Giant tortoise

Giant tortoises are from two remote groups of tropical islands: the Aldabra Atoll and Fregate Island in Seychelles and the Galápagos Islands in Ecuador. These tortoises 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 Giant tortoises originally made their way to islands from the mainland via oceanic dispersal; for example, the Aldabra Atoll and Mascarenes giant tortoises are related to Madagascan tortoises while the Galapagos giant tortoises are related to South American mainland tortoises. Tortoises are aided in such dispersal by their ability to float with their heads up, and to survive up to six months without food or fresh water.[2]

The phenomenon of animal species evolving in cache to unusually large size on islands (in comparison to continental relatives) is known as island 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.[3][4] 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 the Quaternary extinctions) across the Cenozoic faunas of Eurasia, Africa and the Americas.[5][6]

Although often considered examples of island gigantism, prior to the arrival of Homo sapiens giant tortoises also occurred in non-island locales, as well as on a number of other, more accessible islands. During the Pleistocene, and mostly during the last 50,000 years, tortoises of the mainland of southern Asia (Megalochelys atlas),[5] North America[5] and South America,[6] Indonesia,[5] Madagascar (Aldabrachelys),[5] and even the island of Malta[5] became extinct. The giant tortoises formerly of Africa died out somewhat earlier, during the Late Pliocene.[7] 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 giant meiolaniid turtle Meiolania damelipi in Vanuatu.[8][9] One interesting relic is the shell of an extinct giant tortoise found in a submerged sinkhole in Florida with a wooden spear piercing it, carbon dated to 12,000 years ago.[10] Today, only one of the species of the Indian Ocean survives in the wild; the Aldabra giant tortoise (two more are claimed to exist in captive or re-released populations, but some genetic studies have cast doubt on the validity of these as separate species) and 10 extant species in the Galápagos.

Giant tortoises are among the world's longest-living animals, with an average lifespan of 100 years or more.[11] The Madagascar radiated tortoise Tu'i Malila was 188 at death in Tonga in 1965. Harriet (initially thought to be one of the three Galápagos tortoises brought back to England from Charles Darwin's Beagle voyage but later shown to be from an island not even visited by Darwin) was reported by the Australia Zoo to be 176 years old when she died in 2006. Also, on 23 March 2006, an Aldabra giant tortoise named Adwaita died at Alipore Zoological Gardens in Kolkata. He was brought to the zoo in the 1870s from the estate of Lord Clive and is thought to have been around 255 years old when he died.[12] Around the time of its discovery, they were caught for food in such large numbers that they became virtually extinct by 1900. Giant tortoises are now protected by strict conservation laws and are categorized as threatened species.

The Aldabra giant tortoise lives on the remote Aldabra Atoll, one of the Seychelles group of islands in the Indian Ocean. It is the only Indian Ocean giant tortoise species alive today, others having become extinct soon after the arrival of human settlers (including the Seychelles giant tortoise which is now thought to be extinct in the wild, although the Aldabra giant tortoise and the Seychelles giant tortoise are so similar genetically that they are thought by some to be the same species).[12][13]

Today, the Aldabra giant tortoise is listed as an animal that is vulnerable to extinction in the wild. However, the Aldabra atoll has now been protected from human influence after having been declared a World Heritage Site, and is home to some 152,000 Aldabra giant tortoises, the world's largest population of this species. Another isolated population of the Aldabra giant tortoise resides on the island of Zanzibar, and other captive populations exist in conservation parks in Mauritius and Rodrigues. The captive breeding programs on these other islands are trying to revive the species, and populations on them today appear to be thriving.[12]

Distribution and habitat

Aldabra giant tortoise cooling down in a freshwater pond on Curieuse, Seychelles

Aldabra giant tortoises have large dome-shaped shells 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 is mainly inhabits grasslands and swamps on the 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 to extinction in the 1700s and 1800s. 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.[12] Aldabra tortoises tend to spend their lives grazing, but will cover surprising distances in search of food and are also observed on bare rock and thin soil. They can drink from very shallow pools through their nostrils; the former name Dipsochelys refers to this adaptation.[15]

Restoration of the Tenerife giant tortoise Centrochelys burchardi in Museo de la Naturaleza y el Hombre of Santa Cruz de Tenerife

Remains have been found in the Canary Islands of two extinct species of large tortoises: the Tenerife giant tortoise (Centrochelys burchardi) and the Gran Canaria giant tortoise (Centrochelys vulcanica). These species are believed to have died out due to volcanic eruptions.[16]

The Canary Islands tortoises were similar to those currently found in the Galapagos and Seychelles. C. burchardi had a larger shell, with a length of approximately 65 to 94 cm, while G. vulcanica had a 61 cm long shell.[16]

The remains date mostly to the Miocene. It is believed that these species survived until the Late Pleistocene.[16]

The closest living relative of the Galapagos giant tortoise is the small Chaco tortoise from South America, although it is not a direct ancestor. Scientists believe the first tortoises arrived to Galapagos 2–3 million years ago by drifting 600 miles from the South American coast on vegetation rafts or on their own. They were already large animals before arriving in Galapagos. Colonizing the easternmost islands of Española and San Cristóbal first, they then dispersed throughout the archipelago, eventually establishing about 16 separate populations on 10 of the largest Galapagos Islands. Currently there are only 10 species of Galapagos giant tortoises left of the original 16 species. During the 16th and 17th centuries, the Galápagos were frequented by buccaneers 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: "whaling skippers were almost lyrical in their praise of tortoise meat, terming it far more delicious than chicken, pork or beef". They said the meat of the giant tortoise was 'succulent meat and the oil from their bodies as pure as butter, but best of all, the giants could hibernate in a ship's damp for a year or more."[17] The tortoises also conveniently held water in their necks that could be used as drinking water.

Lonesome George, the last known individual of the Pinta Island tortoise (C. abingdonii)

These buccaneers took giant tortoises not only because of their meat and oil but because of their incredible adaption that allows these animals to survive one year without food or water. Once buccaneers, whalers and fur sealers discovered that they could have fresh meat for their long voyages by storing live giant tortoises in the holds of their ships, massive exploitation of the species began. Tortoises were also exploited for their oil, 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 species have been extinct for some time, and a fourth species lost its last member, Lonesome George, in June 2012.[18] In February 2019, a tortoise species once thought to have been extinct since 1906, the Fernandina giant tortoise, was discovered on its namesake island in the Galapagos.[19] It is estimated that 20,000–25,000 wild tortoises live on the islands today.[18]

The Mascarene Islands of Mauritius, Réunion and Rodrigues once harboured five species of giant tortoise, 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 ground predators. 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 name Cylindraspis 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.

Around the 16th century, with human arrival and the subsequent introduction of domestic animals, particularly pigs, the tortoises were rapidly hunted to extinction. Unfortunately, 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, which was very valuable around that time because it provided a cure for many ailments, including scurvy.

On Mauritius, the giant tortoise disappeared from the mainland by the end of the 17th century and the very last tortoises survived until the 1730s on the islets in the north. Around the late 1800s, large number of tortoises bones were discovered in the Mare aux Songes excavations. These resulted in the description of the two species of giant tortoise endemic to Mauritius, the saddle-backed Mauritius (Cylindraspis inepta) and the domed Mauritius (Cylindraspis triserrata).[21]

Today, the only remains from these five species are a number of fossil bones and shells, a few drawings of live animals, and one stuffed saddle-backed Rodrigues giant tortoise in France's National Museum of Natural History.[22]

• Island gigantism

• A giant Aldabra tortoise
• Galapagos Giant Tortoise