Aldabra Tortoise

The Aldabra giant tortoise is one of the largest tortoises in the world and is native to the Seychelles, with the main population on Aldabra Atoll. Its carapace is brown or tan and high-domed, and its long neck lets it reach branches up to a metre above the ground. It mainly eats grasses, leaves, woody stems and fruit, though it occasionally takes small invertebrates and carrion. Because fresh water is scarce, it gets most of its moisture from food. Its grazing shapes a plant community called tortoise turf, and as the largest animal in its habitat it plays a role similar to that of an elephant.

About the Aldabra tortoise

The Aldabra tortoise is a species of tortoise in the family Testudinidae, placed in its own genus, Aldabrachelys. It is endemic to the Seychelles, and the main population belongs to the nominate subspecies, native to Aldabra atoll itself. Among the largest tortoises alive, it rivals the more widely known Galápagos giant tortoise in size. Its carapace averages 122 cm (48 in) in length; males average around 250 kg (550 lb) in weight, while females run smaller, averaging 91 cm (36 in) in carapace length and 159 kg (351 lb) in weight, though medium captive specimens have been recorded between 70 and 110 kg (150 to 240 lb), and one study found masses up to 132 kg (291 lb) fairly common.

The shell is brown or tan and high-domed, sitting atop stocky, heavily scaled legs built to carry that bulk. What sets the animal apart physically is its neck: unusually long even for a tortoise this size, letting it browse tree branches up to a meter off the ground. Shell shape actually tracks food source. Tortoises living where food grows at ground level tend toward more dome-shaped shells that extend down over the neck, while those in habitats with higher browse have flatter-topped shells with a raised front, freeing the neck to reach upward.

Fossil evidence shows giant tortoises once lived on nearly every continent and countless islands, and were common across the western Indian Ocean and Madagascar before European sailors nearly wiped them out through overexploitation. By 1840 almost all of those island populations were gone, leaving the Aldabra giant tortoise as the sole large-scale survivor. Today its main stronghold is the protected Aldabra Atoll, home to roughly 100,000 individuals, the largest population of the species anywhere. Smaller groups live on Frégate Island and within the Sainte Anne Marine National Park, and conservationists have reintroduced tortoises to Aride Island. An isolated population also lives on Changuu Island near Zanzibar, with additional captive groups kept in conservation parks in Mauritius and Rodrigues, even though the genus is not native there.

Behavior and diet

Aldabra tortoises live both alone and in loose herds, which tend to gather on open grasslands. They are most active in the morning, spending those hours grazing and browsing, then retreat from the heat of the day by digging wallows, resting in shade or small caves, or submerging in pools to cool off. Their heavy grazing has shaped the landscape around them: a distinctive habitat called "tortoise turf" has coevolved on Aldabra, a mix of more than twenty species of grasses and herbs that have adapted to grow their seeds close to the ground, out of reach of the tortoises' cropping jaws. As the largest animal in its environment, the species plays a role similar to an elephant's, felling trees in its search for food and creating pathways other animals use.

The diet is primarily herbivorous: grasses, leaves, woody plant stems, and fruit. Aldabra tortoises will also take small invertebrates and carrion, including, occasionally, the bodies of other dead tortoises. Captive animals are commonly fed fruits such as apples and bananas along with compressed vegetable pellets. In 2020, a female on Frégate Island was observed hunting and eating a juvenile lesser noddy, a bird, suggesting the species may be learning to catch prey beyond plants. Because little fresh water is available in its natural habitat, the tortoise gets most of its moisture from the food it eats.

Mating takes place between February and May. Between July and September, females dig a nest about 30 cm deep and lay between 9 and 25 hard-shelled eggs, usually with fewer than half fertile; a female can produce multiple clutches in a year. After roughly eight months of incubation, independent hatchlings emerge between October and December. Tulsa Zoo, which has kept a small herd since well before 1999, has recorded repeated reproduction, typically with eggs laid in November and again in January when weather allows outdoor nesting; the zoo also incubates eggs artificially, running two incubators at 27°C (81°F) and 30°C (86°F), with the warmer one producing hatchlings after an average of 107 days.

Range and habitat

The core range of the Aldabra tortoise is the Aldabra Atoll in the Seychelles, an island group long protected from human disturbance. Beyond Aldabra itself, smaller wild and semi-wild populations exist on Frégate Island, within the Sainte Anne Marine National Park, and on Aride Island following reintroduction efforts. An additional isolated population lives on Changuu, near Zanzibar, and captive conservation groups are kept in the Mascarene Islands of Mauritius and Rodrigues, even though the genus never occurred there naturally.

Within this range the species uses a variety of habitats, including grasslands, low scrub, mangrove swamps, and coastal dunes. Its grazing pressure has helped create "tortoise turf," a specialized plant community found where the tortoises concentrate, and its movement through vegetation in search of food opens pathways that benefit other species sharing its habitat.

Conservation status

The Aldabra giant tortoise is listed as Vulnerable on the IUCN Red List. That listing dates from a 1996 assessment by the IUCN's Tortoise and Freshwater Turtle Specialist Group, made under an older version of the Red List criteria and under the former scientific name Geochelone gigantea. The species is also listed on Appendix II of CITES, which regulates its international trade.

Its numbers today are concentrated in one protected place: Aldabra Atoll in the Seychelles, sheltered from most human influence, holds some 180,000 giant tortoises, the largest population of the species anywhere. Smaller populations live on Frégate Island and in Sainte Anne Marine National Park, and conservationists have moved 50 young tortoises to Aride Island to join 10 larger ones released there earlier.

The species also has an unusually long history of organized conservation. In the late 19th century, Albert Günther of what became the Natural History Museum of London worked with the government of Mauritius, enlisting Charles Darwin and other scientists, to establish a preserve for the tortoises. More recently, the Nature Protection Trust of Seychelles has run captive-breeding and reintroduction programs for the closely related Seychelles giant tortoise and Arnold's giant tortoise, both now generally treated as part of this same species. Genetic studies mapping differences within and among the Aldabra islands have been used to trace the likely island of origin for individual tortoises in captivity.