Aldabra Giant Tortoise (Aldabrachelys gigantea): Facts

What does an Aldabra giant tortoise look like?

The Aldabra tortoise has a high, domed shell, or carapace, that is brown, tan or dark gray, often worn smooth on older animals. As in all tortoises, the shell is fused to the ribs and backbone, so it is part of the skeleton and cannot be removed. The head and legs can be drawn partly inside for protection.

Its legs are thick and pillar-like, covered with heavy, overlapping scales, and built to carry a huge weight over rough coral rock. The neck is remarkably long for a tortoise. Stretching upward, an Aldabra tortoise can reach leaves and branches roughly 1 m (3 ft) above the ground, and large tortoises sometimes push over small trees to get at the leaves.

Shell shape varies with food supply. Tortoises that feed mostly on ground plants tend to have a rounded dome with the front edge curving down over the neck. Where more of the food grows higher up, the shell is flatter on top and the front rim is raised, giving the neck more room. Males are larger than females, have longer and thicker tails, and have a lower shell (plastron) that curves inward, which helps them balance during mating.

How big do Aldabra tortoises get?

Aldabra giant tortoises are similar in size to the Galápagos giant tortoise, and the two species share the title of the largest living tortoises. Size varies a lot with food and population density.

On parts of Aldabra where tortoises are packed closely together and food is scarce, adults stay much smaller than tortoises living on richer ground or in zoos. The heaviest animals are usually captive males, which get regular food all year.

Where do Aldabra giant tortoises live?

The natural home of the species is Aldabra, a raised coral atoll in the outer Seychelles, about 400 km (250 mi) northwest of Madagascar. The atoll is a ring of low coral islands around a large, shallow lagoon, and it is a UNESCO World Heritage Site with very little human disturbance. It holds the largest population of giant tortoises in the world, commonly estimated at around 100,000 animals, with most living on the largest island, Grande Terre.

Smaller populations have been introduced or are managed on other islands, including Curieuse, Frégate, Moyenne and Bird Island in the Seychelles, Île aux Aigrettes and Round Island off Mauritius, Rodrigues, and Changuu (Prison Island) near Zanzibar. A full guide to these places is in Where do Aldabra giant tortoises live?

On Aldabra the tortoises use open grassland, dense scrub, the edges of mangrove swamps and coastal dunes. Fresh water is scarce, so they get most of their moisture from food and from rain pools.

Diet: grazing and “tortoise turf”

Aldabra tortoises are herbivores. They crop grasses and herbs, browse leaves and woody stems, and eat fallen fruit when they find it. Like many tortoises, they occasionally eat carrion, and they have been recorded eating the remains of other dead tortoises. In 2020, a female on Frégate Island was filmed catching and eating a young seabird, a lesser noddy chick, which surprised researchers; it is an unusual behavior rather than a normal part of the diet.

The tortoises graze so heavily that they have shaped their own habitat. On Aldabra they keep patches of grassland cropped very short, creating a mixed lawn of more than 20 kinds of grasses and herbs known as tortoise turf. Several of these plants are dwarfed forms that flower and set seed close to the ground, out of reach of the tortoises’ jaws.

Because they eat so much plant material, knock down shrubs and carry seeds in their guts, Aldabra tortoises act as ecosystem engineers, playing a role similar to that of elephants on the African mainland. This is why conservationists have introduced them to islands whose own giant tortoises were wiped out.

Behavior and daily life

Aldabra tortoises are not territorial, and they may be found alone or in loose groups, especially on open grassland where food is good. They are most active in the cooler hours of the morning and late afternoon, when they graze and walk between feeding areas.

The midday heat on a coral atoll can be dangerous even for a large reptile. To keep cool, the tortoises rest under shade trees, crowd into small caves and rock overhangs, or lie in muddy pools and wallows. On some parts of Aldabra, shade is so limited that tortoises compete for it, and animals caught in the open sun for too long can overheat.

Giant tortoises are generally quiet, but males make loud groaning or bellowing sounds while mating. They move slowly, at well under 1 km/h (0.6 mph), yet they can cover a surprising distance over a day of feeding.

Reproduction and life cycle

Mating on Aldabra takes place mainly between February and May. Between about June and September, females dig a nest in soil about 30 cm (12 in) deep and lay a clutch of roughly 9 to 25 hard-shelled eggs, often fewer than half of which are fertile. A female may lay more than one clutch in a year when food is good.

The eggs incubate in the warm ground for around eight months, and the young hatch between October and December, at the start of the rainy season when fresh plant growth is available. There is no parental care: hatchlings dig their way out and are independent from the start.

Young tortoises grow slowly and are vulnerable to predators such as coconut crabs and other land crabs, birds and introduced rats. Growth and age at maturity depend on food, and tortoises in crowded areas of Aldabra take longer to reach breeding size than those living where food is plentiful.

How long do Aldabra tortoises live?

Giant tortoises are among the longest-lived animals on Earth. Aldabra tortoises routinely live more than 100 years, and captives with good records have passed 150 years. Exact ages are hard to prove, because most tortoises outlive the people who first recorded them.

Some famous claims are much larger. Adwaita, an Aldabra tortoise that died at Kolkata Zoo in India in 2006, was said to have been about 255 years old, but this was based on stories rather than documents and is not accepted as proven. Esmeralda, a large male (despite the name) on Bird Island in the Seychelles, is often cited as one of the oldest living Aldabra tortoises.

The oldest known living land animal, Jonathan the tortoise on Saint Helena, belongs to the Seychelles giant tortoise, a form now usually placed in this species as the subspecies A. g. hololissa. He is estimated to have hatched around 1832.

Island giants: history and relatives

Giant tortoises once lived on many western Indian Ocean islands, including Madagascar, the granitic Seychelles and the Mascarene Islands of Mauritius, Réunion and Rodrigues. From the 1600s onward, sailors collected them in huge numbers as living food that could be stored on ships for months. By the mid-1800s, every Indian Ocean giant tortoise had disappeared from the wild except those on remote Aldabra.

The Mascarene tortoises belonged to a different genus, Cylindraspis, and are all extinct. Genetic studies suggest that the Aldabra tortoise’s closest known relative is an extinct giant tortoise from Madagascar, and that its ancestors reached Aldabra by floating across the sea, something giant tortoises can do surprisingly well (see Can tortoises swim?).

Today most authorities recognize several subspecies of A. gigantea. Besides the Aldabra form, these include the Seychelles giant tortoise (A. g. hololissa) and Arnold’s giant tortoise (A. g. arnoldi), forms from the granitic Seychelles that were thought extinct until a few captive animals were identified. Their status as separate forms is still debated.

Predators and threats

Adult Aldabra tortoises have no natural predators on their islands; their size and shell protect them. Eggs and hatchlings are taken by land crabs, birds and, on some islands, introduced rats and cats.

Because almost all wild tortoises live on a single atoll, the species is vulnerable to anything that affects Aldabra as a whole: a disease outbreak, invasive species, or climate change. Rising sea levels and more extreme droughts could reduce grassland and shade on low coral islands. Shade is already a limiting factor in hot weather, and a run of severe dry years can reduce numbers. In the past, introduced goats competed with the tortoises for food on Aldabra; they were removed in a long eradication program completed in the early 2010s.

Conservation status

The IUCN Red List classifies the Aldabra giant tortoise as Vulnerable. That assessment was made in 1996 and is out of date, but it reflects the species’ tiny natural range rather than a steep decline. Aldabra itself has been strictly protected for decades and is managed by the Seychelles Islands Foundation. International trade is controlled under CITES Appendix II.

Aldabra has a long conservation history. In the 1870s, scientists including Charles Darwin and Albert Günther of the British Museum urged the government of Mauritius, which then ran the Seychelles, to protect the last giant tortoises. In the 1960s, a plan to build a military airbase on the atoll was dropped after strong opposition from scientists.

Relationship with people

Aldabra tortoises have been moved around the world for centuries, which is why they are now one of the most familiar giant tortoises in zoos in Europe and North America. Many large zoos keep and breed them, and the free-roaming tortoises on islands such as Curieuse and La Digue in the Seychelles are a major attraction for visitors.

They are also important in rewilding. On Île aux Aigrettes off Mauritius, tortoises introduced from 2000 onward replaced the extinct Mauritian giant tortoises as grazers and seed dispersers, and studies there showed that they spread the seeds of a rare native ebony tree. For how these giants differ from aquatic turtles, see turtle vs tortoise, and for their Pacific counterparts see the Galápagos giant tortoise.