Galapagos Giant Tortoise

=== Early classification === The Galápagos Islands were discovered in 1535, but first appeared on the maps, of Gerardus Mercator and Abraham Ortelius, around 1570. The islands were named "Insulae de los Galopegos" (Islands of the Tortoises) in reference to the giant tortoises found there. Initially, the giant tortoises of the Indian Ocean and those from the Galápagos were thought to be the same subspecies. Naturalists thought that sailors had transported the tortoises there. In 1676, the pre-Linnaean authority Claude Perrault referred to both subspecies as Tortue des Indes. In 1783, Johann Gottlob Schneider classified all giant tortoises as Testudo indica ("Indian tortoise"). In 1812, August Friedrich Schweigger named them Testudo gigantea ("gigantic tortoise"). In 1834, André Marie Constant Duméril and Gabriel Bibron classified the Galápagos tortoises as a separate subspecies, which they named Testudo nigrita ("black tortoise").

The first systematic survey of giant tortoises was by the zoologist Albert Günther of the British Museum, in 1875. This hypothesis was later disproven by the understanding that the Galápagos, the atolls of Seychelles, and the Mascarene islands are all of recent volcanic origin and have never been linked to a continent by land bridges. Galápagos tortoises are now thought to have descended from a South American ancestor, At the end of the 19th century, Georg Baur recognised five more populations of Galápagos tortoise. In 1905–06, an expedition by the California Academy of Sciences, with Joseph R. Slevin in charge of reptiles, collected specimens which were studied by Academy herpetologist John Van Denburgh. He identified four additional populations, and proposed the existence of 15 subspecies. Van Denburgh's list still guides the taxonomy of the Galápagos tortoise, though now 10 populations are thought to have existed.

The current specific designation of niger, (originally spelled nigra - meaning "black" - by Quoy & Gaimard, 1824 because it was in the feminine genus Testudo) was resurrected as a valid species name in 1984 after it was discovered to be the senior synonym (an older taxonomic synonym taking historical precedence) for the then commonly used subspecies name of elephantopus ("elephant-footed" – Harlan, 1827). Quoy and Gaimard's Latin description explains the use of nigra: "Testudo toto corpore nigro" means "tortoise with completely black body". Quoy and Gaimard described it from a living specimen, but no evidence indicates they knew of its accurate provenance within the Galápagos – the locality was in fact given as California. Garman proposed synonymizing it with the extinct Floreana subspecies. Later, Pritchard deemed it convenient to accept this designation, despite its tenuousness, for minimal disruption to the already confused nomenclature of the subspecies. The even more senior subspecies synonym of californiana ("californian" – Quoy & Gaimard, 1824) is considered a nomen oblitum ("forgotten name"). In most of the 1900s, the Galápagos tortoise was considered to belong to the genus Geochelone, known as 'typical tortoises' or 'terrestrial turtles'. In the 1980s, the former subgenus Chelonoidis was elevated to generic status, a conclusion later supported by phylogenetic evidence which grouped the South American members of Geochelone into an independent clade (branch of the tree of life). This nomenclature has been adopted by several authorities. The names of several species names in the genus Chelonoidis, including this one, have often been misspelled, beginning in the 1980s when Chelonoidis was elevated to genus and mistakenly treated as feminine rather than masculine, an error recognized and fixed in 2017.

showing the ranges of currently recognized Galápagos tortoise subspecies. Islands with extant subspecies are shaded yellow.]] Within the archipelago, 14-15 subspecies of Galápagos tortoises have been identified, although only 12 survive to this day. Five are found on separate islands; five of them on the volcanoes of Isabela Island. Several of the surviving subspecies are seriously endangered. is thought to have been hunted to extinction by 1850, However, recent DNA testing shows that an intermixed, non-native population currently existing on the island of Isabela is of genetic resemblance to the subspecies native to Floreana, suggesting that C. niger has not gone entirely extinct. Prior to widespread knowledge of the differences between the populations (sometimes called races) from different islands and volcanoes, captive collections in zoos were indiscriminately mixed. Fertile offspring resulted from pairings of animals from different races. However, captive crosses between tortoises from different races have lower fertility and higher mortality than those between tortoises of the same race, and captives in mixed herds normally direct courtship only toward members of the same race. Prior to 2021, all subspecies were classified as distinct species from one another, but a 2021 study analyzing the level of divergence within the extinct West Indian Chelonoidis radiation and comparing it to the Galápagos radiation found that the level of divergence within both clades may have been significantly overestimated, and supported once again reclassifying all Galápagos tortoises as subspecies of a single species, C. niger. This was followed by the Turtle Taxonomy Working Group and the Reptile Database later that year. The taxonomic status of the various races is not fully resolved. * Testudo californiana Quoy & Gaimard, 1824a (nomen dubium) * Testudo clivosa Garman, 1917 * Geochelone (Chelonoidis) elephantopus Pritchard, 1967 * Chelonoidis elephantopus Bour, 1980 ; Chelonoidis nigra nigra * Testudo nigra Quoy & Gaimard, 1824 * Testudo californiana Quoy & Gaimard, 1824 * Testudo galapagoensis Baur, 1889 * Testudo elephantopus galapagoensis Mertens & Wermuth, 1955 * Geochelone elephantopus galapagoensis Pritchard, 1967 * Chelonoidis galapagoensis Bour, 1980 * Chelonoidis nigra Bour, 1985 * Chelonoidis elephantopus galapagoensis Obst, 1985 * Geochelone nigra Pritchard, 1986 * Geochelone nigra nigra Stubbs, 1989 * Chelonoidis nigra galapagoensis David, 1994 * Chelonoidis nigra nigra David, 1994 * Geochelone elephantopus nigra Bonin, Devaux & Dupré, 1996 * Testudo california Paull, 1998 (ex errore) * Testudo californianana Paull, 1999 (ex errore) ; Chelonoidis nigra abingdonii * Testudo ephippium Günther, 1875 * Testudo abingdonii Günther, 1877 * Testudo abingdoni Van Denburgh, 1914 (ex errore) * Testudo elephantopus abingdonii Mertens & Wermuth, 1955 * Testudo elephantopus ephippium Mertens & Wermuth, 1955 * Geochelone abingdonii Pritchard, 1967 * Geochelone elephantopus abingdoni Pritchard, 1967 * Geochelone elephantopus ephippium Pritchard, 1967 * Geochelone ephippium Pritchard, 1967 * Chelonoidis abingdonii Bour, 1980 * Chelonoidis ephippium Bour, 1980 * Geochelone elephantopus abingdonii Groombridge, 1982 * Geochelone abingdoni Fritts, 1983 * Geochelone epphipium Fritts, 1983 (ex errore) * Chelonoidis nigra ephippium Pritchard, 1984 * Chelonoidis elephantopus abingdoni Obst, 1985 * Chelonoidis elephantopus ephippium Obst, 1985 * Geochelone nigra abingdoni Stubbs, 1989 * Chelonoidis nigra abingdonii David, 1994 * Chelonoidis elephantopus abingdonii Rogner, 1996 * Chelonoidis nigra abingdonii Bonin, Devaux & Dupré, 1996 * Chelonoidis nigra abdingdonii Obst, 1996 (ex errore) * Geochelone abdingdonii Obst, 1996 * Geochelone nigra abdingdoni Obst, 1996 (ex errore) * Geochelone nigra ephyppium Caccone, Gibbs, Ketmaier, Suatoni & Powell, 1999 (ex errore) * Chelonoidis nigra ahingdonii Artner, 2003 (ex errore) * Chelonoidis abingdoni Joseph-Ouni, 2004 ; Chelonoidis nigra becki * Testudo becki Rothschild, 1901 * Testudo bedsi Heller, 1903 (ex errore) * Geochelone becki Pritchard, 1967 * Geochelone elephantopus becki Pritchard, 1967 * Chelonoidis becki Bour, 1980 * Chelonoidis elephantopus becki Obst, 1985 * Chelonoidis nigra beckii David, 1994 (ex errore) * Chelonoidis elephantopus beckii Rogner, 1996 * Chelonoidis nigra becki Obst, 1996 ; Chelonoidis nigra chathamensis * Testudo wallacei Rothschild, 1902 * Testudo chathamensis Van Denburgh, 1907 * Testudo elephantopus chathamensis Mertens & Wermuth, 1955 * Testudo elephantopus wallacei Mertens & Wermuth, 1955 * Testudo chatamensis Slevin & Leviton, 1956 (ex errore) * Geochelone chathamensis Pritchard, 1967 * Geochelone elephantopus chathamensis Pritchard, 1967 * Geochelone elephantopus wallacei Pritchard, 1967 * Geochelone wallacei Pritchard, 1967 * Chelonoidis chathamensis Bour, 1980 * Chelonoidis elephantopus chathamensis Obst, 1985 * Chelonoidis elephantopus wallacei Obst, 1985 * Chelonoidis elephantopus chatamensis Gosławski & Hryniewicz, 1993 * Chelonoidis nigra chathamensis David, 1994 * Chelonoidis nigra wallacei Bonin, Devaux & Dupré, 1996 * Geochelone cathamensis Obst, 1996 (ex errore) * Geochelone elephantopus chatamensis Paull, 1996 * Testudo chathamensis chathamensis Pritchard, 1998 * Cherlonoidis nigra wallacei Wilms, 1999 * Geochelone nigra chatamensis Caccone, Gibbs, Ketmaier, Suatoni & Powell, 1999 * Geochelone nigra wallacei Chambers, 2004 ; Chelonoidis nigra darwini * Testudo wallacei Rothschild, 1902 * Testudo darwini Van Denburgh, 1907 * Testudo elephantopus darwini Mertens & Wermuth, 1955 * Testudo elephantopus wallacei Mertens & Wermuth, 1955 * Geochelone darwini Pritchard, 1967 * Geochelone elephantopus darwini Pritchard, 1967 * Geochelone elephantopus wallacei Pritchard, 1967 * Geochelone wallacei Pritchard, 1967 * Chelonoidis darwini Bour, 1980 * Chelonoidis elephantopus darwini Obst, 1985 * Chelonoidis elephantopus wallacei Obst, 1985 * Chelonoidis nigra darwinii David, 1994 (ex errore) * Chelonoidis elephantopus darwinii Rogner, 1996 * Chelonoidis nigra darwini Bonin, Devaux & Dupré, 1996 * Chelonoidis nigra wallacei Bonin, Devaux & Dupré, 1996 * Cherlonoidis nigra wallacei Wilms, 1999 * Geochelone nigra darwinii Ferri, 2002 * Geochelone nigra wallacei Chambers, 2004 ; Chelonoidis nigra duncanensis * Testudo duncanensis Garman, 1917 (nomen nudum) * Geochelone nigra duncanensis Stubbs, 1989 * Geochelone nigra duncanensis Garman, 1996 * Chelonoidis nigra duncanensis Artner, 2003 * Chelonoidis duncanensis Joseph-Ouni, 2004 ; Chelonoidis nigra hoodensis * Testudo hoodensis Van Denburgh, 1907 * Testudo elephantopus hoodensis Mertens & Wermuth, 1955 * Geochelone elephantopus hoodensis Pritchard, 1967 * Geochelone hoodensis Pritchard, 1967 * Chelonoidis hoodensis Bour, 1980 * Chelonoidis elephantopus hoodensis Obst, 1985 * Chelonoidis nigra hoodensis David, 1994 ; Chelonoidis nigra phantastica * Testudo phantasticus Van Denburgh, 1907 * Testudo phantastica Siebenrock, 1909 * Testudo elephantopus phantastica Mertens & Wermuth, 1955 * Geochelone elephantopus phantastica Pritchard, 1967 * Geochelone phantastica Pritchard, 1967 * Chelonoidis phantastica Bour, 1980 * Geochelone phantasticus Crumly, 1984 * Chelonoidis elephantopus phantastica Obst, 1985 * Chelonoidis nigra phantastica David, 1994 ; Chelonoidis nigra porteri * Testudo nigrita Duméril & Bibron, 1835 * Testudo porteri Rothschild, 1903 * Testudo elephantopus nigrita Mertens & Wermuth, 1955 * Geochelone elephantopus porteri Pritchard, 1967 * Geochelone nigrita Pritchard, 1967 * Chelonoidis nigrita Bour, 1980 * Geochelone elephantopus nigrita Honegger, 1980 * Geochelone porteri Fritts, 1983 * Chelonoidis elephantopus nigrita Obst, 1985 * Geochelone nigra porteri Stubbs, 1989 * Chelonoidis elephantopus porteri Gosławski & Hryniewicz, 1993 * Chelonoidis nigra nigrita David, 1994 * Geochelone nigra perteri Müller & Schmidt, 1995 (ex errore) * Chelonoidis nigra porteri Bonin, Devaux & Dupré, 1996 ; Chelonoidis nigra vicina * Testudo elephantopus Harlan, 1827 * Testudo microphyes Günther, 1875 * Testudo vicina Günther, 1875 * Testudo macrophyes Garman, 1917 * Testudo vandenburghi de Sola, 1930 * Testudo elephantopus elephantopus Mertens & Wermuth, 1955 * Geochelone elephantopus Williams, 1960 * Geochelone elephantopus elephantopus Pritchard, 1967 * Geochelone elephantopus guentheri Pritchard, 1967 * Geochelone elephantopus guntheri Pritchard, 1967 (ex errore) * Geochelone elephantopus microphyes Pritchard, 1967 * Geochelone elephantopus vandenburgi Pritchard, 1967 (ex errore) * Geochelone guntheri Pritchard, 1967 * Geochelone microphyes Pritchard, 1967 * Geochelone vandenburghi Pritchard, 1967 * Geochelone vicina Pritchard, 1967 * Geochelone elephantopus microphys Arnold, 1979 (ex errore) * Geochelone elephantopus vandenburghi Pritchard, 1979 * Chelonoides elephantopus Obst, 1980 * Chelonoidis elephantopus Bour, 1980 * Chelonoidis guentheri Bour, 1980 * Chelonoidis microphyes Bour, 1980 * Chelonoidis vandenburghi Bour, 1980 * Geochelone guentheri Fritts, 1983 * Chelonoidis elephantopus elephantopus Obst, 1985 * Chelonoidis elephantopus guentheri Obst, 1985 * Chelonoidis elephantopus microphyes Obst, 1985 * Chelonoidis elephantopus vandenburghi Obst, 1985 * Geochelone elephantopus vicina Swingland, 1989 * Geochelone elephantopus vicini Swingland, 1989 (ex errore) * Chelonoidis elephantopus guntheri Gosławski & Hryniewicz, 1993 * Chelonoidis nigra guentheri David, 1994 * Chelonoidis nigra microphyes David, 1994 * Chelonoidis nigra vandenburghi David, 1994 * Geochelone nigra elephantopus Müller & Schmidt, 1995 * Chelonoidis elephantopus vicina Rogner, 1996 * Geochelone elephantopus vandenburghii Obst, 1996 (ex errore) * Geochelone vandenburghii Obst, 1996 * Chelonoidis nigra microphyies Bonin, Devaux & Dupré, 1996 (ex errore) * Geochelone elephantopus microphytes Paull, 1996 (ex errore) * Geochelone elephantopus vandenbergi Paull, 1996 (ex errore) * Testudo elephantopus guntheri Paull, 1999 * Chelonoidis nigra vicina Artner, 2003 * Chelonoidis vicina Joseph-Ouni, 2004 * Geochelone nigra guentheri Chambers, 2004 Modern DNA methods have revealed new information on the relationships between the subspecies: Isabela Island The five populations living on the largest island, Isabela, are the ones that are the subject of the most debate as to whether they are true subspecies or just distinct populations of a subspecies. It is widely accepted that the population living on the northernmost volcano, Volcan Wolf, is genetically independent from the four populations to the south and is therefore a separate subspecies. The fifth population living on the southernmost volcano (C. n. vicina) is thought to have split off from the Sierra Negra population more recently and is therefore not as genetically different as the other two. 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 transported by humans from Floreana to Isabela, or from individuals thrown overboard from ships to lighten the load. Furthermore, individuals from the subspecies possibly are still extant. Genetic analysis from a sample of tortoises from Volcan Wolf found 84 first-generation C. n. niger hybrids, some less than 15 years old. The genetic diversity of these individuals is estimated to have required 38 C. n. niger parents, many of which could still be alive on Isabela Island. Pinta Island at the Charles Darwin Research Station, in 2006]] The Pinta Island subspecies (C. n. abingdonii, now extinct) has been found to be most closely related to the subspecies on the islands of San Cristóbal (C. n. chathamensis) and Española (C. n. hoodensis) which lie over 300 km (190 mi) away, This discovery informed further attempts to preserve the C. n. abingdonii lineage and the search for an appropriate mate for Lonesome George, which had been penned with females from Isabela. Hope was bolstered by the discovery of a C. n. abingdonii hybrid male in the Volcán Wolf population on northern Isabela, raising the possibility that more undiscovered living Pinta descendants exist. Santa Cruz Island Mitochondrial DNA studies of tortoises on Santa Cruz show up to three genetically distinct lineages found in nonoverlapping population distributions around the regions of Cerro Montura, Cerro Fatal, and La Caseta. Although traditionally grouped into a single subspecies (C. n. porteri), the lineages are all more closely related to tortoises on other islands than to each other: Cerro Montura tortoises are most closely related to C. n. duncanensis from Pinzón, Cerro Fatal to C. n. chathamensis from San Cristóbal, Prior to the identification of this subspecies through genetic analysis, it was noted that there existed differences in shells between the Cerro Fatal tortoises and other tortoises on Santa Cruz. By classifying the Cerro Fatal tortoises into a new taxon, greater attention can be paid to protecting its habitat, according to Adalgisa Caccone, who is a member of the team making this classification. Española Island On the southern island of Española, only 14 adult tortoises were found, two males and 12 females. The tortoises apparently were not encountering one another, so no reproduction was occurring. Between 1963 and 1974, all 14 adult tortoises discovered on the island were brought to the tortoise center on Santa Cruz and a tortoise breeding program was initiated. In 1977, a third Española male tortoise was returned to Galapagos from the San Diego Zoo and joined the breeding group. After 40 years' work reintroducing captive animals, a detailed study of the island's ecosystem has confirmed it has a stable, breeding population. Where once 15 were known, now more than 1,000 giant tortoises inhabit the island of Española. One research team has found that more than half the tortoises released since the first reintroductions are still alive, and they are breeding well enough for the population to progress onward, unaided. In January 2020, it was widely reported that Diego, a 100-year-old male tortoise, resurrected 40% of the tortoise population on the island and is known as the "Playboy Tortoise". Fernandina Island The C. n. phantasticus subspecies from Fernandina was originally known from a single specimen—an old male from the voyage of 1905–06. In 2019, an elderly female specimen was finally discovered on Fernandina and transferred to a breeding center, and trace evidence found on the expedition indicates that more individuals likely exist in the wild. It has been theorized that the rarity of the subspecies may be due to the harsh habitat it survives in, such as the lava flows that are known to frequently cover the island.

The extinct Santa Fe subspecies has not yet been described and thus has no binomial name, having been identified from the limited evidence of bone fragments (but no shells, the most durable part) of 14 individuals, old eggs, and old dung found on the island in 1905–06. Later genetic studies of the bone fragments indicate that the Santa Fe subspecies was distinct, and was most closely related to C. n. hoodensis. Subspecies of doubtful existence The purported Rábida Island subspecies (C. n. wallacei) was described from a single specimen collected by the California Academy of Sciences in December 1905,

The discoverer of the Galápagos Islands, Fray Tomás de Berlanga, Bishop of Panama, wrote in 1535 of "such big tortoises that each could carry a man on top of himself." Naturalist Charles Darwin remarked after his trip three centuries later in 1835, "These animals grow to an immense size ... several so large that it required six or eight men to lift them from the ground". The largest recorded individuals have reached weights of over and lengths of . Size overlap is extensive with the Aldabra giant tortoise, however taken as a subspecies, the Galápagos tortoise seems to average slightly larger, with weights in excess of being slightly more commonplace. Weights in the larger bodied subspecies range from in mature males and from in adult females. The tortoises' gigantism was probably a trait useful on continents that was fortuitously helpful for successful colonisation of these remote oceanic islands rather than an example of evolved insular gigantism. Large tortoises would have a greater chance of surviving the journey over water from the mainland as they can hold their heads a greater height above the water level and have a smaller surface area/volume ratio, which reduces osmotic water loss. Their significant water and fat reserves would allow the tortoises to survive long ocean crossings without food or fresh water, and to endure the drought-prone climate of the islands. A larger size allowed them to better tolerate extremes of temperature due to gigantothermy. Fossil giant tortoises from mainland South America have been described that support this hypothesis of gigantism that pre-existed the colonization of islands.

C. n. abingdonii) | image2 = Chathamensis.jpg | alt2 = Tortoise of the chathamensis subspecies has a slightly saddle-shaped shell. | caption2 = Intermediate (C. n. chathamensis) | image3 = GNigrita.jpg | alt3 = Tortoise of the C. n. porteri subspecies has a rounded shell shaped like a dome. | caption3 = Domed (C. n. porteri) }} Galápagos tortoises possess two main shell forms that correlate with the biogeographic history of the subspecies group. They exhibit a spectrum of carapace morphology ranging 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 a saddleback tortoise withdraws its head and forelimbs into its shell, a large unprotected gap remains over the neck, evidence of the lack of predation during the evolution of this structure. Larger islands with humid highlands over in elevation, such as Santa Cruz, have abundant vegetation near the ground. Tortoises native to these environments tend to have domed shells and are larger, with shorter necks and limbs. Saddleback tortoises originate from small islands less than in elevation with dry habitats (e.g. Española and Pinzón) that are more limited in food and other resources. Saddlebacks are more territorial and smaller than domed varieties, possibly adaptations to limited resources. Alternatively, larger tortoises may be better-suited to high elevations because they can resist the cooler temperatures that occur with cloud cover or fog. The shell distortion and elongation of the limbs and neck in saddlebacks is probably an evolutionary compromise between the need for a small body size in dry conditions and a high vertical reach for dominance displays. Saddleback tortoises are, therefore, not necessarily more closely related to each other than to their domed counterparts, as shape is not determined by a similar genetic background, but by a similar ecological one.

The tortoises are ectothermic (cold-blooded), so they 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. They travel mostly in the early morning or late afternoon between resting and grazing areas. They have been observed to walk at a speed of .  Others have been observed sleeping 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.

Regarding their senses, Charles Darwin observed, "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." Although they are not deaf, tortoises depend far more on vision and smell as stimuli than hearing.

Tortoises share a mutualistic relationship with some subspecies of Galápagos finch and mockingbirds. The birds benefit from the food source and the tortoises get rid of irritating ectoparasites. 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. Some tortoises have been observed to exploit this mutualistic relationship to consume birds seeking to groom them. 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.

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.

Females 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.

Young animals emerge from the nest after four to eight months and may weigh only and measure . Life expectancy in the wild is thought to be over 100 years, making it one of the longest-lived species in the animal kingdom. Harriet, a specimen kept in Australia Zoo, was the oldest known Galápagos tortoise, having reached an estimated age of more than 170 years before her death in 2006. Chambers notes that Harriet was probably 169 years old in 2004, although media outlets claimed the greater age of 175 at death based on a less reliable timeline.

All subspecies of Galápagos tortoises evolved from common ancestors that arrived from mainland South America by overwater dispersal. Genetic studies have shown that the Chaco tortoise of Argentina and Paraguay is their closest living relative. Mitochondrial DNA analysis indicates that the oldest existing islands (Española and San Cristóbal) were colonised first, and that these populations seeded the younger islands via dispersal in a "stepping stone" fashion via local currents. Restricted gene flow between isolated islands then resulted in the independent evolution of the populations into the divergent forms observed in the modern subspecies. The evolutionary relationships between the subspecies thus echo the volcanic history of the islands.

aged about 30, with straight brown hair receding from his high forehead and long side-whiskers, smiling quietly, in wide lapelled jacket, waistcoat and high collar with cravat|Charles Darwin as a young man, probably subsequent to the Galápagos visit]] Charles Darwin visited the Galápagos for five weeks on the second voyage of HMS Beagle in 1835 and saw Galápagos tortoises on San Cristobal (Chatham) and Santiago (James) Islands. They appeared several times in his writings and journals, and played a role in the development of the theory of evolution. Darwin wrote in his account of the voyage: 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 ... The inhabitants, as I have said, state that they can distinguish the tortoises from the different islands; and that they differ not only in size, but in other characters. Captain Porter has described* those from Charles and from the nearest island to it, namely, Hood Island, as having their shells in front thick and turned up like a Spanish saddle, while the tortoises from James Island are rounder, blacker, and have a better taste when cooked. 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. It has been suggested that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal (C. chathamensis) and Santiago (C. 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 C. niger subspecies found there was already nearly extinct and he was unlikely to have seen any mature animals. 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. Although the British Museum had a few specimens, their provenance within the Galápagos was unknown. However, conversations with the naturalist Gabriel Bibron, who had seen the mature tortoises of the Paris Natural History Museum confirmed to Darwin that distinct varieties occurred. Darwin later compared the different tortoise forms with those of mockingbirds, in the first tentative statement linking his observations from the Galapagos with the possibility of subspecies 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 archipelagos will be well worth examining; for such facts would undermine the stability of subspecies. His views on the mutability of subspecies 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." These observations served as counterexamples to the prevailing contemporary view that subspecies were individually created. Darwin also found these "antediluvian animals" 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".

Several waves of human exploitation of the tortoises as a food source caused a decline in the total wild population from around 250,000 The subspecies C. n. niger became extinct by human exploitation in the 19th century. Another subspecies, C. n. abingdonii, became extinct on 24 June 2012 with the death in captivity of the last remaining specimen, a male named Lonesome George, the world's "rarest living creature". All the other surviving subspecies are listed by the IUCN as at least "vulnerable" in conservation status, if not worse.

An estimated 200,000 animals were taken before the 20th century. while Captain James Colnett of the Royal 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." US Navy captain David Porter declared, "after once tasting the Galapagos tortoises, every other animal food fell off greatly in our estimation ... The meat of this animal 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." In the 17th century, pirates started to use the Galápagos Islands as a base for resupply, restocking on food and water, and repairing vessels before attacking Spanish colonies on the South American mainland. However, the Galápagos tortoises did not struggle for survival at this point because the islands were distant from 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. Between the 1790s and the 1860s, whaling ships and fur sealers systematically collected tortoises in far greater numbers than the buccaneers preceding them. Some were used for food and many more were killed for high-grade "turtle oil" from the late 19th century onward for lucrative sale to continental Ecuador. 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. 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 energy source. Galápagos tortoise exploitation dramatically increased with the onset of the California Gold Rush in 1849. Tortoises and sea turtles were imported into San Francisco, Sacramento and various other Gold Rush towns throughout Alta California to feed the gold mining population. Galápagos tortoise and sea turtle bones were also recovered from the Gold Rush-era archaeological site, Thompson's Cove (CA-SFR-186H), in San Francisco, California. 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 subspecies. 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. The tortoises are down from 15 different types of subspecies when Darwin first arrived to the current 11 subspecies. Threats * Introduced mammals * Poachers * Destruction of habitat * Characteristics that make tortoises vulnerable ** Slow growth rate ** Late sexual maturity ** Can only be found on the Galápagos Islands ** Large size and slow-moving

]] The remaining subspecies of tortoise range in IUCN classification from extinct in the wild to vulnerable. Slow growth rate, late sexual maturity, and island endemism make the tortoises particularly prone to extinction without help from conservationists. and established the Charles Darwin Foundation. In 1970, capturing or removing many subspecies from the islands (including tortoises and their eggs) was banned. 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). A 1971 Ecuadorian decree made it illegal to damage, remove, alter, or disturb any organism, rock, or other natural object in the national park.

With the establishment of the Galapagos National Park and the CDF in 1959, a review of the status of the tortoise populations began. Only 11 of the 14 original populations remained and most of these were endangered if not already on the brink of extinction. The breeding and rearing program for giant tortoises began in response to the condition of the population on Pinzón, where fewer than 200 old adults were found. All of the hatchlings had been killed by introduced black rats, for perhaps more than a century. Without help, this population would eventually disappear. The only thing preserving it was the longevity of the tortoise. Its genetic resistance to the negative effects of inbreeding would be another. Fruitless attempts to breed one of the tortoises, Lonesome George for example, is speculated to be attributed to a lack of postnatal cues, and confusion over which would be the most appropriate genetic subspecies would be the most appropriate to mate him with on the islands. The 15 remaining tortoises were brought to the Charles Darwin Research Station in 1971 for a captive breeding program and, in the following 33 years, they gave rise to over 1,200 progeny which were released onto their home island and have since begun to reproduce naturally. One of the tortoises, Diego, is one of the main drivers of a remarkable recovery of the hoodensis subspecies, having fathered between 350 and 800 progeny.

The Galápagos National Park Service systematically culls feral predators and competitors. Goat eradication on islands, including Pinta, was achieved by the technique of using "Judas goats" with radio location collars to find the herds. Marksmen then shot all the goats except the Judas, and then returned weeks later to find the "Judas" and shoot the herd to which it had relocated. Goats were removed from Pinta Island after a 30-year eradication campaign, the largest removal of an insular goat population using ground-based methods. Over 41,000 goats were removed during the initial hunting effort (1971–82). This process was repeated until only the "Judas" goat remained, which was then killed. Other measures have included dog eradication from San Cristóbal, and fencing off nests to protect them from feral pigs. Sterile tortoises were released so the problem of interbreeding between subspecies would be avoided if any fertile tortoises were to be released in the future. It is hoped that with the recent identification of a hybrid C. n. abingdonii 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.

The Galapagos Tortoise Movement Ecology Programme is a collaborative project coordinated by Dr Stephen Blake of the Max Planck Institute for Ornithology. Its goal is to assist the Galapagos National Park to effectively conserve giant tortoises by conducting cutting-edge applied science, and developing an inspirational tortoise-based outreach and education programme. Since 2009, the project team have been analysing the movements of giant tortoises by tracking them via satellite tags. As of November 2014, the team have tagged 83 tortoises from four subspecies on three islands. They have established that giant tortoises conduct migrations up and down volcanoes, primarily in response to seasonal changes in the availability and quality of vegetation. In 2015 they started to track the movements of hatchling and juvenile tortoises, supported by the UK's Galapagos Conservation Trust.