Giant tortoises are any of several species of various large landtortoises, which include a number of extinct species,[1] as well as two extant species with multiple subspecies formerly common on the islands of the westernIndian Ocean and on theGalápagos Islands.[2]
As of February 2024, two different species of giant tortoise are found on two remote groups of tropical islands:Aldabra Atoll and Fregate Island in theSeychelles and theGalápagos Islands inEcuador. Thesetortoises can weigh as much as 417 kg (919 lb) and can grow to be 1.3 m (4 ft 3 in) long. Giant tortoises originally made their way to islands from the mainland viaoceanic dispersal. Tortoises are aided in such dispersal by their ability to float with their heads up and to survive for up to six months without food or fresh water.[3]
Giant tortoises were once all placed in a single genus (often referred to asTestudo orGeochelone), but more recent studies have shown that giant tortoises represent several distinct lineages that are not closely related to one another.[3] These lineages appear to have developed large size independently and, as a result, giant tortoises arepolyphyletic. For example, theAldabra Atoll (Aldabrachelys) giant tortoises are related to Malagasy tortoises (Asterochelys) while theGalapagos giant tortoises are related to South American mainland tortoises, particularly theChaco tortoise (Chelonoidis chiliensis). The recently extinctMascarene giant tortoises (Cylindraspis) are thought to have belonged to their own branch of the tortoise family, beingsister to all other modern tortoise genera aside fromManouria,Gopherus, andTestudo.
Giant tortoises are classified into several distinct genera, includingAldabrachelys,Centrochelys (in part, often excluding the extantAfrican spurred tortoise (Centrochelys sulcata)),Chelonoidis (in part), †Cylindraspis (extinct c. 1840), †Hesperotestudo (extinct c. 9,000 yearsBefore Present), †Megalochelys, †Solitudo, and †Titanochelon. BothMegalochelys andTitanochelon reached sizes substantially greater than modern giant tortoises, with up to 2.4 m (7 ft 10 in) and 2 m (6 ft 7 in) shell lengths respectively.
The phenomenon of animal species evolving to unusually large size on islands (in comparison to continental relatives) is known asisland gigantism or insular gigantism. This may occur due to factors such as relaxed predation pressure, competitive release, or as an adaptation to increased environmental fluctuations on islands.[4][5] However, giant tortoises are no longer considered to be classic examples of island gigantism, as similarly massive tortoises are now known to have once been widespread. Giant tortoises were formerly common (prior to theQuaternary extinctions) across theCenozoic faunas ofEurasia,Africa and theAmericas.[6][7]
Giant tortoises are notably absent fromAustralia and theSouth Pacific. However, extinct giant horned turtles (Meiolaniidae) likely filled a similar niche, with LatePleistocene-Holocene meiolaniid species being known fromAustralia,New Caledonia,Lord Howe Island,Vanuatu, and theFijian Archipelago. The identity of the Vanuatu meiolaniid has been controversial, however, with some studies concluding the remains actually belong to a giant tortoise, which are otherwise unknown from this region.[8] Older (EarlyMiocene) meiolaniids are also known from theSt. Bathans fauna inNew Zealand.
Although often considered examples ofisland gigantism, prior to the arrival ofHomo sapiens giant tortoises also occurred in non-island locales, as well as on a number of other, more accessible islands. During thePleistocene, and mostly during the last 50,000 years, tortoises of the mainland of southern Asia (†Megalochelys atlas),[6] North America (†Hesperotestudo spp.)[6] and South America (Chelonoidis spp.),[7]Indonesia,[6]Madagascar (†Aldabrachelys)[6] and even the island ofMalta[6] all became extinct.[1]
Giant tortoises (†Titanochelon) also inhabited mainland Europe until the Early Pleistocene (2.0 Mya).[9] The giant tortoises formerly of Africa died out somewhat earlier, during theLate Pliocene.[10] While the timing of the disappearances of various extinct giant tortoise species seems to correlate with the arrival of humans, direct evidence for human involvement in these extinctions is usually lacking; however, such evidence has been obtained in the case of the distantly-related giantmeiolaniid turtleMeiolania damelipi inVanuatu.[11][12] One interesting relic is the shell of an extinct giant tortoise found in a submerged sinkhole in Florida with a wooden spear piercing through it, carbon dated to 12,000 years ago.[13][better source needed]
Today, only one of the subspecies of the Indian Ocean survives in the wild; theAldabra giant tortoise[1] (two more are claimed to exist in captive or re-released populations, but some[vague] genetic studies have cast doubt on the validity of these as separate species)[citation needed] and 10extant species in theGalápagos.
Giant tortoises are among the world's longest-living animals, with an average lifespan of 100 years or more.[14] TheMadagascarradiated tortoiseTu'i Malila was 188 at her death inTonga in 1965.[citation needed]Harriet (initially thought to be one of the threeGalápagos tortoises brought back to England fromCharles Darwin'sBeagle voyage, but later shown to be from an island not even visited by Darwin) was reported by theAustralia Zoo to be 176 years old when she died in 2006.[15]
On 23 March 2006, anAldabra giant tortoise namedAdwaita died at theAlipore Zoological Gardens inKolkata. He was brought to the zoo in the 1870s from the estate ofLord Clive and is thought to have been around 255 years old when he died.[16][better source needed] Around the time of its discovery, they were caught for food in such large numbers that they became virtually extinct by 1900.[citation needed] Giant tortoises are now protected by strict conservation laws and are categorized as threatened species.
Taxonomy of extant and extinct insular giant tortoise species follows Rhodinet al. (2021),[17] unless otherwise noted.
| Archipelago | Island | Species |
|---|---|---|
| Seychelles | Granitic Seychelles[a][b] |
|
| Aldabra Atoll | Aldabra giant tortoise (A. gigantea gigantea) | |
| Cosmoledo | †Aldabrachelys sp.[19] | |
| Denis Island | ||
| Assumption Island | ||
| Astove Atoll | ||
| Glorioso Islands | Glorioso Islands | †Aldabrachelys sp.[19] |
| Comoro Islands | Comoro Islands | †Aldabrachelys sp.[19] |
| Madagascar | Madagascar |
|
| Archipelago | Island | Species |
|---|---|---|
| Mascarene Islands | Réunion | †Réunion giant tortoise (Cylindraspis indica) |
| Rodrigues |
| |
| Mauritius |
| |
| Malta | Malta | †Centrochelys robusta[c] |
| Canary Islands | Tenerife | †Tenerife giant tortoise ("Centrochelys" burchardi)[22] |
| Gran Canaria | †Gran Canaria giant tortoise ("Centrochelys" vulcanica)[22] |
TheAldabra giant tortoise (Aldabrachelys gigantea) lives on the remoteAldabra Atoll, one of theSeychelles group of islands in theIndian Ocean It is the only living species in the genusAldabrachelys. Two other species in the genus,Aldabrachelys abrupta, andAldabrachelys grandidieri were formerly endemic to Madagascar, but became extinct after the arrival of people.
Aldabra giant tortoises have large dome-shapedshells in order to protect their delicate bodies that lie beneath their shells. They also have long necks in order to eat leaves from the higher branches of trees. The males, although not much bigger than the females, weigh nearly 100 kg (220 lbs) more. They move slowly and have small, thick legs and round, almost flat feet that assist them in walking on sand.
The Aldabra giant tortoise mainly inhabitsgrasslands andswamps on Aldabra Atoll's islands, which form a part of the Seychelles island chain in the Indian Ocean. In the past, they shared the islands with multiple other giant tortoise species, but many of them were hunted toextinction in the 1700s and 1800s.[citation needed] Despite the fact that they are usually found in regions of dense low-lying vegetation, they have been known to wander into areas with more sparse vegetation and rocks when food is scarce. They can also be seen resting in shaded areas or shallow pools of water in order to cool themselves on hot days.[16][better source needed] Aldabra giant tortoises tend to spend their livesgrazing, but will cover surprising distances in search of food and have also been observed on bare rock and thin soil. They can drink from very shallow pools through theirnostrils; the former genusDipsochelys refers to thisadaptation.[23][full citation needed]
The closest living relative of theGalápagos giant tortoise (Chelonoidis niger) is the smallChaco tortoise fromSouth America, although it is not a directancestor. Scientists believe the first tortoises arrived to the Galápagos 2–3 million years ago by drifting 600 miles from the South American coast on vegetation rafts or on their own.[citation needed] They were already large animals before arriving in the Galápagos. Colonizing the easternmost islands of Española and San Cristóbal first, they then dispersed throughout thearchipelago, eventually establishing about 16 separate populations on 10 of the largest of the Galápagos Islands. Currently, there are only 10 subspecies of Galápagos giant tortoises left of the original 16 subspecies. During the 16th and 17th centuries, the Galápagos were frequented bybuccaneers preying on Spanish treasure ships. Filling a ship's hold with tortoises was an easy way to stock up on food, a tradition that was continued by whalers in the centuries that followed.[24][better source needed] The tortoises also conveniently held water in their necks that could be used as drinking water.
These buccaneers stocked giant tortoises not only because of their meat but because of these animals' ability to survive for six months to one year without food or water.[citation needed] Once buccaneers, whalers and fur sealers discovered that they could have fresh meat for their longvoyages by storing live giant tortoises in the holds of their ships, massive exploitation of the species began. Tortoises were also exploited for their oil,[citation needed] which was used to light the lamps of Quito.
Two centuries of exploitation resulted in the loss of between 100,000 and 200,000 tortoises. Three subspecies have been extinct since the 19th century, and a fourth subspecies lost its last member,Lonesome George, in June 2012.[25] In February 2019, a tortoise subspecies once thought to have been extinct since 1906, the Fernandina giant tortoise, was discovered on its namesake island in the Galápagos.[26] It is estimated that 20,000–25,000 wild tortoises live on the islands today.[25][better source needed]
Galápagos tortoises are mainlyherbivorous, feeding primarily on cactus pads, grasses, and native fruit, but have been recorded eating baby birds in the case of the aldabran species. They drink large quantities of water when available that they can store in theirbladders for long periods of time. There are two main types of shell among them, the saddle-backed shell and the domed shell. They both provide special adaption to different environments. The saddle-backed tortoises are the smallest Galápagos tortoises, but present a very long neck and pairs of legs. They live onarid zone and feed oncactus. The domed tortoises are bigger with shorter neck and legs, they are found in the more vegetated islands and feed on grass.[27]
They spend an average of 16 hours a day resting. Their activity level is driven by ambient temperature and food availability. In the cool season, they are active at midday, sleeping in during the morning and afternoon. In the hot season, their active period is early morning and late afternoon, while midday finds them resting and trying to keep cool under the shade of a bush or half-submerged in muddy wallows.[citation needed]
Tortoises breed primarily during the hot season from January to May; however, tortoises can be seen mating any month of the year. During the cool season (June to November), female tortoises migrate tonesting zones, which are generally located in low lands of the islands, to lay their eggs. A female can lay from 1–4 nests over a nesting season from June to December. She digs the hole with her hind feet, then lets the eggs drop down into the nest, and finally covers it again with her hind feet. The number of eggs ranges from 2 to 7 for saddle-backed tortoises to sometimes more than 20 to 25 eggs for domed tortoises.[citation needed]
The eggs incubate from 110 to 175 days (incubation periods depend on the month the clutch was produced, with eggs laid early in the cool season requiring longer incubation periods than eggs laid at the end of the cool season, when the majority of their incubation will occur at the start of the hot season). After hatching, the young hatchlings remain in the nest for a few weeks before emerging out a small hole adjacent to the nest cap. Usually, the temperature of the nest influences on the sex of the hatchling. Warm temperatures would yield more females, while colder temperatures would yield more males.[citation needed]
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TheMascarene Islands ofMauritius,Réunion andRodrigues once harboured five species of giant tortoise belonging to the extinct genusCylindraspis, comprising two species occurring on Mauritius, another two on Rodrigues, and one on Réunion. The tortoises were unique to these islands and had gained a number of special adaptations in the absence of groundpredators. They differed from any other giant tortoise species because of their modified jaws, reduced scales on the legs and shells averaging just 1mm thick. The shells of the giant tortoises were open-ended; the nameCylindraspis actually means "cylinder-shaped". This was a specific adaptation in response to the lack of predators, where thick, heavily armored shells were no longer necessary.[citation needed]
They belonged to a far more ancient lineage than the two extant giant tortoises, having diverged from all other tortoises during theEocene, with divergence between the individual species far greater than that between the insular subspecies of the extant tortoises. The divergences between someCylindraspis species are thought to be even older than the geologic history of the modern Mascarenes themselves, indicating thatCylindraspis originally inhabited several now-submerged island chains of theMascarene Plateau before colonizing the modern Mascarene islands following their formation.[17][28]
Around the 16th century, with human arrival and the subsequent introduction of domestic animals, particularlypigs, the tortoises were hunted toextinction.[citation needed] The thin shells were of no protection against these new invaders;rats,cats and pigs devoured the eggs and young and thousands were collected alive for provisioning ships. Sometimes they were even hunted for their oil,[citation needed] which was very valuable around that time because it provided a cure for manyailments, includingscurvy.
On Mauritius, the giant tortoise disappeared from the main island by the end of the 17th century and the very last tortoises survived until the 1730s on theislets in the north. Around the late 1800s, large number of tortoise bones were discovered in theMare aux Songes excavations.[citation needed] These resulted in the description of the two species of giant tortoise endemic to Mauritius, theMauritius saddle-backed (Cylindraspis inepta) and theMauritius domed (Cylindraspis triserrata).[29]
Today, the only remains from these five species are a number offossil bones and shells, a few drawings of live animals and one stuffedRodrigues saddle-backed giant tortoise in France'sNational Museum of Natural History.[30][better source needed]
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