animal Behaviour Science, 51, 251–265.</ref>

Young animals emerge from the nest after 4 to 8 months (December–April) and may weigh only {{convert|50|g}} and measure {{convert|6|cm}}<ref name='R. Perry pp. 145'/>.  When the young tortoises emerge from their shells, they must dig their way to the surface, which can take  up several weeks, though their yolk sac can sustain them for up to seven months<ref name='MacFarland 1972'/>. In particularly dry conditions, the hatchlings may die underground if they are encased by hardened soil, while flooding of the nest area can drown them. Subspecies are initially indistinguishable as they all have domed carapaces. The young stay in warmer lowland areas for the their first 10–15 years<ref name='Swingland 1989'/>, encountering hazards such as falling into cracks, being crushed by falling rock, or excessive heat stress. The [[Galápagos Hawk]] was formerly the sole native predator of the tortoise hatchlings, as Darwin wrote: 'The young tortoises, as soon as they are hatched, fall prey in great numbers to the buzzard<ref name='Darwin 1839 pp462-466'/>'. The hawk is now much rarer, but introduced feral pigs, dogs, cats and black rats have become predators of eggs and young tortoises<ref name='Cayot 1994'/>. The adult tortoises have no natural predators apart from humans, as Darwin noted: 'The old ones seem generally to die from accidents, as from falling down precipices. At least several of the inhabitants told me, they had never found one dead without some such apparent cause<ref name='Darwin 1839 pp462-466'/>.'

Sex can be determined only when the tortoise is about 15 years old, and sexual maturity is reached at around 20–25 in captivity, possibly 40 years in the wild (when they reach their full size). Life expectancy in the wild is thought to be over 100 years<ref name='NatGeo'/>.

==Human disturbance==
As a consequence of several waves of human exploitation of the tortoises as a food source, their total population declined from around 250,000<ref name='amnh.org'/> when first discovered in the 16th century to around 20,000<ref>Galapagos National Park. [http://www.galapagospark.org/nophprg.php?page=parque_nacional_nativas_endemicas_tortugas Reproduction, breeding, repatriation, and monitoring of tortoises.] Retrieved 2010-09-04.</ref> in the modern day. An estimated 200,000 animals were taken before the 20the  century<ref name='Pritchard 1996'/><ref name='Swingland 1989'/>.

The relatively immobile and defenceless tortoises were collected and stored live on board ships where they could survive for at least a year without food or water (some anecdotal reports suggest individuals surviving two years<ref name='Townsend 1925'/>), providing valuable fresh meat, whilst their diluted urine and water stored in their neck bags could also be used as drinking water. The 17th century British pirate, explorer and naturalist [[William Dampier]] wrote that 'They are so extraordinarily large and fat, and so sweet, that no pullet eats more pleasantly<ref>[http://www.galapagos.to/TEXTS/DAMPIER-1.HTM#galapagos Dampier 1697. ''A New Voyage Around the World''], reprinted in 1919 by Dover Publications (USA)</ref>,' whilst Captain [[James Colnett]] of the British Navy wrote of 'the land tortoise which in whatever way it was dressed, was considered by all of us as the most delicious food we had ever tasted.<ref>Colnett 1798. ''A Voyage to the South Atlantic'', reprinted in 1968 by Da Capo Press (USA)</ref>' US Navy captain David porter declared that, 'after once tasting the Gallipagos tortoises, every other animal food fell off greatly in our estimation ... The meat of this animals is the easiest of digestion, and a quantity of it, exceeding that of any other food, can be eaten without experiencing the slightest of inconvenience.<ref name='Porter 1815'/>' Darwin was less enthusiastic about tortoise meat, writing 'the breast-plate roasted (as the Gauchos do "carne con cuero"), with the flesh on it, is very good; and the young tortoises make excellent soup; but otherwise the meat to my taste is indifferent<ref>Darwin 1839. ''Narrative of the Surveying Voyages of His Majesty's Ships Adventure and Beagle'' p.[http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F10.3&pageseq=478 459].</ref>.'  

In the 17th century, [[pirate]]s started to use the Galápagos islands as a base for resupply, restocking on food, water and repairing vessels before attacking [[Spanish colonization of the Americas|Spanish colonies]] in the South American mainland. However, the Galápagos tortoises did not struggle for survival at this point because the islands were distant to busy shipping routes and harboured few valuable natural resources. As such they remained unclaimed by any nation, uninhabited and uncharted. In comparison, the tortoises of the islands in the Indian Ocean were already facing extinction by the late 17th century<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p85</ref>.

Between the 1790s and the 1860s, [[whaling]] ships and [[Fur seal#Exploitation|fur-sealers]] systematically collected tortoises in far greater numbers than the buccanneers preceding them<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p94</ref>. They were used for food and many more were killed for high grade 'turtle oil' from the late 19th century onward for lucrative export to continental Ecuador<ref>Beck 1903. ''In the home of the giant tortoise'', 7th Annual Report of the New York Zoological Society New York pp. 1–17</ref>. A total of over 13,000 tortoises is recorded in the logs of whaling ships between 1831 and 1868, and an estimated 100,000 were taken before 1830<ref name='Townsend 1925'>[http://mysite.du.edu/~ttyler/ploughboy/townsendgaltort.htm Townsend 1925. ''The Galapagos tortoises in their relation to the whaling industry: a study of old logbooks.''] Zoologica. 4:55–135</ref>. Since it was easiest to collect tortoises around coastal zones, females were most vulnerable to depletion during the nesting season. The collection by whalers came to a halt eventually through a combination of the scarcity of tortoises that they had created and the competition from crude oil as a cheaper and more abundant energy source<ref>Chambers 2004. ''A sheltered life: The Unexpected History of the Giant Tortoise''. John Murray. London. ISBN 0719565286. p104</ref>.

Population decline accelerated with the early settlement of the islands in the early 19th century leading to unregulated hunting for meat, habitat clearance for agriculture and the introduction of alien mammal species<ref name='MacFarland 1974'/>. Feral pigs, dogs, cats and black rats have become predators of eggs and young tortoises, whilst goats, donkeys and cattle compete for grazing and trample nest sites. Scientific collection expeditions took 661 tortoises between 1888 and 1930, and more than 120 tortoises have been taken by poachers since 1990. Threats continue today with the rapid expansion of the tourist industry and increasing size of human settlements on the islands<ref>[http://www.darwinfoundation.org/english/_upload/Galapagos_At-Risk-2007.pdf Watkins and Cruz 2007. ''Galapagos at risk: a Socioeconomic Analysis.''] Charles Darwin Foundation.</ref>.

===Conservation===
The remaining subspecies of tortoise range in IUCN classification from [[extinct in the wild]] to [[Vulnerable species|vulnerable]]. Slow growth rate, late sexual maturity and island endemism make the tortoises particularly prone to extinction without the efforts of conservationists<ref name='Chiari 2009'/>. The Galápagos giant tortoise is now strictly protected. ''Geochelone nigra'' is listed on Appendix I of the [[CITES|Convention on International Trade in Endangered Species of Wild Fauna and Flora]]<ref name='CITES Appendices I, II and III 2010'/>. The listing requires that trade in the [[taxon]] and its products is subject to strict regulation by ratifying states and international trade for primarily commercial purposes is prohibited. In 1936 the Ecuadorian government listed the giant tortoise as a protected species. In 1959, it declared all uninhabited areas in the Galápagos to be a National Park<ref>Ecuador 1959. ''Decreto ley de emergencia, por el cual se declaran parques nacionales de reserva de exclusivo dominio del estado, para la preservación de la fauna y flora, todas las tierras que forman las islas del Archipiélago de Colón o Galápagos, Registro Oficial No. 873 (1959) Quito, 20 de Julio, 1959''</ref> and established the [[Charles Darwin Foundation]]. In 1970, capturing or removing many species from the islands (including tortoises and their eggs) was banned<ref>Ecuador 1970. ''Ley de Protección de la Fauna Silvestre y de los Recursos Ictiológicos, Registro Oficial No. 104 (1970) Quito, 20 de Noviembre 1970''</ref>. To halt trade in the tortoises altogether, it became illegal to export the tortoises from Ecuador, captive or wild, continental or insular in provenance. The banning of their exportation resulted in automatic prohibition of importation to the United States  under Public Law 91-135 (1969)<ref>United States 1969. ''Endangered Species Conservation Act.'' 83 Stat. 275; 16 USC 668 cc-1 to 668 cc-6 (1969)</ref>. A 1971 Ecuadorian decree made it illegal to damage, remove, alter or disturb any organism, rock or other natural object in the National Park<ref>Ecuador 1971. ''Decreto Supremo No. 1306 (1971) Quito, 27 de Agosto 1971''</ref>.

[[File:BabyGalapagosTortoises.jpg|thumb|right|A 2–3 month old tortoise with a tortoise egg]]
Breeding and repatriation programs began in 1965 and have successfully brought 10 of the 11 endangered subspecies up to guarded population levels. Young tortoises are raised at one of several breeding centres across the islands to improve their survival during their vulnerable early development. Eggs are collected from threatened nesting sites, and the hatched young are given a head start by being kept in captivity for four to five years to reach a size with a much better chance of survival to adulthood before re-release onto their native ranges<ref name='Cayot 1994'>Cayot 1994. ''Conservation biology of Galápagos reptiles: twenty-five years of successful research and management.'' In: J. B. Murphy, K. Adler, and J. T. Collins (eds.). Captive Management and Conservation of Amphibians and Reptiles, pp. 297–305. Ithaca, New York: Society for the Study of Amphibians and Reptiles. Contributions to Herpetology. vol. 11. ISBN 0916984338.</ref><ref>{{cite web|url=http://www.galapagospark.org/nophprg.php?page=parque_nacional_nativas_endemicas_tortugas |title=''Reproduction, breeding, repatriation, and monitoring of tortoises.'' |publisher=galapagospark.org |date= |accessdate=2010-08-30}}</ref>.

The most significant population recovery was that of the Española Tortoise (''hoodensis''), which was saved from near-certain extinction. The population had been depleted to 3 males and 12 females that had been so widely dispersed that no mating in the wild had occurred<ref name='MacFarland 1972'/>. They were brought to the Charles Darwin Research Station in 1971 for a captive breeding program<ref>[http://www.tortoise.org/archives/pinzon.html Caporaso 1991. ''The Galapagos tortoise conservation program: the plight and future for the Pinzón Island tortoise.''] In: K. R. Beaman, F. Caporaso, S. McKeown, and M. D. Graff (eds.), Proceedings of the first International Symposium on Turtles & Tortoises: Conservation and Captive Husbandry, pp. 113–116. California: Chapman University.</ref>. In the following 33 years, these 15 tortoises gave rise to over 1200 tortoises released onto their home island, which themselves have begun to reproduce naturally<ref name='Pritchard 1996'/><ref>[http://rspb.royalsocietypublishing.org/content/271/1537/341.full.pdf Milinkovitch 2004. ''Genetic analysis of a successful repatriation programme: giant Galapagos tortoises.''] Proceedings of the Royal Society of London B. 271:341–345</ref>

The Galápagos National Park Service also systematically culls feral predators and competitors. Goat eradication on islands including Pinta was achieved by the technique of using 'Judas' goats with electronic collars to locate the herds. Marksmen shot all the goats except the Judas, then returned weeks later to find the Judas and shoot the herd to which it had relocated, repeating until only the Judas remained, which was then killed<ref name='guardian.co.uk'>[http://www.guardian.co.uk/environment/2010/jun/27/giant-tortoise-galapagos-saved-extinction Carroll 27-06-2010. ''Galápagos giant tortoise saved from extinction by breeding programme''] The Observer. Retrieved 2010-09-04</ref>. Other measures have included dog eradication from San Cristóbal, and fencing off nests to protect from feral pigs<ref name='Cayot 1994'/>.

Efforts are now underway to repopulate islands formerly inhabited by tortoises in order to [[Island restoration|restore the insular ecosystems]] to their pre-human condition. The tortoises are a [[keystone species]], acting as [[ecosystem engineer]]s<ref name='guardian.co.uk'/> which help in plant seed dispersal, trampling down brush and thinning the understory of vegetation (allowing light to penetrate and germination to occur). Birds such as flycatchers perch on and fly around tortoises in order to hunt the insects it displaces from the brush<ref name='MacFarland 1972'/>. 39 sterilised tortoises of hybrid origin were introduced in May 2010 to Pinta Island, the first tortoises there since the evacuation of Lonesome George 38 years ago in 1972<ref>Galapagos Conservancy. [http://www.galapagos.org/2008/index.php?id=68 Project Pinta]. Retrieved 2010-09-04</ref>. It is hoped that with the recent identification of a hybrid ''abingdoni'' tortoise, the approximate genetic constitution of the original inhabitants of Pinta may eventually be restored with the identification and relocation of appropriate specimens to this island<ref name='Russello 2007'/>. This approach may also be used to 're-tortoise' Floreana in the future, since captive individuals have been found to be descended from the extinct original stock<ref name='Russello 2010'/>.

==Notable individuals==
[[File:Lonesome George -Pinta giant tortoise -Santa Cruz.jpg|thumb|right|Lonesome George]]

* [[Lonesome George]] is the only known living specimen of the Pinta Island Tortoise and the world's 'rarest living creature'<ref>Footman ed. 2001 ''Guinness World Records''. Guinness Records. ISBN 0851121020. p. 182</ref>.
* [[Harriet (turtle)|Harriet]] was the oldest known Galápagos tortoise, having reached an estimated age of more than 170 years old at its death in 2006<ref>[http://www.newscientist.com/article/mg18324645.000-the-origin-of-harriet.html?full=true Chambers 2004. ''The origin of Harriet''] 11 September 2004, New Scientist. Retrieved 2010-09-13. Chambers notes that Harriet was probably 169 years old in 2004. The individual died in 2006 (though media outlets claimed the greater age of 175 based on a less reliable timeline): [http://www.abc.net.au/news/newsitems/200606/s1670600.htm Harriet the tortoise dies. 23/06/2006 ABC News Online.] Retrieved 2010-09-13</ref>.

==References==<!-- CurrBiol17:317. -->
{{Reflist|2}}

==External links==
*ARKive – [http://www.arkive.org/species/GES/reptiles/Geochelone_spp/ images and movies of the Galápagos giant tortoise ''(Geochelone spp.)'']
*[http://www.amnh.org/nationalcenter/Endangered/tortoise/tortoise.html American Museum of Natural History, Galápagos Giant Tortoise webpage]

{{Use dmy dates|date=October 2010}}

{{DEFAULTSORT:Galapagos Tortoise}}
[[Category:Tortoises]]
[[Category:Endemic fauna of the Galápagos Islands|Tortoise, Galapagos]]
[[Category:Megafauna]]

[[br:Baot ramz Galapagos]]
[[bg:Слонска костенурка]]
[[ca:Tortuga gegant de les Galápagos]]
[[cs:Želva sloní]]
[[da:Elefantskildpadde]]
[[de:Galápagos-Riesenschildkröte]]
[[nv:Chʼééh digháhiitsoh]]
[[et:Elevantkilpkonn]]
[[es:Geochelone nigra]]
[[fr:Tortue géante des Galapagos]]
[[ko:갈라파고스땅거북]]
[[hi:गैलापागोस कछुआ]]
[[hr:Galapagoška kornjača]]
[[it:Geochelone nigra]]
[[hu:Galápagosi óriásteknős]]
[[nl:Galapagosreuzenschildpad]]
[[ja:ガラパゴスゾウガメ]]
[[no:Elefantskilpadde]]
[[pl:Żółw słoniowy]]
[[pt:Tartaruga-das-galápagos]]
[[ru:Слоновая черепаха]]
[[sk:Korytnačka slonia]]
[[sr:Geochelone nigra ssp. darwini]]
[[fi:Galápagoksenjättiläiskilpikonna]]
[[sv:Galapagossköldpadda]]
[[tr:Dev Galapagos kaplumbağası]]
[[uk:Галапагоська черепаха]]
[[zh:加拉帕戈斯象龜]]