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Galápagos tortoise
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The Galápagos tortoise (or Galápagos giant tortoise), is the largest living tortoise, native to seven islands of the Galápagos archipelago. Fully grown adults can weigh over and measure long. They are long-lived with a life expectancy in the wild estimated to be 100-150 years. Populations fell dramatically because of hunting and the introduction of predators and grazers by humans since the seventeenth century. Now only ten subspecies of the original twelve exist in the wild.
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The Galápagos tortoise (or Galápagos giant tortoise), is the largest living tortoise, native to seven islands of the Galápagos archipelago. Fully grown adults can weigh over and measure long. They are long-lived with a life expectancy in the wild estimated to be 100-150 years. Populations fell dramatically because of hunting and the introduction of predators and grazers by humans since the seventeenth century. Now only ten subspecies of the original twelve exist in the wild. However, conservation efforts since the establishment of the Galápagos National Park and the Charles Darwin Foundation have met with success, and hundreds of captive-bred juveniles have been released back onto their home islands. They have become one of the most symbolic animals of the fauna of the Galápagos Islands.
Anatomy and morphology
Shell
The tortoises have very large shells (carapace) made of bone. The bony plates of the shell are integral to the skeleton, fused with the ribs in a rigid protective structure.
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." . This is due to the phenomenon of island gigantism whereby in the absence of natural predation, the largest tortoises had a survival advantage and no disadvantage in fleeing or fending off predators.
When threatened, it can withdraw its head, neck and forelimbs into its shell for protection, presenting a protected shield to a would-be predator. The legs have hard scales that also provide armour when withdrawn. Tortoises keep a characteristic scute pattern on their shell throughout life. These have annual growth bands but are not useful for aging as the outer layers are worn off. There is little variation in the dull-brown colour of the shell or scales.
Physical features (including shape of the shell) relate to the habitat of each of the subspecies. These differences were noted by Captain Porter even before Charles Darwin. Larger islands with more wet highlands such as Santa Cruz and the Alcedo Volcano on Isabela have lush vegetation near the ground. Tortoises here tend to have 'dome-back' shells. These animals have restricted upward head movement due to shorter necks, and also have shorter limbs. These are the heaviest and largest of the subspecies.
Smaller, drier islands such as Española and Pinta are inhabited by tortoises with 'saddleback' shells comprising a flatter carapace which is elevated above the neck and flared above the hind feet. Along with longer neck and limbs, this allows them to browse taller vegetation. On these drier islands the Galápagos Opuntia cactus (a major source of their fluids) has evolved a taller, tree-like form. This is evidence of an evolutionary arms race between progressively taller tortoises and correspondingly taller cacti. Saddlebacks are smaller in size than domebacks. They tend to have a yellowish color on lower mandible and throat. At one extreme, the Sierra Negra volcano population that inhabits southern Isabela Island has a very flattened "tabletop" shell. However, there is no saddleback/domeback dualism; tortoises can also be of 'intermediate' type with characteristics of both.
Sexual dimorphism is more apparent in the 'intermediate' and saddleback populations since males have more angled and higher front openings. Males also have a longer tail and a shorter and concave undershell and which has thickened knobs at the back edge, which facilitates mating. Male are larger than females: adult males weigh in at 272-317 kg; females 136-181 kg ).
Range and distribution
The Galápagos tortoise is unique to the Galápagos Islands, a group of thirteen major islands and many smaller islets, all of volcanic origin lying west of Ecuador in South America.
Ecology and behavior
The tortoises are slow-moving reptiles with an average long-distance walking speed of 0.3 km/h (0.18 mph). Although feeding giant tortoises browse with no apparent direction, when moving to water-holes or nesting grounds, they can move at a surprising speed for their size. Marked individuals have been reported to have traveled 13 km in two days.
On the wetter islands, the tortoises migrate down gentle mountain slopes after the wet season to feed on the grass-covered plains. In the dry season they climb back up to feed on grasses of the mountain meadows. On these islands, the tortoises appear to be gregarious, often found in large 'herds'.
Being cold-blooded, the tortoises bask for two hours after dawn, absorbing the energy through their shells, then becoming active for 8-9 hours a day. They may sleep for about sixteen hours in a mud wallow partially or submerged in rain-formed pools (sometimes dew ponds formed by garua-moisture dripping off trees). This may be both a thermoregulatory response and a protection from parasites such as mosquitoes and ticks. Some rest in a 'pallet'- a snug depression in soft ground or dense brush- which probably helps to conserve heat and may aid digestion. On the Alcedo Volcano, repeated use of the same sites by the large resident population has resulted in the formation of small sandy pits.
Darwin observed that: "The inhabitants believe that these animals are absolutely deaf; certainly they do not overhear a person walking close 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." The tortoises can vocalise in aggressive encounters, whilst righting themselves if turned upside down and, in males, during mating. The latter is described as "rhythmic groans".
Diet
The tortoises are herbivorous animals with a diet comprising cactus, grasses, leaves, vines, and fruit. Fresh young grass is a favorite food of the tortoises, and others are the 'poison apple' (Hippomane mancinella) (toxic to humans), the endemic guava (Psidium galapageium), the water fern (Azolla microphylla), and the bromeliad (Tillandsia insularis). Tortoises eat a large quantity of food when it is available at the expense of incomplete digestion.
By acquiring most of their moisture from the dew and sap in vegetation (particularly the Opuntia cactus), tortoises can go for long periods without actually drinking. They can also survive for over a year being forcefully deprived of all liquids, by breaking down their body fat to produce water.
Mutualism
Tortoises have a classic example of a mutualistic symbiotic relationship with some species of Galápagos finch. The finch hops in front of the tortoise to show that it is ready and the tortoise then raises itself up high on its legs and stretches out its neck so that the bird can pick off ticks that are hidden in the folds of the skin (especially on the rear legs, cloacal opening, neck, and skin between plastron and carapace), thus freeing the tortoise from harmful parasites and providing the finch with an easy meal. Other birds, including Galápagos Hawk and flycatchers, use tortoises as observation posts from which to sight their prey.
Reproduction
Mating occurs at any time of the year, although it does have seasonal peaks between January and August. When two mature males meet in the mating season they will face each other, rise up on their legs and stretch up their necks with their mouths open to assess dominance. Occasionally, head-biting occurs, but usually the shorter 'loser' tortoise will back off, leaving the other to mate with the female. In groups of tortoises from mixed island populations, saddleback males have an advantage over domebacks. Frustrated non-dominant males have been observed attempting to mate with other males and boulders.
The male sniffs the air when seeking a female, bellows loudly, and bobs his head. The male then rams the female with the front of his shell and bites her exposed legs until she withdraws them, immobilizing her. Copulation can last several hours with roaring vocalisations from the males. Their concave shell base allows males to mount the females from behind. It brings its tail which houses the penis into the female's cloaca.
After mating (June-December), the females journey up to several kilometres to reach nesting areas of dry, sandy ground (often near the coast). Nest digging can last from hours to days and is elaborate and exhausting. It is carried out blindly using only the hind legs to dig a 30 cm deep hole, into which she lays up to sixteen hard-shelled eggs the size of tennis balls. The female makes a muddy plug for the nest hole out of soil mixed with urine and leaves the eggs to incubate. In rocky areas, the eggs are deposited randomly into cracks.
The young emerge from the nest after 120 to 140 days gestation later (December-April) and may weigh only 80 grams (2.8 oz) and measure 6 centimetres (2.4 in). Temperature plays a role in the sex of the hatchling: if the nest temperature is lower, more males will hatch; if it is high, more females will hatch. When the young tortoises emerge from their shells, they must dig their way to the surface, which can take up to a month. All have domed carapaces, and subspecies are indistinguishable. Galápagos Hawk used to be the only native predator of the tortoise hatchlings, as Darwin remarked: "The young tortoises, as soon as they are hatched, fall prey in great numbers to buzzards".
Sex can be determined only when the tortoise is 15 years old, and sexual maturity is reached at 20 to 25 years old. The tortoises grow slowly for about 40 years until they reach their full size. Reproductive prime is considered to be from the ages of 60-90.
Etymology and taxonomic history
The shape of the carapace of some subspecies of the tortoises is said to have reminded the early Spanish explorers of a kind of saddle they called a "galápago," and for these saddle-shaped tortoises they named the archipelago. Up to 250,000 tortoises inhabited the islands when they were discovered. Today only about 15,000 are left.
There were probably twelve subspecies of Geochelone nigra in the Galápagos Islands, although some recognise up to 15 subspecies. Now only 11 subspecies remain, five on Isabela Island, and the other six on Santiago, Santa Cruz, San Cristóbal, Pinzón, Española and Pinta. Of these, the Pinta Island subspecies is extinct in the wild and is represented by a single individual. In the past, zoos took animals without knowing their island of origin. Production of fertile offspring from various pairings of tortoises largely confirmed that they are subspecies and not different species.
All the subspecies of giant tortoise evolved in Galápagos from a common ancestor that arrived from the mainland, floating on the ocean currents (the tortoises can drift for long periods of time as they are buoyant and can stretch head upwards to breathe). Only a single pregnant female or breeding pair needed to arrive in this way, and then survive, for Galápagos to be colonised. The closest living relative of the Galápagos giant tortoises is Geochelone chilensis, a small tortoise found in Chile. The split between G. chilensis and the Galápagos lineage probably occurred 6-12 million years ago based on mitochondrial DNA analysis, before the origin of the oldest extant Galápagos island.
Espanola and San Cristobal, the oldest islands, were colonized first; this was followed by several migration events to and between other islands via local currents. The four named southern subspecies on the largest island, Isabela, are possibly not distinct genetic units, whereas a genetically distinct northernmost Isabela subspecies is probably the result of a separate colonization. Unexpectedly, the lone survivor of the abingdoni subspecies from Pinta Island ("Lonesome George") is very closely related to tortoises from San Cristobal and Espanola, the islands farthest from the island of Pinta. This informed breeding attempts more successful in the future as breeding had been attempted with Isabela Island populations
It is thought that the saddle-backed type carapace evolved independently several times as a reaction to dry environments, although extinction of crucial populations by human activities confounds whether domed versus saddleback carapaces of different populations are mono- or polyphyletic.
Pending further discussion the more conservative subspecies arrangement is retained here. Population details for each subspecies are summarised below.
| Subspecies | Native range | Picture | Conservation issues | Population estimate |
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| Abingdon Island Tortoise G. n. abingdoni Günther, 1877 | Southern slopes of Pinta (Abingdon) Island | | Only one known purebred individual is alive ("Lonesome George"), and is currently maintained at the Charles Darwin Research Station. This population was severely depleted by whalers and fishermen, and the introduction of goats in 1958 resulted in massive destruction of vegetation. Tortoise droppings, probably not more than a few years old, were found in the island in 1981, so there is a possibility that a second individual of this subspecies still exists, though this appears increasingly unlikely. Recent reports of a clutch of eggs in Lonesome George's enclosure have garnered excitement that the lineage may continue. | Extinct in the wild, hybrid eggs have been laid and at least one hybrid adult is also known | | Chatham Island Tortoise G. n. chathamensis Van Denburgh, 1907 | San Cristóbal (Chatham) Island: now confined to the northeast | | Heavily exploited and completely eliminated over much of its original range. Trampling of nests by feral donkeys, and the predation of young by feral dogs decimated populations, but the breeding program had led to successful releases. | 700 | | James Island Tortoise G. n. darwini Van Denburgh, 1907 | West-central San Salvador (aka James or Santiago) Island | | Large numbers of tortoises were removed from the island in the early nineteenth century by whaling vessels, and introduced goats reduced the coastal lowlands to deserts, restricting the remaining tortoises to the interior. The sex ratio is strongly imbalanced in favour of the males and most nests and young are destroyed by feral pigs. Some nests are now protected by lava corals and since 1970 eggs have been transported to the Charles Darwin Research Station for hatching and rearing. Release programs have been successful. | 800 | Duncan Island Tortoise G. n. duncanensis Garman, 1917
(syn. ephippium Günther, 1875) | Southwestern Pinzón (Duncan) Island | | Although relatively undisturbed by whalers, fairly large numbers of tortoises were removed by expeditions in the latter half of the nineteenth century and early twentieth. After the introduction of black rats some time before 1900, there was no natural breeding. Since 1965, eggs have been transported to the Charles Darwin Research Station for hatching and rearing. | 300 | Charles Island Tortoise G. n. galapagoensis Baur, 1889
(syn. nigra/elephantopus) | Floreana (aka Charles or Santa Maria) Island | | Formerly abundant but heavily exploited by visiting ships and a penal colony in the twentieth century. Darwin saw them in 1835, and noted that tortoises comprised the main food item in the Floreana colony; "two days hunting will find food for the other five in the week." Although he commented on how the numbers had been obviously reduced from those in years past ("not many years since the Ship's company of a Frigate brought down to the Beach in one day more then 200"), he did mention Vice Governor Lawson's prediction that "there is yet sufficient for 20 years." Indeed there is a well-documented record of heavy collecting in the years leading up to Darwin's visit, but then just three years later, a visiting ship could find no tortoises and in 1846, another visitor declared them extinct. Descriptions of the Floreana race are based on skeletal material from individuals who fell down into lava tubes and died. However in 2008, research into mitochondrial DNA in museum specimens of the Floreana race by Dr Caccone of Yale University suggested that a population from Floreana may have been transposed to Isabela. Theoretically, a breeding programme could be established to 'resurrect' the pure Floreana race from the hybrid subpopulation. Using marker-assisted selection for a captive breeding population, it is estimated that the project would last a century. | Extinct, hybrid subpopulation exists on Isabela | | Hood Island Tortoise G. n. hoodensis Van Denburgh, 1907 | Española (Hood) Island | | This population was very heavily exploited by whalers in the nineteenth century and collapsed around 1850. 13 adults were found in the early 1970s and held at the Charles Darwin Research Station as a breeding colony. The 2 males and 11 females were initially brought to the Darwin Station. Fortuitously, a third male was discovered at the San Diego Zoo and joined the others in a captive breeding program. These 13 tortoises have given rise to over 1000 tortoises now released into their home island. Mating had not occurred naturally for some time because the individuals were so scattered that they did not meet. | 120 | Indefatigable Island TortoiseG. n. porteri Rothschild, 1903
(syn. nigrita) | Santa Cruz (Indefatigable) Island: the main population occurs in southwest with a smaller population in the northwest | | Depleted by heavy exploitation for oil at least until the 1930s. Reproductive success severely hampered for many years by the presence of feral dogs and pigs, but breeding programs are steady. MtDNA evidence shows that there are actually three genetically distinct populations on Santa Cruz island. | 3000 | | Volcán Wolf Tortoise G. n. becki Rothschild, 1901 | Northern Isabela (Albermarle) Island: northern and western slopes of Volcano Wolf | | Reproduction is successful | 2000 | | Iguana Cove Tortoise G. n. vicina Günther, 1875 | Eastern Isabela Island: Cerro Azul | | A typical dome-shelled tortoise. Range overlaps with G. n. guentheri. This population was depleted by seamen in the last two centuries and by extensive slaughter in the late 1950s and 60's by employees of cattle companies based at Iguana Cove. | 700 | | Sierra Negra Tortoise G. n. guntheri Baur, 1889 | Isabela Island: Volcano Sierra Negra, one group in the east and another over the western and southwestern slopes | | Severely depleted by settlement and exploitation for tortoise oil which continued until the 1950s. The wild reproduction is successful in the east but in the western-southwestern area pigs, dogs, rats and cats are present as predators. It is one of the most threatened of the existing subspecies, and 20 adults were taken into captivity for a breeding program in 1998 following the threat of a volcanic eruption from the nearby Cerro Azul volcano. | 500 | | Volcán Darwin Tortoise G. n. microphyes Günther, 1875 | Isabela Island: southern and western slopes of Volcano Darwin | | Heavily exploited in the nineteenth century by whaling vessels, but wild reproduction is successful. | 1000 | | Volcán Alcedo TortoiseG. n. vandenburghi DeSola, 1930 | Central Isabela Island: caldera and southern slopes of Volcano Alcedo | | The largest population in the archipelago, wild reproduction successful. | 5000 | | G. n. phantastica Van Denburgh, 1907 | Fernandina (Narborough) Island (purportedly) | | Known from only one male specimen found (and killed) by members of the 1906 San Francisco Academy of Sciences expedition. There was a discovery of putative tortoise droppings in 1964. However, no other tortoises or even remains have been found on Fernandina and it is entirely possible that that one lone male was a stray or a release. Fernandina is the most pristine of the islands and any tortoise population would not be likely to have become extinct at the hands of introduced animals. If G. n. phantastica was, indeed, a real subspecies, then it is the only one to become extinct by natural means. | Possible existence | | Santa Fe Island Tortoise | Santa Fe Island (purportedly) | | There are only 2 records of whalers removing tortoises, and there are two eye-witness accounts of locals removing tortoises in 1876 and 1890. These accounts, however, were given 15 and 30 years after the incident. Expeditions found old bones but no shell fragments, the most durable part of a tortoise skeleton, casting strong doubt on the validity of this subspecies. | Doubtful existence | | G. n. wallacei | Rabida Island (purportedly) | | This putative subspecies is known from only one specimen. Tracks were seen on Rabida in 1897 and a single individual was removed by the Academy of Sciences in 1906. No logs from whaling or sealing vessels make any mention of collecting at Rabida. Rabida has a good anchorage and near which is found a corral in which tortoises, perhaps from other islands, were temporarily held. The type specimen of G. n. wallacei, the individual from which the race was named, actually has an unknown provenance: it was assigned to Rabida because it resembled the one removed in 1906. | Doubtful existence |
Notable specimens
- Lonesome George is the only known living specimen of the Pinta Island Tortoise.
- Harriet was the second oldest tortoise ever authenticated with an estimated age of 175 years at the time of her death in 2006 in Australia Zoo.
Human disturbance
In the seventeenth 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. The tortoises were collected and stored live on board ships where they could survive for at least a year without food or water, providing valuable fresh meat, whilst their diluted urine and water stored in their neck bags could also be used as drinking water. Of the meat, Darwin wrote: "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 nineteenth century, whaling ships and fur-sealers collected tortoises for food and many more were killed for high grade 'turtle oil' from the late 1800s onward. Darwin described this process thus: "beautifully clear oil is prepared from the fat. When a tortoise is caught, the man makes a slit in the skin near its tail, so as to see inside its body, whether the fat under the dorsal plate is thick. If it is not, the animal is liberated and it is said to recover soon from this strange operation." A total of over 15,000 tortoises is recorded in the logs of 105 whaling ships between 1811 and 1844. As hunters found it easiest to collect the tortoises living round the coastal zones, the least decimated populations tended to be those in the highlands.
Population decline accelerated with the early settlement of the islands, when they were hunted for meat, their habitat was cleared for agriculture and alien mammal species were introduced. Feral pigs, dogs, cats and black rats are effective predators of eggs and young tortoises, whilst goats, donkeys and cattle compete for grazing. In the twentieth century, increasing human settlement and urbanisation and collection of tortoises for zoo and museum specimens depleted numbers even more.
Conservation
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. Appendix I 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. A period of inaction followed until 1959, when it declared all uninhabited areas in the Galápagos to be a National Park and established the Charles Darwin Foundation. In 1970 it was made illegal to capture or remove many species from the islands, including tortoises and their eggs. To halt the trade altogether, it became illegal to export the tortoises from Ecuador, captive or wild, continental or insular in provenance. United States Public Law 91-135 (1969) automatically prohibited the importation of Galápagos tortoises into the U.S.A. as their export was declared illegal. A 1971 decree made it illegal to damage, remove, alter or disturb any organism, rock or other natural object in the National Park. Today, all tour groups must be guided and are asked to stay on the paths to avoid disruption of all flora and fauna.
Young tortoises are raised in a programme by the Charles Darwin Research Station in order to bolster the numbers of the extant subspecies. Eggs are collected from places on the islands where they are threatened and when the tortoises hatch they are kept in captivity until they have reached a size that ensures a good chance of survival and are returned to their original ranges. The Galápagos National Park Service systematically culls feral predators and competitors where necessary such as the complete eradication of goats from Pinta.
The conservation project begun in the 1970s successfully brought 10 of the 11 endangered subspecies up to guarded population levels. The most incredible recovery was that of the Española Tortoise, whose breeding stock comprised 2 males and 11 females brought to the Darwin Station. Fortuitously, a third male was discovered at the San Diego Zoo and joined the others in a captive breeding program. These 13 tortoises gave rise to over 1000 tortoises now released into their home island. In all, 2500 individuals of all breeds have been reintroduced to the islands.
However, persecution still continues on a much smaller scale; more than 120 tortoises have been killed by poachers since 1990 and they have been taken hostage as political leverage by local fishermen.
See also
- Chambers, Paul. A Sheltered Life: The Unexpected History of the Giant Tortoise. John Murray (Publishers), London. 2004. ISBN 0719565286.
Bibliography
External links
- ARKive -
- 'Harriet' the tortoise dies at 175 http://news.bbc.co.uk/1/hi/world/asia-pacific/5109342.stm
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