Giant tortoises are characteristic reptiles that are currently found on two groups of tropical islands: the Aldabra Atoll in Seychelles and the Galapagos Islands in Ecuador (a population at the Mascarene Islands was exterminated by the 1900s). 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 Giant tortoises originally made their way to islands from the mainland; for example, Aldabra Atoll and Mascarenes giant tortoises are related to Madagascar tortoises while Galapagos giant tortoises are related to Ecuador mainland tortoises. This phenomenon of excessive growth is known as island gigantism or insular gigantism. It occurs when the size of the animals that are isolated on an island increases dramatically in comparison to their mainland relatives. This is due to several 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 have been examples of island gigantism, as they originally evolved their massive sizes on the mainland. Giant tortoises were once common across the Cenozoic faunas of Eurasia, Africa and the Americas.
These animals belong to an ancient group of reptiles, appearing about 250 million years ago. By the Upper Cretaceous, 70 or 80 million years ago, some had already become gigantic. About 1 million years ago tortoises reached the Galápagos Islands. Most of the gigantic species began to disappear about 100,000 years ago. Only 250 years ago there were at least 20 species and subspecies in islands of the Indian Ocean and 14 or 15 subspecies in the Galápagos Islands.
|Aldabra giant tortoise|
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 (Colossochelys atlas), North and South America, Indonesia, Madagascar (Dipsochelys), 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 (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. 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. Around the time of its discovery, they were caught for food in such large quantities that they became virtually extinct by 1900. Giant tortoises are now under strict conservation laws and are categorised 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).
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 programmes on these other islands are trying to revive the species, and populations on them today appear to be thriving.
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 particularly vulnerable to being eaten by predators. It is thought that female Aldabra giant tortoises are able to produce more than one clutch a year. The hatchlings emerge after an incubation period of 8 months. The baby Aldabra giant tortoises tend to all emerge during the same two week period which coincides with the arrival of the rainy season. They reach sexual maturity at around the age of 30 years. Although some individuals have been known to live for more than 250 years, their life expectancy is between 80 and 120 years old.
The Aldabra giant tortoise has a dome-shaped shell which acts as protective armor for their soft, vulnerable body that lies underneath. They have incredibly long necks which they use to tear leaves from the branches higher up trees. The males, although not really that much bigger, are also known to weigh nearly 100 kg (220 lb) more than females. They are slow moving animals with thick, short legs and round, almost flat feet that help them when they are walking on the sand.
The Aldabra giant tortoise is primarily found inhabiting grasslands and swamps on the islands of the Aldabra atoll, which forms part of the Seychelles island chain in the Indian Ocean. They once shared these islands with a number of other giant tortoise species, but many of these were hunted to extinction in the 1700s and 1800s. Although they are usually found in areas of dense low-lying vegetation they are also known to wander into more sparse and rocky regions when food is in short supply. They can also be found often resting in the shade or in a very shallow pool of water to cool themselves down from the heat. Aldabra tortoises tend to spend their lives grazing but will cover surprising distances in search of food and are also very much at home on bare rock and thin soil. They can drink from very shallow pools through their nostrils, and the name Dipsochelys refers to this remarkable adaptation.
Remains have been found in the Canary Islands of two extinct species of large tortoises: Geochelone burchardi and Geochelone vulcanica. 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. G. burchardi had a larger shell, with a length of approximately 65 to 94 cm, while G. vulcanica had a 61 cm long shell. Both are related to North African species of Geochelone.
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 eastern-most islands of Española and San Cristóbal first, they then dispersed throughout the archipelago, eventually establishing about 16 separate populations on ten 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 neck that could be used as drinking water.
These buccaneers took giant tortoises not only because of their meat and oil but because of their incredible adaption that allows this 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. It is estimated that 20,000–25,000 wild tortoises live on the islands today.
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 the saddle-back and the domed shell. They both provide special adaption to different environments. The saddle-back shell 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 shell 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.
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 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 2–7 for saddle-backed tortoises to sometimes more than 20–25 eggs for domed tortoises. The eggs incubate from 110 to 175 days (incubation periods depend on the month the nest was laid, 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.
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 the human arrival and the subsequent introduction of domestic species, particularly pigs, the tortoises were rapidly hunted to extinction. Unfortunately, the thin shells were of no protection against these new invaders; pigs, rats and cats 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).
Today all we have 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.