Species Profile: Aldabra Giant Tortoise

A tortoise seeks shade from the hot midday sun. Credit: Maeve Quaid

A tortoise seeks shade from the hot midday sun. Credit: Maeve Quaid

Name: Aldabra Giant Tortoise (Aldabrachelys gigantean)
Range: Indian Ocean islands of the Aldabra Atoll in the Seychelles and captive populations reside in conservation parks on the island of Mauritius.
Diet: Herbivorous
Conservation Status: IUCN – Vulnerable

The giant Aldabra tortoise of the Indian Ocean islands, are some of the largest species of terrestrial tortoise, rivalling those from the Galapagos Islands, with whom they shared a common ancestor ~20 million years ago. The Aldabra tortoise originally dispersed from Madagascar to the islands of Seychelles, Mauritius, Réunion, Rodrigues and Zanzibar ~17.5 million years, and since then the islands have experienced successive colonization events, with the last wave of  colonists arriving ~80,000 years ago. Although historically, the Aldabra tortoises shared the Indian Ocean islands with a dozen other species of giant tortoises, today it is the sole survivor of a clade of giants. Once widespread amongst the Indian Ocean islands, like those on the Galapagos Islands, the Aldabra tortoises were easy targets for hunters and merchants, and were ruthlessly exploited until extinction occurred in most of their range. By the 1800s their populations were reduced to the Seychelles and the Aldabra Atoll. By the 1900s they occurred only on the Aldabra Atoll. There is a population of ~100,000 currently residing on Aldabra.

Aldabran Tortoise at the  Tsimbazaza Zoo . Credit: Rob Gandola

Aldabran Tortoise at the Tsimbazaza Zoo . Credit: Rob Gandola

The large size of these turtles made them appealing to hunters and merchant sailors, because they provided a significant stockpile of fresh meat, without requiring frequent access to fresh food or water. In a time before modern refrigeration, this proved an invaluable way to feed a crew of men on a long voyage. Keeping other livestock, such as poultry or goats, would require a large supply of fresh food and water, as well as storage space for both the animals and their food. In contrast, giant tortoises could be “shelved”, literally racked, stacked, and strapped on top of each other and kept for months at a time.

This gigantism, that was so appealing to the sailors, is also the very trait that allowed them to colonise the islands in the first place. There has been much debate on the origins of gigantism in tortoises; they are unusual in that these terrestrial giants are strictly insular and don’t occur on the continental mainland. Some early theories suggested that the giants found today, are dwarfs compared to some of the extinct species which once occurred even on the large continents. Other theories suggested that the large size helps these cold blooded animals to retain body heat for longer, a phenomenon known as gigantothermy. This suggestion has been quashed by the discovery of  giant turtles in the fossil record, which lived in warmer climates than today.

Young tortoises being transported to a nursery on Ile aux Aigrettes Credit: Maeve Quaid

Young tortoises being transported to a nursery on Ile aux Aigrettes Credit: Maeve Quaid

The explanation currently favoured, is a form of genetic drift known as the founder effect. This occurs when a few individuals, representing only a sub-sample of the original population disperse and colonise an isolated habitat, such as an island. Within the original population, if a few individuals express abnormal traits, such as gigantism, gene flow among the larger population will suppress these traits and prevent them becoming widespread within the gene pool. However, on a small island, these mutations can quickly become typical due to inbreeding within the small population. This trait amplification, either dooms the population (for example, if the trait makes them more susceptible to predation) or is advantageous. In the case of these tortoises, the trait for gigantism became fixed and remained, as natural selection favoured this trait on these island habitats. So while the original mainland population may not have been giants, only the largest tortoises could survive the long sea journeys to colonise these distant islands.

Credit: Rob Gandola

Credit: Rob Gandola

The extinction of Pleistocene megafauna such as mammoths, giant ground sloths, and giant tortoises, has had a profound effect on the ecosystems they once occupied. Pleistocene rewilding is a concept of re-introducing megafauna from nearby areas to habitats they once occupied during the Pleistocene epoch. In the absence of the original species, their closely related ecological counterparts can be introduced in their stead to restore ecological balance. These “ecosystem engineers” create and maintain a specific ecosystem structure through services such as grazing, browsing and seed dispersal.

A tortoise that has been introduced to Ile aux Aigrettes. Credit: Maeve Quaid.

A tortoise that has been introduced to Round island. Credit: Maeve Quaid.

On the island of Ile aux Aigrettes, Mauritius, two giant tortoises once provided this ecosystem service by maintaining this delicate balance as the dominant grazing and browsing animals. The saddleback tortoises, had long necks that allowed them to reach up and pluck leaves from bushes, while the dome-shaped tortoise had a short neck for grazing. In their absence, competitors flourished and over populated, causing a shift of balance in the ecosystem. By re-wilding Ile aux Aigrettes with giant Aldabra tortoises from neighbouring islands, this delicate balance can be restored in time. In 2000, 20 tortoises were released to free roam the island, and to date the re-wilding project has been successful in re-shaping the landscape with the natural dispersal of vegetation and reproduction of tortoises.

JP

About the author: John is an Irish herpetologist working at the Herpetarium in the Alistair Reid Venom Research Unit, at the Liverpool School of Tropical Medicine. He is also a Science officer with the Herpetological Society of Ireland.

For further reading: http://www.mauritian-wildlife.org/application/index.php?tpid=30&tcid=80

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