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Galápagos tortoise

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Galápagos tortoise
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Order:
Testudines
Family:
Testudinidae
Genus:
Geochelone
Species:
G. nigra
Binomial name
Geochelone nigra
(Quoy & Gaimard, 1824b[2])
Subspecies

12, see Subspecies of Geochelone nigra

Synonyms

The Galápagos tortoise or Galápagos giant tortoise (Geochelone nigra) is the largest living species of tortoise, reaching weights of over Template:Kg to lb and lengths of Template:M to ft. It is among the longest lived of all vertebrates. Life expectancy in the wild is over 100 years, and the oldest known individual is estimated to have reached at least 170 years of age. The tortoise is endemic to seven volcanic islands of the Galápagos archipelago, lying west of Ecuador. Spanish explorers who discovered the islands in the 16th century were purportedly struck by the resemblance of the local tortoises' shells to the shape of a saddle known as a 'galápago', and this is popularly said to be the origin of the name of the archipelago[4][5][6]. The size and shell shape of the tortoise differs between populations on different islands according to habitat. This variation in form played a role in the inception of Charles Darwin's theory of evolution.

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

Taxonomy

The current species designation of nigra ("black"- Quoy & Gaimard, 1824b[2]) was resurrected in 1984[7], after it was discovered to be the senior synonym for the commonly used elephantopus ("elephant-footed"- Harlan, 1827[3]). The even more senior synonym of californiana ("californian" Quoy & Gaimard, 1824a[8]) is considered a nomen oblitum ("forgotten name")[9].

Before a sufficient number of specimens became available for scientific scrutiny and comparison, the giant tortoises of the Indian Ocean and those from the Galápagos were considered to be the same species Testudo gigantea ("gigantic tortoise"-Schweigger, (1812[10]), then Testudo indica ("indian tortoise") by Gray (1831[11]), who would later become Keeper of Zoology at the British Museum. It was postulated that animals from the Indian Ocean had been transported by sailors to the Galápagos[12]. Later, the Galápagos tortoise was considered by Duméril and Bibron (1834[13]) as a different species, Testudo nigrita ("black tortoise"), from the Indian Ocean's elephantina (Aldabra), daudinii (Seychelles) and peltastes (Mascarene).

The first truly systematic comparison of giant tortoises which recognised their diversity and endemism was by Albert Günther, Gray's successor as Keeper of Zoology, in 1875[14]. He identified at least five distinct populations from the Galápagos, and three in the Indian Ocean. This list was expanded by Gunther in 1877 to six from the Galápagos, four from the Seychelles, and four from the Mascarenes. He theorised that the tortoises of the Galápagos and the Indian Ocean were derived from a single ancestral population which spread to the isolated islands by sunken land bridges[15]. This theory was later definitively disproven by the understanding that the Galápagos, Aldabra and Mascarene islands are all of volcanic origin and have risen from the sea in recent geological history; thus, they could not have been linked by land bridges. It is now thought that the Galápagos tortoises descended from a common ancestor from the South American mainland[16], whilst those of the Indian Ocean derived from two colonisations from nearby Madagascar – one to the Seychelles, and another to the Mascarenes[17][18].

Rollo Beck, an American explorer who visited the Galápagos on collecting expeditions organised by Rothschild and the Academy of Sciences. He is pictured mounting a Galápagos tortoise.

Five more populations of Galápagos tortoise were first recognised by Baur[19] and Rothschild[20][21] [22]. Specimens collected by an Academy of Sciences expedition in 1906 were studied by Van Denburgh, who identified four additional new populations[23], and proposed the existence of 15 species[24]. This definitive list still guides the taxonomy of the Galápagos tortoise in the modern day, though with some modifications.

Elevation of genus Chelonoidis

There are taxonomic justifications for elevating Chelonoidis (Fitzinger 1835) from subgenus to genus status based on molecular phylogenetic analyses of testudinids[25]. If the subgenus is elevated, South American tortoises including the Galápagos tortoise would belong in the genus Chelonoidis rather than Geochelone, a convention which is being adopted by some authorities[1][26][27].

Phylogeny

External phylogeny

All subspecies of Galápagos tortoise evolved from a common ancestor that arrived from mainland South America by overwater dispersal of a pregnant female or a breeding pair[16]. Survival on the 1000 km oceanic journey is accounted for by the fact that the tortoises are buoyant and can breathe by extending their necks above the water, able to survive months without food or fresh water. As they are poor swimmers, the journey was probably a passive one facilitated by the Humboldt Current, which diverts westwards towards the Galápagos Islands from the mainland[28].

The closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Chaco tortoise (Geochelone chilensis), a much smaller species from South America. The divergence between G. chilensis and G. nigra probably occurred 6–12 million years ago[16], an evolutionary event preceding the volcanic formation of the oldest modern Galápagos islands 5 million years ago[29]. Mitochondrial DNA analysis indicates that the oldest existing islands (Española and San Cristóbal) were colonised first, and that these populations seeded the younger islands formed by subsequent volcanism via dispersal events on local currents in a 'stepping stone' fashion[30][31]. Restricted gene flow between isolated populations then resulted in the radiation into the divergent forms observed in the modern subspecies. The species phylogeny thus echoes the paleogeography of the islands.

Internal phylogeny

Galápagos archipelago annotated with ranges of currently recognised subspecies of giant tortoise. Islands with extant subspecies are shaded.
Main article: Subspecies of Geochelone nigra

There were probably 12 subspecies of Geochelone nigra in the Galápagos Islands, although some recognise up to 15 subspecies[28]. Only ten subspecies now exist in the wild, one on each of the islands of Santiago, Santa Cruz, San Cristóbal, Pinzón, and Española, and one on each of the five main volcanoes of the largest island Isabela (Wolf, Darwin, Alcedo, Sierra Negra, and Cerro Azul). The eleventh extant subspecies, abingdoni from Pinta Island, is considered extinct in the wild and is represented by a single living specimen, 'Lonesome George'.

Prior to widespread knowledge of the differences between the populations from different islands and volcanoes (sometimes called 'races'), captive collections in zoos were indiscriminately mixed. Fertile offspring resulted from pairings of animals from different races, confirming that they are subspecies and not distinct species. However captive crosses between tortoises from different races have lower fertility and higher mortality than those between tortoises of the same race[32][33] and captives in mixed herds normally direct subadult courtship only toward members of the same race[33]. The taxonomic status of the various races is not fully resolved[34], and the valid scientific names of each the individual populations[9][35][36][37], whilst some researchers consider the subspecies to be full species[38][39].

Modern molecular methods have revealed new information on the relationships between the subspecies. The Pinta Island subspecies (abingdoni) is most closely related to the subspecies on the islands of San Cristóbal (chathamensis) and Española (hoodensis) which lie over 300 km away[16], rather than neighbouring Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta[40]. 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[41]. 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[42].

Mitochondrial DNA studies of tortoises on Santa Cruz show up to three genetically distinct lineages found in non-overlapping population distributions around Cerro Monturra, Cerro Fatal and La Caseta[43]. Although currently united in a single subspecies (porteri), the lineages are all more closely related to tortoises on other islands than to each other[44]; 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[45].

The four southern subspecies on Isabela, though separated from each other by barren stretches of lava between volcanoes, are likely to be a single genetic unit derived from colonists from Santa Cruz. The genetically distinct Volcán Wolf subspecies in northern Isabela (becki) is probably the result of a separate colonisation event from Santiago[16]. Tortoises from Sierra Negra in southern Isabela (guentheri) are putatively the ancestral source of dispersal to the volcanoes Darwin (microphyes), Alcedo (vandenburghi) and Cerro Azul (vicina)[46]. On this basis it has been suggested that the southern populations on Isabela may be considered as a single subspecies vicina, with morphological differences attributable to geographical variation[9].

Phylogenetic analysis may also help to 'resurrect' the subspecies of Floreana (nigra), which became extinct from human exploitation by 1850[47], just years after Charles Darwin's landmark visit in 1835[9]. The subspecies was only known from subfossil remains[48]. 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[39] either by deliberate moving between the islands[9] or from individuals thrown overboard ships to lighten loads[19]. Nine Floreana descendants have also been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz[49]. This permits the possibility of re-establishing a reconstructed subspecies from selective breeding of the hybrid animals.

Subspecies of doubtful existence

Subspecies were described from three other islands, but their existence is based on scant evidence. The purported Rabida island subspecies (wallacei) was described from a single specimen removed by the Academy of Sciences in 1906[24], which has since been lost. This individual was probably an artificial introduction from another island that was originally corralled on Rabida next to a good anchorage, as no contemporary whaling or sealing logs mention removing tortoises from this island[9][50]. The phantastica subspecies from Fernandina is also known from a single specimen from the voyage of 1906, an old male[24]. No other tortoises or remains have been found on the island, suggesting the specimen was an artificial introduction from elsewhere[9][35][50]. 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[35]. The Santa Fe subspecies has no binomial name, having been described from the limited evidence of bone fragments (but no shells) of 14 individuals, old eggs and old dung found on the island in 1906[24]. These are also considered artificial introductions[35].

Description

Galápagos tortoise shell varieties
G. n. abingdoni
Saddleback (abingdoni)
G. n. chathamensis
Intermediate (chathamensis)
G. n. cporteri
Domed (porteri)

The discoverer of the Galápagos islands, Fray Tomás de Berlanga, Bishop of Panama, wrote of 'such big tortoises that each could carry a man on top of himself[51]. Later, naturalist Charles Darwin remarked 'These animals grow to an immense size ... several so large that it required six or eight men to lift them from the ground'[52]. The largest recorded individuals have reached weights of over 400 kg[53] and lengths of 1.87 m[54]. The tortoises' gigantism was probably a preadapted condition (allowing significant water and fat storage) for successful colonisation of these remote oceanic islands whose annual rainfall is extremely variable and prone to drought, rather than an evolutionary adaptation to the island environment[9]. Indeed, fossil giant tortoises from mainland South America have been described which are consistent with current phylogenetic groupings[55].

The tortoises have a large bony shell (carapace) of a dull brown colour. The plates of the carapace are fused with the ribs in a rigid protective structure that is integral to the skeleton. Lichen can grow on the shells of the slow-moving animals[56]. Tortoises keep a characteristic scute pattern on their shell throughout life, though the annual growth bands are not useful for aging as the outer layers are worn off with time. A tortoise can withdraw its head, neck and forelimbs into its shell for protection. The legs are large and stumpy, with dry scaly skin and hard scales. The front legs are five-clawed, and the back legs are four-clawed[24].

Galapágos tortoises are the only lineage of giant tortoise exhibiting different types of shell shape[44]. They exhibit a spectrum of carapace morphology from 'saddleback' (denoting upward arching of the front edge of the shell resembling a saddle) to 'domed' (denoting a rounded convex surface resembling a dome). When saddleback tortoises withdraw their head and forelimbs into their shells a large unprotected gap remains over their neck, evidence of the lack of predation during the evolution of this structure. There is no saddleback/domed dualism, as tortoises can be of intermediate type with characteristics of both. Larger islands with humid (mesic) highlands over 800 m in elevation, such as Santa Cruz have abundant vegetation near the ground[37]. 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 m in elevation with xeric habitats (e.g. Española and Pinzón) that are more limited in food and other resources[28].

In combination with proportionally longer necks and limbs[24], the unusual saddleback carapace structure is thought to be an adaptation to increase vertical reach, which enables the tortoise to browse tall vegetation such as the arboreal Opuntia (prickly pear) cactus which grows in arid environments[57]. Saddlebacks are more territorial[54][58] 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[36].

Rather than principally a feeding adaptation, the distinctive saddle shape and longer extremities might have been an adaptive change for infraspecific competition over limited food, water and shade on the dry islands, which is settled by dominance displays on the basis of vertical neck height rather than body size[36] (see below). This correlates with the observation that saddleback males are more aggressive than domed males[59]. The shell distortion and elongation of the limbs and neck in saddlebacks is probably the evolutionary compromise between the need for a small body size in xeric conditions and a high vertical reach for agonistic encounters[36].

Sexual dimorphism is most pronounced in saddleback populations, in which males have more angled and higher front openings, giving a more extreme saddled appearance[59]. Males of all varieties generally have a longer tail and a shorter, concave undershell with thickened knobs at the back edge to facilitate mating. Males are also larger than females: adult males weigh around 272–317 kilograms (600–699 lb) whilst females are 136–181 kilograms (300–399 lb)[36].

The saddleback carapace probably evolved independently several times in the context of xeric biomes[54], since genetic similarity between populations does not correspond to carapace shape[60]. Saddleback tortoises are therefore not necessarily more closely related to each other than to domed counterparts, as the shape reflects not evolutionary proximity but ecological parity[36]. However, the extinction of crucial populations by human activities confounds studies investigating whether domed and saddleback populations are definitively mono- or polyphyletic[28].

Role in the inception of the theory of evolution

Charles Darwin visited the Galápagos for five weeks on the second voyage of HMS Beagle in 1835. In the first edition of his published account of the voyage he wrote: 'It was confidently asserted, that the tortoises coming from different islands in the archipelago were slightly different in form; and that in certain islands they attained a larger average size than in others. Mr. Lawson [the Vice-Governor] maintained that he could at once tell from which island any one was brought.[52]' He noted in the addenda of the first edition that 'there is every reason for believing that several of the islands possess their own peculiar varieties or species of tortoise[61]'. He expanded on these thoughts in the second edition, writing, 'I have not as yet noticed by far the most remarkable feature in the natural history of this archipelago; it is, that the different islands to a considerable extent are inhabited by a different set of beings. My attention was first called to this fact by the Vice-Governor, Mr. Lawson, declaring that the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought.[62]'. 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[62]'.

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[63]. It has been suggested[64] that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal[65] (chathamensis) and Santiago[66] (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[9].

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)[67]. 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[68]. Although the British Museum had a few specimens, their provenance within the Galápagos was unknown[15]. 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[69].

He later compared the different tortoise forms with those of mockingbirds, in the first[70] tentative statement linking his observations from the Galapagos with the possibility of species transmuting: 'When I recollect the fact that [from] the form of the body, shape of scales and general size, the Spaniards can at once pronounce from which island any tortoise may have been brought; when I see these islands in sight of each other and possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure and filling the same place in nature; I must suspect they are only varieties ... If there is the slightest foundation for these remarks, the zoology of archipelagoes will be well worth examining; for such facts would undermine the stability of species.[71]'. 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[72]. These observations served as coutnerexamples to the prevailing contemporary view that species were individually created.

Darwin also found these 'antediluvian animals[65]' 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[52]'.

Biology and ecology

Behaviour

The tortoises are 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[35]. They travel mostly in the early morning or late afternoon. They have been observed to walk at a speed of 0.3 kilometres per hour (0.19 mph)[52]. On the larger and more humid islands, the tortoises seasonally migrate down between low elevations, which become grassy plains in the wet season, to meadowed areas of higher elevation (up to 2,000 ft[24]) in the dry season. The same routes have been used for many generations, creating well-defined paths through the undergrowth known as 'tortoise highways'[37]. 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.

Several tortoises bathing in a pool on Santa Cruz

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[37]. Parasites are also countered by taking dust baths in loose soil. Tortoises have been noted to shelter at night under overhanging rocks[73]. 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.[74]

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[52]'. Although they are not deaf[24], tortoises depend far more on vision and smell as stimuli[37]. In addition to hissing as they withdraw into their shells due to the forceful expulsion of air[75], the tortoises also vocalise when struggling to right themselves and during aggressive encounters. Males emit 'rhythmic groans' during sexual congress[37].

Diet

A tortoise feeding.

The tortoises are herbivores that consume a diet of cactus, grasses, leaves, lichen, 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.[76]. A tortoise eats an average of 70–80 pounds (32–36 kg) per day, though inefficient digestion means that much of this passes through without nutritional extraction[77].

Tortoises acquire most of their moisture from the dew and sap in vegetation (particularly the Opuntia cactus); therefore, they can spend long periods without drinking water. They can also endure 18 months when deprived of all food and water[78], 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[35]), both of which used to make them useful water sources on ships[78]. On arid islands, tortoises lick morning dew from boulders, and the repeated action over many generations has formed half-sphere depressions in the rock[35].

Mutualism

File:Galapagos Tortoise and finch symbiosis.jpg
A finch and a tortoise exhibiting mutualistic behaviour.

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, cloacal opening, and skin between plastron and carapace. The birds benefit from the food source and the tortoises get rid of irritating ecto-parasites[79].

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[80].

Reproduction and life cycle

File:Galapagos tortoise dominance display.jpg
A pair of tortoises engaging in a dominance display.

Mating occurs at any time of the year, although it does have seasonal peaks between February and June in the humid uplands during the rainy season[37]. 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[59].

The prelude to mating can be very aggressive, as the male forcefully rams the female's shell with his own and nips her legs[81]. Mounting is an awkward process and the male must stretch and tense to maintain equilibrium in a slanting position. The concave underside of the male's shell helps him to balance when straddled over the female's shell, and brings his cloacal vent (which houses the penis) closer to the female's dilated cloaca. The male vocalises with hoarse bellows and grunts during mating[73].

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[37]. It is carried out blindly using only the hind legs to dig a 30 cm (12 in) deep cylindrical hole, into which she lays up to sixteen spherical, hard-shelled eggs ranging from 82 to 157 g in mass[35], and the size of a billiard ball[56]. 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)[50]. The female makes a muddy plug for the nest hole out of soil mixed with urine and seals the nest by pressing down firmly with her plastron. Females may lay 1–4 clutches per season, and leaves them to be incubated by the sun. Temperature 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)[82]

Young animals emerge from the nest after 4 to 8 months (December–April) and may weigh only 50 grams (1.8 oz) and measure 6 centimetres (2.4 in)[37]. 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[56]. In particularly dry conditions, the hatchlings may die underground if they are encased by hardened soil, while flooding of the nest area can drown them. Subspecies are initially indistinguishable as they all have domed carapaces. The young stay in warmer lowland areas for the their first 10–15 years[35], 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[52]'. The hawk is now much rarer, but introduced feral pigs, dogs, cats and black rats have become predators of eggs and young tortoises[83]. 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[52].'

Sex can be determined only when the tortoise is about 15 years old, and sexual maturity is reached at around 20–25 in captivity, possibly 40 years in the wild (when they reach their full size). Life expectancy in the wild is thought to be over 100 years[5].

Human disturbance

As a consequence of several waves of human exploitation of the tortoises as a food source, their total population declined from around 250,000[6] when first discovered in the 16th century to around 20,000[84] in the modern day. An estimated 200,000 animals were taken before the 20the century[9][35].

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[85]), providing valuable fresh meat, whilst their diluted urine and water stored in their neck bags could also be used as drinking water. The 17th century British pirate, explorer and naturalist William Dampier wrote that 'They are so extraordinarily large and fat, and so sweet, that no pullet eats more pleasantly[86],' whilst Captain James Colnett of the British Navy wrote of 'the land tortoise which in whatever way it was dressed, was considered by all of us as the most delicious food we had ever tasted.[87]' US Navy captain David porter declared that, 'after once tasting the Gallipagos tortoises, every other animal food fell off greatly in our estimation ... The meat of this animals is the easiest of digestion, and a quantity of it, exceeding that of any other food, can be eaten without experiencing the slightest of inconvenience.[78]' 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[88].'

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 in the South American mainland. However, the Galápagos tortoises did not struggle for survival at this point because the islands were distant to busy shipping routes and harboured few valuable natural resources. As such they remained unclaimed by any nation, uninhabited and uncharted. In comparison, the tortoises of the islands in the Indian Ocean were already facing extinction by the late 17th century[89].

Between the 1790s and the 1860s, whaling ships and fur-sealers systematically collected tortoises in far greater numbers than the buccanneers preceding them[90]. They were used for food and many more were killed for high grade 'turtle oil' from the late 19th century onward for lucrative export to continental Ecuador[91]. 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[85]. Since it was easiest to collect tortoises around coastal zones, females were most vulnerable to depletion during the nesting season. The collection by whalers came to a halt eventually through a combination of the scarcity of tortoises that they had created and the competition from crude oil as a cheaper and more abundant energy source[92].

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[50]. Feral pigs, dogs, cats and black rats have become predators of eggs and young tortoises, whilst goats, donkeys and cattle compete for grazing and trample nest sites. Scientific collection expeditions took 661 tortoises between 1888 and 1930, and more than 120 tortoises have been taken by poachers since 1990. Threats continue today with the rapid expansion of the tourist industry and increasing size of human settlements on the islands[93].

Conservation

The remaining subspecies of tortoise range in IUCN classification from extinct in the wild to vulnerable. Slow growth rate, late sexual maturity and island endemism make the tortoises particularly prone to extinction without the efforts of conservationists[44]. The Galápagos giant tortoise is now strictly protected. Geochelone nigra is listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora[27]. The listing requires that trade in the taxon and its products is subject to strict regulation by ratifying states and international trade for primarily commercial purposes is prohibited. In 1936 the Ecuadorian government listed the giant tortoise as a protected species. In 1959, it declared all uninhabited areas in the Galápagos to be a National Park[94] and established the Charles Darwin Foundation. In 1970, capturing or removing many species from the islands (including tortoises and their eggs) was banned[95]. 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)[96]. A 1971 Ecuadorian decree made it illegal to damage, remove, alter or disturb any organism, rock or other natural object in the National Park[97].

File:BabyGalapagosTortoises.jpg
A 2–3 month old tortoise with a tortoise egg

Breeding and repatriation programs began in 1965 and have successfully brought 10 of the 11 endangered subspecies up to guarded population levels. Young tortoises are raised at one of several breeding centres across the islands to improve their survival during their vulnerable early development. Eggs are collected from threatened nesting sites, and the hatched young are given a head start by being kept in captivity for four to five years to reach a size with a much better chance of survival to adulthood before re-release onto their native ranges[83][98].

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[56]. They were brought to the Charles Darwin Research Station in 1971 for a captive breeding program[99]. In the following 33 years, these 15 tortoises gave rise to over 1200 tortoises released onto their home island, which themselves have begun to reproduce naturally[9][100]

The Galápagos National Park Service also systematically culls feral predators and competitors. Goat eradication on islands including Pinta was achieved by the technique of using 'Judas' goats with electronic collars to locate the herds. Marksmen shot all the goats except the Judas, then returned weeks later to find the Judas and shoot the herd to which it had relocated, repeating until only the Judas remained, which was then killed[101]. Other measures have included dog eradication from San Cristóbal, and fencing off nests to protect from feral pigs[83].

Efforts are now underway to repopulate islands formerly inhabited by tortoises in order to restore the insular ecosystems to their pre-human condition. The tortoises are a keystone species, acting as ecosystem engineers[101] which help in plant seed dispersal, trampling down brush and thinning the understory of vegetation (allowing light to penetrate and germination to occur). Birds such as flycatchers perch on and fly around tortoises in order to hunt the insects it displaces from the brush[56]. 39 sterilised tortoises of hybrid origin were introduced in May 2010 to Pinta Island, the first tortoises there since the evacuation of Lonesome George 38 years ago in 1972[102]. 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[42]. This approach may also be used to 're-tortoise' Floreana in the future, since captive individuals have been found to be descended from the extinct original stock[49].

Notable individuals

Lonesome George
  • Lonesome George is the only known living specimen of the Pinta Island Tortoise and the world's 'rarest living creature'[103].
  • Harriet was the oldest known Galápagos tortoise, having reached an estimated age of more than 170 years old at its death in 2006[104].

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