Galápagos tortoise
Encyclopedia
The Galápagos tortoise or Galápagos giant tortoise
(Chelonoidis nigra) is the largest living species of tortoise
, reaching weights of over 400 kg (881.8 lb) and lengths of over 1.8 metres (5.9 ft). With life spans in the wild of over 100 years, it is one of the longest-lived vertebrates. A captive individual lived at least 170 years.
The tortoise is native to seven of the Galápagos Islands
, a volcanic archipelago
about 1000 km (621.4 mi) west of the Ecuador
ian mainland. Spanish explorers who discovered the islands in the 16th century named them after the Spanish galápago, meaning tortoise.
Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks. On islands with dry lowlands, the tortoises are smaller, with "saddleback" shells and long necks. These island-to-island differences played a role in the inception of Charles Darwin's theory of evolution
.
Tortoise numbers declined from over 250,000 in the 16th century to a low of around 3,000 in the 1970s. The decline was caused by hunting for tortoise meat and oil, habitat clearance
for agriculture, and introduction of non-native animals such as rats, goats, and pigs. Ten subspecies
of the original fifteen survive in the wild. An eleventh subspecies (C. n. abingdoni) has only a single living individual, in captivity, nicknamed Lonesome George
. Conservation efforts beginning in the 20th century have resulted in thousands of captive-bred
juveniles being released onto their home islands, and it is estimated that numbers exceeded 19,000 at the start of the 21st century. Despite this rebound, the species as a whole is classified as "Vulnerable" by the International Union for Conservation of Nature (IUCN).
The Galápagos Islands were discovered in 1535, but first appeared on maps, of Gerardus Mercator
and Abraham Ortelius
, in about 1570. The islands were named "Insulae de los Galopegos" (Islands of the Tortoises) in reference to the giant tortoises found there.The first navigation chart showing the individual islands was done by the pirate Ambrose Cowley
in 1684. He named them after fellow pirates or English noblemen. More recently, the Ecuadorian government gave most of the islands Spanish names. While the Spanish names are official, many researchers continue to use the older English names, particularly as those were the names used when Darwin visited. This article uses the Spanish island names.
Initially, the giant tortoise
s of the Indian Ocean and those from the Galápagos were considered to be the same species. Naturalists thought that sailors had transported the tortoises. In 1676, the pre-Linnaean authority Claude Perrault
referred to both species 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 species, which they named Testudo nigrita ("black tortoise").
Recognition of subpopulations
The first systematic survey of giant tortoises was by Albert Günther of the British Museum
, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles
, and four from the Mascarenes
. Günther theorised that all the giant tortoises descended from a single ancestral population which spread by sunken land bridge
s. This theory was later disproven by the understanding that the Galápagos, Seychelles and Mascarene islands are all of recent volcanic origin and therefore could not have been linked by land bridges. It is now thought that the Galápagos tortoises descended from an ancestor from South America. The Indian Ocean tortoises derived from Madagascar
.
At the end of the 19th century, Georg Baur and Walter Rothschild
recognised five more populations of Galápagos tortoise. In 1906, the Academy of Sciences
collected specimens and gave them to John Van Denburgh
for study. He identified four additional populations, and proposed the existence of 15 species. His list still guides the taxonomy
of the Galápagos tortoise, though now ten populations are thought to have existed.
Current species and genus names
The current species designation of nigra ("black" – Quoy
& Gaimard
, 1824b) was resurrected in 1984 after it was discovered to be the senior synonym (an older taxonomic synonym taking historical precedence) for the then commonly used species name of elephantopus ("elephant footed" – Harlan
, 1827). The use of nigra is explained by Quoy and Gaimard's Latin description: "Testudo toto corpore nigro", meaning "tortoise with completely black body". Quoy and Gairmard described nigra from a living specimen, but there is no evidence that they knew of its accurate provenance within the Galápagos – the locality was in fact given as California. Garman proposed the linking of nigra with the extinct Floreana subspecies. Later, Pritchard deemed it was convenient to accept this designation despite its tenuousness because this decision allowed minimal disruption to the already-confused nomenclature of the species. The even more senior species synonym of californiana ("californian" – Quoy
& Gaimard
, 1824a) is considered a nomen oblitum
("forgotten name").
Previously, the Galápagos tortoise was considered to belong to the genus Geochelone
, known as typical tortoises or terrestrial turtles. Subsequently, subgenus Chelonoidis
was elevated to generic status based on 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.
Subspecies
The exact number of subspecies
of Chelonoidis nigra that have existed in history is still under debate among scientists, but some recognize up to 15. Only ten subspecies now exist in the wild, one on Santiago
, Santa Cruz
, San Cristóbal
, Pinzón
and Española
, and five on Isabela
, although this number is controversial and may be fewer. An eleventh surviving subspecies, abingdoni from Pinta Island, is considered extinct in the wild
and is represented by a single living specimen, "Lonesome George". The subspecies inhabiting Floreana island (C. n. nigra) is thought to have been hunted to extinction by 1850, only years after Charles Darwin
's landmark visit of 1835 in which he saw shells but no live tortoises on the island. The phantastica subspecies of Fernandina is of disputed existence as it was described from a single specimen which may have been an artificial introduction to the island.
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, 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 and captives in mixed herds normally direct courtship only toward members of the same race.
The valid scientific names of each of the individual populations are not universally accepted, and some researchers consider each subspecies to be a full species. The taxonomic status of the various races is not fully resolved.
. 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 closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Argentine tortoise
(Chelonoidis chilensis), 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 (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.
Isabela Island
The five populations living on the largest island, Isabela are the ones that are the most highly debated on whether they are true subspecies or just distinct populations. It is widely accepted that the population living on the most northern volcano, Volcan Wolf are genetically independent from the four other southern populations and are therefore their own subspecies. They are 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 (G. becki) and the second from the island of Santa Cruz resulting in the four southern populations. It is thought that they were first colonized on the Sierra Negra volcano, which was the first volcano formed on the island. The tortoises then spread north to each subsequently forming volcano, resulting in the populations living on Volcan Alcedo and then Volcan Darwin. Recent genetic evidence shows these two populations are genetically distinct from each other and from the population living on Sierra Negra and therefore form the subspecies G. vandenburghi (Alcedo) and G. microphyes (Darwin). 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 youngest island which tortoises inhabit, therefore none of them have had the same amount of time to independently evolve as populations have one other islands, but according to Ciofi and associates they are all genetically different and should each be considered their own subspecies.
Floreana Island
Phylogenetic analysis may help to "resurrect" the extinct subspecies of Floreana (nigra). The subspecies was only known 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 speculated to be caused either by deliberate moving between the islands or from individuals thrown overboard ships to lighten loads. Nine Floreana descendants have been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz. This permits the possibility of re-establishing a reconstructed subspecies
from selective breeding
of the hybrid animals.
Pinta Island
The Pinta Island subspecies (abingdoni, now extinct in the wild) is most closely related to the subspecies on the islands of San Cristóbal (chathamensis) and Española (hoodensis) which lie over 300 kilometres (186.4 mi) away, rather than neighbouring Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta. 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. 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.
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 united in a single subspecies (porteri), 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 Rabida island subspecies (wallacei) was described from a single specimen removed by the 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 Rabida 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 specimen from the voyage of 1906, an old male. 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 (but no shells, the most durable part) 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.
) 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
s can grow on the shells of the slow-moving animals. Tortoises keep a characteristic scute
(shell segment) 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, stumpy, with dry scaly skin and hard scales
. The front legs are five-clawed, and the back legs are four-clawed.
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 400 kilograms (881.8 lb) and lengths of 1.87 metres (6.1 ft). The tortoises' gigantism was probably a preadapted condition 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 preadapted gigantism.
Evolutionary implications
In combination with proportionally longer necks and limbs, 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 Opuntia
(prickly pear) cactus that grows in arid environments. Saddlebacks are more territorial 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.
A competing hypothesis is that rather than being principally a feeding adaptation, the distinctive saddle shape and longer extremities might have been a secondary sexual characteristic of saddleback males. Male competition over mates is settled by dominance displays on the basis of vertical neck height rather than body size (see below). This correlates with the observation that saddleback males are more aggressive than domed males. 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 dry conditions and a high vertical reach for dominance displays.
The saddleback carapace probably evolved independently several times in dry habitats, since genetic similarity between populations does not correspond to carapace shape. Saddleback tortoises are therefore not necessarily more closely related to each other than to domed counterparts, as shape is not determined by a similar genetic background, but by a similar ecological one.
Sexual dimorphism
Sexual dimorphism
is most pronounced in saddleback populations, in which males have more angled and higher front openings, giving a more extreme saddled appearance. 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 larger than females: adult males weigh around 272 – while females are 136 –.
The tortoises are ectotherm
ic (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. 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 0.3 kilometre per hour (0.186411357820329 mph).
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 2000 ft (609.6 m)) in the dry season. The same routes have been used for many generations, creating well-defined paths through the undergrowth known as "tortoise highways". 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.
Tortoises sometimes rest in mud wallows or rain-formed pools, which may be both a thermoregulatory
response during cool nights, and a protection from parasites
such as mosquitoes and ticks. Parasites are countered by taking dust baths in loose soil. Tortoises have been noted to shelter at night under overhanging rocks. 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.
Diet
The tortoises are herbivore
s that consume a diet of cacti, grasses, leaves, lichens, and berries. They have been documented to feed on Hippomane mancinella
('poison apple'), the endemic guava
Psidium galapageium, the water fern
Azolla microphylla, and the bromeliad
Tillandsia insularis. A tortoise eats an average of 32 – 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 (particularly the Opuntia
cactus); 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" (the pericardium
), both of which used 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.
Senses
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." Although they are not deaf, tortoises depend far more on vision and smell as stimuli.
Mutualism
Tortoises share a mutualistic relationship with some species of 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, cloaca
l opening, and skin between plastron and carapace. The birds benefit from the food source and the tortoises get rid of irritating ectoparasites.
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.
Mating
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. 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.
The prelude to mating can be very aggressive, as the male forcefully rams the female's shell with his own and nips her legs. 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. During mating, the male vocalises with hoarse bellows and grunts, described as "rhythmic groans". This is one of the few vocalisations the tortoise makes; other noises are made during aggressive encounters, when struggling to right themselves, and hissing as they withdraw into their shells due to the forceful expulsion of air.
Egg-laying
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. It is carried out blindly using only the hind legs to dig a 30 cm (11.8 in) deep cylindrical hole, into which she lays up to sixteen spherical, hard-shelled eggs ranging from 82 to 157 g (2.9 to 5.5 ) in mass, and the size of a billiard ball. 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). The female makes a muddy plug for the nest hole out of soil mixed with urine, seals the nest by pressing down firmly with her plastron, and leaves them to be incubated by the sun. Females may lay 1–4 clutches per season. 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).
Early life and maturation
Young animals emerge from the nest after four to eight months and may weigh only 50 grams (1.8 oz) and measure 6 centimetres (2.4 in). 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. 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 their first 10–15 years, 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". The hawk is now much rarer, but introduced feral pigs, dogs, cats and black rats have become predators of eggs and young tortoises. 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".
Sex can be determined only when the tortoise is about 15 years old, and sexual maturity
is reached at around 20–25 years 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, making it one of the longest lived species in the animal kingdom. Harriet was the oldest known Galápagos tortoise, having reached an estimated age of more than 170 years before her death in 2006 in Australia Zoo
. Chambers notes that Harriet was probably 169 years old in 2004. The individual died in 2006 (although media outlets claimed the greater age of 175 based on a less reliable timeline.
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:
The significance of the differences in tortoises between islands did not strike him as important until it was too late, as he continued,
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. It has been suggested that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal (chathamensis) and Santiago (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.
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). 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.
Darwin later compared the different tortoise forms with those of mockingbird
s, in the first tentative statement linking his observations from the Galapagos with the possibility of species transmuting:
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." 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".
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.
wrote that "They are so extraordinarily large and fat, and so sweet, that no pullet eats more pleasantly," while 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." 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 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." 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."
In the 17th century, pirates started to use the Galápagos Islands as a base for resupply, restocking on food, 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 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.
Between the 1790s and the 1860s, whaling
ships and fur-sealers systematically collected tortoises in far greater numbers than the buccaneers preceding them. They 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.
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. Feral pigs, dogs, cats and black rat
s have become predators of eggs and young tortoises, whilst goats, donkeys and cattle compete for grazing and trample nest sites. The extinction of the Floreana subspecies in the mid-19th century has been attributed to the combined pressures of hunting for the penal colony on the relatively small island, the conversion of the grazing highlands into land for farming and fruit plantations, and the introduction of feral mammals.
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.
to vulnerable
. Slow growth rate, late sexual maturity and island endemism make the tortoises particularly prone to extinction without the efforts of conservationists. The Galápagos giant tortoise has become a flagship species
for conservation efforts throughout the Galápagos.
Legal protection
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 (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.
Captive breeding
Breeding and releasing 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 (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. They were brought to the Charles Darwin Research Station
in 1971 for a captive breeding program. In the following 33 years, these 15 tortoises gave rise to over 1,200 tortoises released onto their home island, which themselves have begun to reproduce naturally.
Island restoration
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 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. 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 (island restoration
) to their pre-human condition. The tortoises are a keystone species
, acting as ecosystem engineer
s 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 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.
Giant tortoise
Giant tortoises are characteristic reptiles of certain tropical islands. Often reaching enormous size—they can weigh as much as 300 kg and can grow to be 1.3 m long—they live, or lived , in the Seychelles, the Mascarenes and the Galapagos...
(Chelonoidis nigra) is the largest living species of tortoise
Tortoise
Tortoises are a family of land-dwelling reptiles of the order of turtles . Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise...
, reaching weights of over 400 kg (881.8 lb) and lengths of over 1.8 metres (5.9 ft). With life spans in the wild of over 100 years, it is one of the longest-lived vertebrates. A captive individual lived at least 170 years.
The tortoise is native to seven of the Galápagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
, a volcanic archipelago
Archipelago
An archipelago , sometimes called an island group, is a chain or cluster of islands. The word archipelago is derived from the Greek ἄρχι- – arkhi- and πέλαγος – pélagos through the Italian arcipelago...
about 1000 km (621.4 mi) west of the Ecuador
Ecuador
Ecuador , officially the Republic of Ecuador is a representative democratic republic in South America, bordered by Colombia on the north, Peru on the east and south, and by the Pacific Ocean to the west. It is one of only two countries in South America, along with Chile, that do not have a border...
ian mainland. Spanish explorers who discovered the islands in the 16th century named them after the Spanish galápago, meaning tortoise.
Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks. On islands with dry lowlands, the tortoises are smaller, with "saddleback" shells and long necks. These island-to-island differences played a role in the inception of Charles Darwin's theory of evolution
Inception of Darwin's theory
The inception of Darwin's theory occurred during an intensively busy period which began when Charles Darwin returned from the survey voyage of the Beagle, with his reputation as a fossil collector and geologist already established...
.
Tortoise numbers declined from over 250,000 in the 16th century to a low of around 3,000 in the 1970s. The decline was caused by hunting for tortoise meat and oil, habitat clearance
Habitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...
for agriculture, and introduction of non-native animals such as rats, goats, and pigs. Ten subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
of the original fifteen survive in the wild. An eleventh subspecies (C. n. abingdoni) has only a single living individual, in captivity, nicknamed Lonesome George
Lonesome George
Lonesome George is a tortoise, the last known individual of the Pinta Island tortoise , which is one of eight to fifteen extant subspecies of Galápagos tortoise, all of which are native to the Galápagos Islands...
. Conservation efforts beginning in the 20th century have resulted in thousands of captive-bred
Captive breeding
Captive breedingis the process of breeding animals in human controlled environments with restricted settings, such as wildlife reserves, zoos and other conservation facilities; sometimes the process is construed to include release of individual organisms to the wild, when there is sufficient...
juveniles being released onto their home islands, and it is estimated that numbers exceeded 19,000 at the start of the 21st century. Despite this rebound, the species as a whole is classified as "Vulnerable" by the International Union for Conservation of Nature (IUCN).
Taxonomy
Early classificationThe Galápagos Islands were discovered in 1535, but first appeared on maps, of Gerardus Mercator
Gerardus Mercator
thumb|right|200px|Gerardus MercatorGerardus Mercator was a cartographer, born in Rupelmonde in the Hapsburg County of Flanders, part of the Holy Roman Empire. He is remembered for the Mercator projection world map, which is named after him...
and Abraham Ortelius
Abraham Ortelius
thumb|250px|Abraham Ortelius by [[Peter Paul Rubens]]Abraham Ortelius thumb|250px|Abraham Ortelius by [[Peter Paul Rubens]]Abraham Ortelius (Abraham Ortels) thumb|250px|Abraham Ortelius by [[Peter Paul Rubens]]Abraham Ortelius (Abraham Ortels) (April 14, 1527 – June 28,exile in England to take...
, in about 1570. The islands were named "Insulae de los Galopegos" (Islands of the Tortoises) in reference to the giant tortoises found there.The first navigation chart showing the individual islands was done by the pirate Ambrose Cowley
Ambrose Cowley
William Ambrosia Cowley was a 17th century English buccaneer who surveyed the Galápagos Islands during his circumnavigation of the world, and published the first chart of the islands in 1684...
in 1684. He named them after fellow pirates or English noblemen. More recently, the Ecuadorian government gave most of the islands Spanish names. While the Spanish names are official, many researchers continue to use the older English names, particularly as those were the names used when Darwin visited. This article uses the Spanish island names.
Initially, the giant tortoise
Giant tortoise
Giant tortoises are characteristic reptiles of certain tropical islands. Often reaching enormous size—they can weigh as much as 300 kg and can grow to be 1.3 m long—they live, or lived , in the Seychelles, the Mascarenes and the Galapagos...
s of the Indian Ocean and those from the Galápagos were considered to be the same species. Naturalists thought that sailors had transported the tortoises. In 1676, the pre-Linnaean authority Claude Perrault
Claude Perrault
Claude Perrault is best known as the architect of the eastern range of the Louvre Palace in Paris , but he also achieved success as a physician and anatomist, and as an author, who wrote treatises on physics and natural history.Perrault was born and died in Paris...
referred to both species as Tortue des Indes. In 1783, Johann Gottlob Schneider
Johann Gottlob Schneider
Johann Gottlob Theaenus Schneider was a German classicist and naturalist.-Biography:Schneider was born at Collm in Saxony...
classified all giant tortoises as Testudo indica ("Indian tortoise"). In 1812, August Friedrich Schweigger
August Friedrich Schweigger
August Friedrich Schweigger was a German naturalist born in Erlangen. He was the younger brother of scientist Johann Salomo Christoph Schweigger ....
named them Testudo gigantea ("gigantic tortoise"). In 1834, André Marie Constant Duméril
André Marie Constant Duméril
André Marie Constant Duméril was a French zoologist. He was professor of anatomy at the Muséum National d'Histoire Naturelle from 1801 to 1812, when he became professor of herpetology and ichthyology...
and Gabriel Bibron
Gabriel Bibron
Gabriel Bibron was a French zoologist. He was born in Paris. Son of an employee of the Museum national d'histoire naturelle, he had a good foundation in natural history and was hired to collect vertebrates in Italy and Sicily. He classified a number of reptile species with André Marie Constant...
classified the Galápagos tortoises as a separate species, which they named Testudo nigrita ("black tortoise").
Recognition of subpopulations
The first systematic survey of giant tortoises was by Albert Günther of the British Museum
British Museum
The British Museum is a museum of human history and culture in London. Its collections, which number more than seven million objects, are amongst the largest and most comprehensive in the world and originate from all continents, illustrating and documenting the story of human culture from its...
, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles
Seychelles
Seychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....
, and four from the Mascarenes
Mascarene Islands
The Mascarene Islands is a group of islands in the Indian Ocean east of Madagascar comprising Mauritius, Réunion, Rodrigues, Cargados Carajos shoals, plus the former islands of the Saya de Malha, Nazareth and Soudan banks...
. Günther theorised that all the giant tortoises descended from a single ancestral population which spread by sunken land bridge
Land bridge
A land bridge, in biogeography, is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonise new lands...
s. This theory was later disproven by the understanding that the Galápagos, Seychelles and Mascarene islands are all of recent volcanic origin and therefore could not have been linked by land bridges. It is now thought that the Galápagos tortoises descended from an ancestor from South America. The Indian Ocean tortoises derived from Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
.
At the end of the 19th century, Georg Baur and Walter Rothschild
Walter Rothschild, 2nd Baron Rothschild
Lionel Walter Rothschild, 2nd Baron Rothschild, Baron de Rothschild FRS , a scion of the Rothschild family, was a British banker, politician, and zoologist.-Biography:...
recognised five more populations of Galápagos tortoise. In 1906, the Academy of Sciences
California Academy of Sciences
The California Academy of Sciences is among the largest museums of natural history in the world. The academy began in 1853 as a learned society and still carries out a large amount of original research, with exhibits and education becoming significant endeavors of the museum during the twentieth...
collected specimens and gave them to John Van Denburgh
John Van Denburgh
John Van Denburgh was a U.S. herpetologist from California.He was born in San Francisco and enrolled at Stanford University in 1891. As of 1895, he organized the herpetology department of the California Academy of Sciences. In 1897 he got a Ph.D. from Stanford University. Additionally, he got an M.D...
for study. He identified four additional populations, and proposed the existence of 15 species. His list still guides the taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
of the Galápagos tortoise, though now ten populations are thought to have existed.
Current species and genus names
The current species designation of nigra ("black" – Quoy
Jean René Constant Quoy
Jean René Constant Quoy was a French zoologist.Along with Joseph Paul Gaimard he served as naturalist aboard La Coquille under Louis Isidore Duperrey during its circumnavigation of the globe , and the Astrolabe under the command of Jules Dumont d'Urville...
& Gaimard
Joseph Paul Gaimard
Joseph Paul Gaimard was a French naval surgeon and naturalist.Along with Jean René Constant Quoy he served as naturalist on the ships L'Uranie under Louis de Freycinet 1817-1820, and L'Astrolabe under Jules Dumont d'Urville 1826-1829...
, 1824b) was resurrected in 1984 after it was discovered to be the senior synonym (an older taxonomic synonym taking historical precedence) for the then commonly used species name of elephantopus ("elephant footed" – Harlan
Richard Harlan
Richard Harlan was an American naturalist, zoologist, physicist and paleontologist....
, 1827). The use of nigra is explained by Quoy and Gaimard's Latin description: "Testudo toto corpore nigro", meaning "tortoise with completely black body". Quoy and Gairmard described nigra from a living specimen, but there is no evidence that they knew of its accurate provenance within the Galápagos – the locality was in fact given as California. Garman proposed the linking of nigra with the extinct Floreana subspecies. Later, Pritchard deemed it was convenient to accept this designation despite its tenuousness because this decision allowed minimal disruption to the already-confused nomenclature of the species. The even more senior species synonym of californiana ("californian" – Quoy
Jean René Constant Quoy
Jean René Constant Quoy was a French zoologist.Along with Joseph Paul Gaimard he served as naturalist aboard La Coquille under Louis Isidore Duperrey during its circumnavigation of the globe , and the Astrolabe under the command of Jules Dumont d'Urville...
& Gaimard
Joseph Paul Gaimard
Joseph Paul Gaimard was a French naval surgeon and naturalist.Along with Jean René Constant Quoy he served as naturalist on the ships L'Uranie under Louis de Freycinet 1817-1820, and L'Astrolabe under Jules Dumont d'Urville 1826-1829...
, 1824a) is considered a nomen oblitum
Nomen oblitum
A nomen oblitum is a technical term, used in zoological nomenclature, for a particular kind of disused scientific name....
("forgotten name").
Previously, the Galápagos tortoise was considered to belong to the genus Geochelone
Geochelone
Geochelone is a genus of tortoises.Geochelone tortoises, which are also known as typical tortoises or terrestrial turtles, can be found in Africa and Asia. They primarily eat plants.The genus consists of three extant species:...
, known as typical tortoises or terrestrial turtles. Subsequently, subgenus Chelonoidis
Chelonoidis
Chelonoidis is a genus of turtles in the tortoise family. They are found in South America and the Galápagos Islands. They were formerly assigned to Geochelone, but a recent comparative genetic analysis has indicated that they are actually most closely related to African hingeback tortoises. Their...
was elevated to generic status based on phylogenetic evidence which grouped the South American members of Geochelone into an independent clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
(branch of the tree of life
Tree of life
The concept of a tree of life, a many-branched tree illustrating the idea that all life on earth is related, has been used in science , religion, philosophy, mythology, and other areas...
). This nomenclature has been adopted by several authorities.
Subspecies
The exact number of subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
of Chelonoidis nigra that have existed in history is still under debate among scientists, but some recognize up to 15. Only ten subspecies now exist in the wild, one on Santiago
Santiago Island (Galápagos)
Santiago Island is an island of the Galápagos Islands. It is also known as San Salvador, named after the first island discovered by Columbus in the Caribbean Sea , or as James Island. The island, which consists of two overlapping volcanoes, has an area of 585 km² and a maximum altitude of...
, Santa Cruz
Santa Cruz Island (Galápagos)
Santa Cruz Island is one of the Galápagos Islands with an area of and a maximum altitude of .Situated in the center of the archipelago, Santa Cruz is the second largest island after Isabela. Its capital is Puerto Ayora, the most populated urban centre in the islands. On Santa Cruz there are some...
, San Cristóbal
San Cristóbal Island
San Cristóbal is the easternmost island in the Galápagos archipelago, and one of the oldest geologically.Its Spanish name "San Cristóbal" comes from the patron saint of seafarers, St. Christopher...
, Pinzón
Pinzón Island
Pinzón Island, sometimes called Duncan Island , is an island in the Galápagos Islands, Ecuador.Pinzón is home to giant tortoises, sea lions and other endemic species...
and Española
Española Island
Española Island is part of the Galápagos Islands. The English named it Hood Island after Viscount Samuel Hood. It is located in the extreme southeast of the archipelago and is considered, along with Santa Fe, one of the oldest, at approximately four million years...
, and five on Isabela
Isabela Island (Ecuador)
Isabela Island is the largest island of the Galápagos with an area of , and length of nearly 4 times larger than Santa Cruz, the next largest of the islands. This island was named in honor of Queen Isabella of Spain, who sponsored the voyage of Columbus. . By the English, it was named Albemarle...
, although this number is controversial and may be fewer. An eleventh surviving subspecies, abingdoni from Pinta Island, is considered extinct in the wild
Extinct in the Wild
Extinct in the Wild is a conservation status assigned to species or lower taxa, the only known living members of which are being kept in captivity or as a naturalized population outside its historic range.-Examples:...
and is represented by a single living specimen, "Lonesome George". The subspecies inhabiting Floreana island (C. n. nigra) is thought to have been hunted to extinction by 1850, only years after Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
's landmark visit of 1835 in which he saw shells but no live tortoises on the island. The phantastica subspecies of Fernandina is of disputed existence as it was described from a single specimen which may have been an artificial introduction to the island.
Prior to widespread knowledge of the differences between the populations (sometimes called races
Race (biology)
In biology, races are distinct genetically divergent populations within the same species with relatively small morphological and genetic differences. The populations can be described as ecological races if they arise from adaptation to different local habitats or geographic races when they are...
) from different islands and volcanoes, 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 and captives in mixed herds normally direct courtship only toward members of the same race.
The valid scientific names of each of the individual populations are not universally accepted, and some researchers consider each subspecies to be a full species. The taxonomic status of the various races is not fully resolved.
Evolutionary history
All subspecies of Galápagos tortoise evolved from common ancestors that arrived from mainland South America by overwater dispersalBiological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
. 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
Humboldt Current
The Humboldt Current , also known as the Peru Current, is a cold, low-salinity ocean current that flows north-westward along the west coast of South America from the southern tip of Chile to northern Peru. It is an eastern boundary current flowing in the direction of the equator, and can extend...
, which diverts westwards towards the Galápagos Islands from the mainland.
The closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Argentine tortoise
Argentine Tortoise
The Argentine tortoise is a species of turtle in the family Testudinidae. It is found in Argentina.- References :* Tortoise & Freshwater Turtle Specialist Group 1996. . Downloaded on 29 July 2007. Out of date.Bibliography...
(Chelonoidis chilensis), 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
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
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
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
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.
Subspecies genetics
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 most highly debated on whether they are true subspecies or just distinct populations. It is widely accepted that the population living on the most northern volcano, Volcan Wolf are genetically independent from the four other southern populations and are therefore their own subspecies. They are 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 (G. becki) and the second from the island of Santa Cruz resulting in the four southern populations. It is thought that they were first colonized on the Sierra Negra volcano, which was the first volcano formed on the island. The tortoises then spread north to each subsequently forming volcano, resulting in the populations living on Volcan Alcedo and then Volcan Darwin. Recent genetic evidence shows these two populations are genetically distinct from each other and from the population living on Sierra Negra and therefore form the subspecies G. vandenburghi (Alcedo) and G. microphyes (Darwin). 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 youngest island which tortoises inhabit, therefore none of them have had the same amount of time to independently evolve as populations have one other islands, but according to Ciofi and associates they are all genetically different and should each be considered their own subspecies.
Floreana Island
Phylogenetic analysis may help to "resurrect" the extinct subspecies of Floreana (nigra). The subspecies was only known from subfossil
Subfossil
Subfossil refers to remains whose fossilization process is not complete, either for lack of time or because the conditions in which they were buried were not optimal for fossilization....
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 speculated to be caused either by deliberate moving between the islands or from individuals thrown overboard ships to lighten loads. Nine Floreana descendants have been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz. This permits the possibility of re-establishing a reconstructed subspecies
Breeding back
Breeding back can be described as either a natural or a human attempt to assemble or re-assemble the genes of an extinct species, subspecies or domesticated breed, which may still be present in the larger gene pool of the overall species or those of multiple interbreedable species.Breeding back is...
from selective breeding
Selective breeding
Selective breeding is the process of breeding plants and animals for particular genetic traits. Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties,...
of the hybrid animals.
Pinta Island
The Pinta Island subspecies (abingdoni, now extinct in the wild) is most closely related to the subspecies on the islands of San Cristóbal (chathamensis) and Española (hoodensis) which lie over 300 kilometres (186.4 mi) away, rather than neighbouring Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta. 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. 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.
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 united in a single subspecies (porteri), 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 Rabida island subspecies (wallacei) was described from a single specimen removed by the Academy of Sciences
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
in 1906, which has since been lost. This individual was probably an artificial introduction from another island that was originally penned on Rabida 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 specimen from the voyage of 1906, an old male. 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
Feral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
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 (but no shells, the most durable part) 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.
Description
The tortoises have a large bony shell (carapaceCarapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
) 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
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
s can grow on the shells of the slow-moving animals. Tortoises keep a characteristic scute
Scute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...
(shell segment) 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, stumpy, with dry scaly skin and hard scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
. The front legs are five-clawed, and the back legs are four-clawed.
Gigantism
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 DarwinCharles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
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 400 kilograms (881.8 lb) and lengths of 1.87 metres (6.1 ft). The tortoises' gigantism was probably a preadapted condition 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
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
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
Gigantothermy
Gigantothermy is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their smaller surface area to volume ratio...
. Fossil giant tortoises from mainland South America have been described that support this hypothesis of preadapted gigantism.
Shell shape
Galapágos tortoises are the only lineage of giant tortoise exhibiting different types of shell shape. 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 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 highlands over 800 metres (2,624.7 ft) in elevation, such as Santa Cruz have abundant vegetation near the ground. 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 metres (1,640.4 ft) in elevation with dry habitats (e.g. Española and Pinzón) that are more limited in food and other resources.Evolutionary implications
In combination with proportionally longer necks and limbs, 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 Opuntia
Opuntia
Opuntia, also known as nopales or paddle cactus , is a genus in the cactus family, Cactaceae.Currently, only prickly pears are included in this genus of about 200 species distributed throughout most of the Americas. Chollas are now separated into the genus Cylindropuntia, which some still consider...
(prickly pear) cactus that grows in arid environments. Saddlebacks are more territorial 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.
A competing hypothesis is that rather than being principally a feeding adaptation, the distinctive saddle shape and longer extremities might have been a secondary sexual characteristic of saddleback males. Male competition over mates is settled by dominance displays on the basis of vertical neck height rather than body size (see below). This correlates with the observation that saddleback males are more aggressive than domed males. 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 dry conditions and a high vertical reach for dominance displays.
The saddleback carapace probably evolved independently several times in dry habitats, since genetic similarity between populations does not correspond to carapace shape. Saddleback tortoises are therefore not necessarily more closely related to each other than to domed counterparts, as shape is not determined by a similar genetic background, but by a similar ecological one.
Sexual dimorphism
Sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
is most pronounced in saddleback populations, in which males have more angled and higher front openings, giving a more extreme saddled appearance. 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 larger than females: adult males weigh around 272 – while females are 136 –.
Behaviour
RoutineThe tortoises are ectotherm
Ectotherm
An ectotherm, from the Greek εκτός "outside" and θερμός "hot", refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...
ic (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. 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 0.3 kilometre per hour (0.186411357820329 mph).
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 2000 ft (609.6 m)) in the dry season. The same routes have been used for many generations, creating well-defined paths through the undergrowth known as "tortoise highways". 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.
Tortoises sometimes rest in mud wallows or rain-formed pools, which may be both a thermoregulatory
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
response during cool nights, and a protection from parasites
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
such as mosquitoes and ticks. Parasites are countered by taking dust baths in loose soil. Tortoises have been noted to shelter at night under overhanging rocks. 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.
Diet
The tortoises are herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s that consume a diet of cacti, grasses, leaves, lichens, and berries. They have been documented to feed on Hippomane mancinella
Manchineel tree
The Manchineel tree, Hippomane mancinella, is a species of flowering plant in the spurge family , native to Florida in the United States, the Bahamas, the Caribbean, Central America, and northern South America...
('poison apple'), the endemic guava
Guava
Guavas are plants in the myrtle family genus Psidium , which contains about 100 species of tropical shrubs and small trees. They are native to Mexico, Central America, and northern South America...
Psidium galapageium, the water fern
Mosquito fern
Azolla is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like conventional ferns but more resembling duckweed or some mosses.-Selected species:Azolla caroliniana Willd. [ Poss. synon...
Azolla microphylla, and the bromeliad
Tillandsia
Tillandsia is a genus of around 540 species in the Bromeliad family , found in the forests, mountains, and deserts, of Central and South America, and Mexico and the southern United States in North America....
Tillandsia insularis. A tortoise eats an average of 32 – 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 (particularly the Opuntia
Opuntia
Opuntia, also known as nopales or paddle cactus , is a genus in the cactus family, Cactaceae.Currently, only prickly pears are included in this genus of about 200 species distributed throughout most of the Americas. Chollas are now separated into the genus Cylindropuntia, which some still consider...
cactus); 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" (the pericardium
Pericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...
), both of which used 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.
Senses
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." Although they are not deaf, tortoises depend far more on vision and smell as stimuli.
Mutualism
Tortoises share a mutualistic relationship with some species of Galápagos finch
Darwin's finches
Darwin's finches are a group of 14 or 15 species of passerine birds. It is still not clear which bird family they belong to, but they are not related to the true finches. They were first collected by Charles Darwin on the Galápagos Islands during the second voyage of the Beagle...
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, cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
l opening, and skin between plastron and carapace. The birds benefit from the food source and the tortoises get rid of irritating ectoparasites.
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.
Mating
Mating
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
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. 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.
The prelude to mating can be very aggressive, as the male forcefully rams the female's shell with his own and nips her legs. 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. During mating, the male vocalises with hoarse bellows and grunts, described as "rhythmic groans". This is one of the few vocalisations the tortoise makes; other noises are made during aggressive encounters, when struggling to right themselves, and hissing as they withdraw into their shells due to the forceful expulsion of air.
Egg-laying
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. It is carried out blindly using only the hind legs to dig a 30 cm (11.8 in) deep cylindrical hole, into which she lays up to sixteen spherical, hard-shelled eggs ranging from 82 to 157 g (2.9 to 5.5 ) in mass, and the size of a billiard ball. Some observations suggest that the average clutch
Clutch (eggs)
A clutch of eggs refers to all the eggs produced by birds or reptiles, often at a single time, particularly those laid in a nest.In birds, destruction of a clutch by predators, , results in double-clutching...
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). The female makes a muddy plug for the nest hole out of soil mixed with urine, seals the nest by pressing down firmly with her plastron, and leaves them to be incubated by the sun. Females may lay 1–4 clutches per season. Temperature plays a role in the sex of the hatchling
Temperature-dependent sex determination
Temperature-dependent sex determination is type of environmental sex determination in which the temperatures experienced during embryonic development determine the sex of the offspring. It is most prevalent and common among amniote vertebrates that are classified under the reptile class, but is...
, 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).
Early life and maturation
Young animals emerge from the nest after four to eight months and may weigh only 50 grams (1.8 oz) and measure 6 centimetres (2.4 in). 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. 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 their first 10–15 years, encountering hazards such as falling into cracks, being crushed by falling rock, or excessive heat stress. The Galápagos Hawk
Galápagos Hawk
The Galapagos Hawk is a large hawk endemic to the Galapagos Islands.-Physical description:Similar in size to the Red-tailed Hawk and the Swainson's Hawk of North America, the Galapagos Hawk is about 55 cm from beak to tail with a wingspan of 120 cm...
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". The hawk is now much rarer, but introduced feral pigs, dogs, cats and black rats have become predators of eggs and young tortoises. 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".
Sex can be determined only when the tortoise is about 15 years old, and sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
is reached at around 20–25 years 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, making it one of the longest lived species in the animal kingdom. Harriet was the oldest known Galápagos tortoise, having reached an estimated age of more than 170 years before her death in 2006 in Australia Zoo
Australia Zoo
Australia Zoo is a zoo located in the Australian state of Queensland on the Sunshine Coast near Beerwah/Glass House Mountains. It is a member of the Zoo and Aquarium Association , and is owned by Terri Irwin, the widow of Steve Irwin, whose wildlife documentary series The Crocodile Hunter made the...
. Chambers notes that Harriet was probably 169 years old in 2004. The individual died in 2006 (although media outlets claimed the greater age of 175 based on a less reliable timeline.
Darwin's development of theory of evolution
Second voyage of HMS Beagle
The second voyage of HMS Beagle, from 27 December 1831 to 2 October 1836, was the second survey expedition of HMS Beagle, under captain Robert FitzRoy who had taken over command of the ship on its first voyage after her previous captain committed suicide...
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".
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. It has been suggested that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal (chathamensis) and Santiago (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.
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
Robert FitzRoy
Vice-Admiral Robert FitzRoy RN achieved lasting fame as the captain of HMS Beagle during Charles Darwin's famous voyage, and as a pioneering meteorologist who made accurate weather forecasting a reality...
(two from Española) and his servant Syms Covington
Syms Covington
Syms Covington was a fiddler and cabin boy on HMS Beagle who became an assistant to Charles Darwin and was appointed as his personal servant in 1833, continuing in Darwin's service after the voyage until 1839. Originally named Simon Covington, he was born in Bedford, Bedfordshire, England, the...
(from Floreana). 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
Gabriel Bibron
Gabriel Bibron was a French zoologist. He was born in Paris. Son of an employee of the Museum national d'histoire naturelle, he had a good foundation in natural history and was hired to collect vertebrates in Italy and Sicily. He classified a number of reptile species with André Marie Constant...
, who had seen the mature tortoises of the Paris Natural History Museum confirmed to Darwin that there were distinct varieties.
Darwin later compared the different tortoise forms with those of mockingbird
Mockingbird
Mockingbirds are a group of New World passerine birds from the Mimidae family. They are best known for the habit of some species mimicking the songs of other birds and the sounds of insects and amphibians, often loudly and in rapid succession. There are about 17 species in three genera...
s, 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."
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." 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".
Conservation
Several 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 (estimate for 1995–2009).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.
Historical exploitation
An estimated 200,000 animals were taken before the 20th century. 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), providing valuable fresh meat, while their diluted urine and water stored in their neck bags could be used as drinking water. The 17th century British pirate, explorer and naturalist William DampierWilliam Dampier
William Dampier was an English buccaneer, sea captain, author and scientific observer...
wrote that "They are so extraordinarily large and fat, and so sweet, that no pullet eats more pleasantly," while Captain James Colnett
James Colnett
James Colnett was an officer of the British Royal Navy, an explorer, and a maritime fur trader. He served under James Cook during Cook's second voyage of exploration...
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." US Navy captain David Porter
David Porter (naval officer)
David Porter was an officer in the United States Navy in a rank of commodore and later the commander-in-chief of the Mexican Navy.-Life:...
declared that, "after once tasting the Gallipagos 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." 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."
In the 17th century, pirates started to use the Galápagos Islands as a base for resupply, restocking on food, water and repairing vessels before attacking Spanish colonies
Spanish colonization of the Americas
Colonial expansion under the Spanish Empire was initiated by the Spanish conquistadores and developed by the Monarchy of Spain through its administrators and missionaries. The motivations for colonial expansion were trade and the spread of the Christian faith through indigenous conversions...
on 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.
Between the 1790s and the 1860s, whaling
Whaling
Whaling is the hunting of whales mainly for meat and oil. Its earliest forms date to at least 3000 BC. Various coastal communities have long histories of sustenance whaling and harvesting beached whales...
ships and fur-sealers systematically collected tortoises in far greater numbers than the buccaneers preceding them. They 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.
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. Feral pigs, dogs, cats and black rat
Black Rat
The black rat is a common long-tailed rodent of the genus Rattus in the subfamily Murinae . The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 1st century and spreading with Europeans across the world.-Taxonomy:The black rat was...
s have become predators of eggs and young tortoises, whilst goats, donkeys and cattle compete for grazing and trample nest sites. The extinction of the Floreana subspecies in the mid-19th century has been attributed to the combined pressures of hunting for the penal colony on the relatively small island, the conversion of the grazing highlands into land for farming and fruit plantations, and the introduction of feral mammals.
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.
Modern conservation
The remaining subspecies of tortoise range in IUCN classification from extinct in the wildExtinct in the Wild
Extinct in the Wild is a conservation status assigned to species or lower taxa, the only known living members of which are being kept in captivity or as a naturalized population outside its historic range.-Examples:...
to vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
. Slow growth rate, late sexual maturity and island endemism make the tortoises particularly prone to extinction without the efforts of conservationists. The Galápagos giant tortoise has become a flagship species
Flagship species
The concept of flagship species is a surrogate species concept with its genesis in the field of conservation biology. The flagship species concept holds that by raising the profile of a particular species, it can successfully leverage more support for biodiversity conservation at large in a...
for conservation efforts throughout the Galápagos.
Legal protection
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
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
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
National park
A national park is a reserve of natural, semi-natural, or developed land that a sovereign state declares or owns. Although individual nations designate their own national parks differently A national park is a reserve of natural, semi-natural, or developed land that a sovereign state declares or...
and established the Charles Darwin Foundation
Charles Darwin Foundation
The Charles Darwin Foundation was founded in 1959, under the auspices of UNESCO and the World Conservation Union. The Foundation is dedicated to the conservation of the Galapagos Islands ecosystems. The Charles Darwin Research Station serves as headquarters for The Foundation, and is used to...
. In 1970, capturing or removing many species 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.
Captive breeding
Breeding and releasing 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 (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. They were brought to the Charles Darwin Research Station
Charles Darwin Research Station
The Charles Darwin Research Station is a biological research station operated by the Charles Darwin Foundation. It is located in Puerto Ayora on Santa Cruz Island in the Galapagos Islands, with satellite offices on Isabela and San Cristóbal islands.- Background :In Puerto Ayora, Santa Cruz Island,...
in 1971 for a captive breeding program. In the following 33 years, these 15 tortoises gave rise to over 1,200 tortoises released onto their home island, which themselves have begun to reproduce naturally.
Island restoration
The Galápagos National Park Service systematically culls
Culling
Culling is the process of removing animals from a group based on specific criteria. This is done either to reinforce certain desirable characteristics or to remove certain undesirable characteristics from the group...
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. 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 (island restoration
Island restoration
The ecological restoration of islands, or island restoration, is the application of the principles of ecological restoration to islands and island groups. Islands, due to their isolation, are home to many of the world's endemic species, as well as important breeding grounds for seabirds and some...
) to their pre-human condition. The tortoises are a keystone species
Keystone species
A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and...
, acting as ecosystem engineer
Ecosystem engineer
An ecosystem engineer is any organism that creates or modifies habitats. Jones et al. identified two different types of ecosystem engineers:...
s which help in plant seed dispersal
Seed dispersal
Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors. Seeds can be dispersed away from the parent plant...
, 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 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.