Giant tortoise
Giant tortoises are any of various large land tortoises formerly common on the islands of the western Indian Ocean and on the Galápagos Islands.
on Santa Cruz Island
History
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 and can grow to be 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 Giant tortoises originally made their way to islands from the mainland via oceanic dispersal; for example, the Aldabra Atoll and Mascarenes giant tortoises are related to Madagascan tortoises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.Island gigantism
The phenomenon of animal species evolving in cache to unusually large size on islands 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. 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 across the Cenozoic faunas of Eurasia, Africa and the Americas.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, North America and South America, Indonesia, Madagascar, and even the island of Malta became extinct. The giant tortoises formerly of Africa died out somewhat earlier, during the Late Pliocene. 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.
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. Today, only one of the species of the Indian Ocean survives in the wild; the Aldabra giant tortoise and 10 extant species in the Galápagos.
Life expectancy
Giant tortoises are among the world's longest-living animals, with an average lifespan of 100 years or more. The Madagascar radiated tortoise Tu'i Malila was 188 at death in Tonga in 1965. Harriet 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. 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.Aldabra giant tortoise
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.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.
Life cycle
Aldabra giant tortoises reproduce and lay up to 25 rubbery eggs between February and May. Mating is a noisy process, with the males bellowing like a bull, and chasing after the females. During copulation, the male's tail guides his penis into position to fertilize the female, with copulation lasting for 10–15 minutes. Eggs are deposited into a dry, shallow nest on the ground, making them vulnerable to being eaten by predators. It is thought that female Aldabra giant tortoises are able to lay more than one clutch of eggs a year. The hatchlings emerge after an incubation period of 8 months. The hatchling Aldabra giant tortoises tend to all emerge during the same two week period, coinciding with the arrival of the rainy season. They reach sexual maturity at around the age of 30 years. Although a few individuals have been said to live for more than 250 years, their average life expectancy is between 80 and 120 years old.Distribution and habitat
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 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. 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.
Species and subspecies
Canary Islands giant tortoises
Remains have been found in the Canary Islands of two extinct species of large tortoises: the Tenerife giant tortoise and the Gran Canaria giant tortoise. These species are believed to have died out due to volcanic eruptions.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.
The remains date mostly to the Miocene. It is believed that these species survived until the Late Pleistocene.
Galapagos giant tortoises
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." The tortoises also conveniently held water in their necks that could be used as drinking water., the last known individual of the Pinta Island tortoise
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 s 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. 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. It is estimated that 20,000–25,000 wild tortoises live on the islands today.
Distribution and habitat
Galapagos tortoises are herbivorous, feeding primarily on cactus pads, grasses, and native fruit. They drink large quantities of water when available that they can store in their bladders 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 Galapagos tortoises, but present a very long neck and pairs of legs. They live on arid zone and feed on cactus. The domed tortoises are bigger with shorter neck and legs, they are found in the more vegetated islands and feed on grass.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 in 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.
Life cycle
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, female tortoises migrate to nesting 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. The eggs incubate from 110 to 175 days. 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.Species
Mascarenes giant tortoises
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 and the domed Mauritius.
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.