AppearanceThe tortoises have a large bony carapace of a dull brown colour. The plates of the shell are fused with the ribs in a rigid protective structure that is integral to the skeleton. Lichens can grow on the shells of these slow-moving animals. Tortoises keep a characteristic scute pattern on their shell throughout life, though the annual growth bands are not useful for determining age because 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 scales. The front legs have five claws, the back legs four.
NamingModern DNA methods have revealed new information on the relationships between the subspecies:
The five populations living on the largest island, Isabela, are the ones that are the subject of most debate as to whether they are true subspecies or just distinct populations. It is widely accepted that the population living on the most northern volcano, Volcan Wolf, is genetically independent from the four other southern populations and is therefore its own subspecies. It is thought to be so genetically different from the others because there were two colonization events — one in the north from the island of Santiago resulting in the Volcan Wolf species and the second from the island of Santa Cruz resulting in the four southern populations. It is thought that they first colonized the Sierra Negra volcano, which was the first of the island's volcanoes to be formed. The tortoises then spread north to each newly formed volcano, resulting in the populations living on Volcan Alcedo and then Volcan Darwin. Recent genetic evidence shows that these two populations are genetically distinct from each other and from the population living on Sierra Negra and therefore form the subspecies ''G. vandenburghi'' and ''G. microphyes'' . The fifth population living on the most southern volcano is thought to have split off from the Sierra Negra population more recently and is therefore not as genetically different as the other two. Isabela is the most recently formed island which tortoises inhabit, therefore none of its populations have had the same amount of time to independently evolve as populations have on other islands, but according to Ciofi and associates they are all genetically different and should each be considered their own subspecies.
Phylogenetic analysis may help to "resurrect" the extinct subspecies of Floreana — a subspecies known only from subfossil remains. 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, resulting either from individuals deliberately transported between the islands, or from individuals thrown overboard from ships to lighten the load. Nine Floreana descendants have been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz; the genetic footprint was identified in the genomes of hybrid offspring. This allows the possibility of re-establishing a reconstructed subspecies from selective breeding of the hybrid animals. Furthermore, it is possible that individuals from the subspecies are still extant. Genetic analysis from a sample of tortoises from Volcan Wolf found 84 first generation ''nigra'' hybrids, some less than 15 years old. The genetic diversity of these individuals is estimated to have required 38 ''nigra'' parents, many of which could still be alive on Isabela island.
The Pinta Island subspecies has been found to be most closely related to the subspecies on the islands of San Cristóbal and Española which lie over 300 kilometers away, rather than that on the neighbouring island of Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta. This discovery informed further attempts to preserve the ''abingdoni'' lineage and the search for an appropriate mate for Lonesome George, who had been penned with females from Isabela. 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 undiscovered living Pinta descendants.
;Santa Cruz Island
Mitochondrial DNA studies of tortoises on Santa Cruz show up to three genetically distinct lineages found in non-overlapping population distributions around the regions of Cerro Monturra, Cerro Fatal and La Caseta. Although currently grouped into a single subspecies , the lineages are all more closely related to tortoises on other islands than to each other: 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.
;Subspecies of doubtful existence
Subspecies were described from three other islands, but their existence is based on scant evidence. The purported Rábida Island subspecies was described from a single specimen collected by the California Academy of Sciences in 1906, which has since been lost. This individual was probably an artificial introduction from another island that was originally penned on Rábida next to a good anchorage, as no contemporary whaling or sealing logs mention removing tortoises from this island. The ''phantastica'' subspecies from Fernandina is known from a single specimin — and old male from the voyage of 1906. No other tortoises or remains have been found on the island, suggesting the specimen was an artificial introduction from elsewhere. 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. The Santa Fe subspecies has no binomial name, having been described from the limited evidence of bone fragments of 14 individuals, old eggs and old dung found on the island in 1906. The island has never been inhabited by man nor had any introduced predators. The remains are considered artificial introductions, possibly from camping at the good anchorage on the island.
StatusSeveral waves of human exploitation of the tortoises as a food source caused a decline in the total wild population from around 250,000 when first discovered in the 16th century to a low of 3,060 individuals in a 1974 census. Modern conservation efforts have subsequently brought tortoise numbers up to 19,317 .
The subspecies ''C. n. nigra'' became extinct by human exploitation in the 19th century. Another subspecies, ''C. n. abingdoni'', is now extinct in the wild and represented in captivity by a single male specimen, Lonesome George. It is the only known living specimen of the Pinta Island tortoise and 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.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. The Galápagos giant tortoise has become a flagship species for conservation efforts throughout the Galápagos.
The Galápagos giant tortoise is now strictly protected and is listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora. The listing requires that trade in the taxon and its products are 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 and established the Charles Darwin Foundation. In 1970, capturing or removing many species from the islands 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 . A 1971 Ecuadorian decree made it illegal to damage, remove, alter or disturb any organism, rock or other natural object in the National Park.
Breeding and release programs began in 1965 and have successfully brought seven of the eight endangered subspecies up to less perilous population levels. Young tortoises are raised at 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 release onto their native ranges.
The most significant population recovery was that of the Española Tortoise , 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. 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.
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. 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.
Efforts are now underway to repopulate islands formerly inhabited by tortoises in order to restore their ecosystems to their pre-human condition. The tortoises are a keystone species, acting as ecosystem engineers which help in plant seed dispersal, trampling down brush and thinning the understory of vegetation . Birds such as Flycatchers perch on and fly around tortoises in order to hunt the insects they displace from the brush. In May 2010, 39 sterilised tortoises of hybrid origin were introduced to Pinta Island, the first tortoises there since the evacuation of Lonesome George 38 years ago in 1972. Sterile tortoises were released so that 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 ''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. This approach may be used to "re-tortoise" Floreana in the future, since captive individuals have been found to be descended from the extinct original stock.
FoodThe tortoises are herbivores that consume a diet of cacti, grasses, leaves, lichens, and berries. They have been documented to feed on ''Hippomane mancinella'' , the endemic guava ''Psidium galapageium'', the water fern ''Azolla microphylla'', and the bromeliad ''Tillandsia insularis''. A tortoise eats an average of 32–36 kilograms per day, though inefficient digestion means that much of this passes through without nutritional extraction.
Tortoises acquire most of their moisture from the dew and sap in vegetation ; therefore, they can spend long periods without drinking water. They can endure 18 months when deprived of all food and water, 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" , both of which served to make them useful water sources on ships. On arid islands, tortoises lick morning dew from boulders, and the repeated action over many generations has formed half-sphere depressions in the rock.
EvolutionAll subspecies of Galápagos tortoise evolved from common ancestors that arrived from mainland South America by overwater dispersal. The minimal founding population was a pregnant female or a breeding pair. Survival on the 1000 km oceanic journey is accounted for by the fact that the tortoises are buoyant, can breathe by extending their necks above the water, and are 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. The ancestors of the genus ''Chelonoidis'' are believed to have similarly dispersed from Africa to South America during the Oligocene.
The closest living relative of the Galápagos giant tortoise is the Argentine tortoise , a much smaller species from South America. The divergence between ''C. chilensis'' and ''C. nigra'' probably occurred 6–12 million years ago, an evolutionary event preceding the volcanic formation of the oldest modern Galápagos Islands 5 million years ago. Mitochondrial DNA analysis indicates that the oldest existing islands 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.
Charles Darwin visited the Galápagos for five weeks on the second voyage of HMS ''Beagle'' in 1835 and saw Galápagos tortoises on San Cristobal and Santiago Islands....hieroglyph snipped... 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".
Even though the ''Beagle'' departed from the Galápagos with over 30 adult tortoises on deck, these weren't for scientific study but a source of fresh meat for the Pacific crossing. Their shells and bones were thrown overboard, leaving no remains with which to test any hypotheses. It has been suggested that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal and Santiago , 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.
However, Darwin did have four live juvenile specimens to compare from different islands. These were pet tortoises taken by himself , his captain FitzRoy and his servant Syms Covington . 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 there were distinct varieties....hieroglyph snipped...
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 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 archipelagos will be well worth examining; for such facts would undermine the stability of species."...hieroglyph snipped...
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."...hieroglyph snipped... These observations served as counterexamples to the prevailing contemporary view that species 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".
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