Where Do Aldabra Giant Tortoises Live? The Aldabra Atoll

Aldabra Atoll: the tortoises’ home

Aldabra is one of the largest coral atolls in the world and the largest raised coral atoll, meaning its ancient reef has been lifted a few meters above the sea. It lies in the far southwest of the Seychelles, about 400 km (250 mi) northwest of Madagascar and more than 1,000 km (620 mi) from Mahé, the main island of the Seychelles.

The atoll is a ring of four large islands and many islets around a broad, shallow lagoon. The main islands are Grande Terre (also called South Island), Malabar (Middle Island), Picard (West Island) and Polymnie. The land is mostly jagged coral limestone, called champignon, with little soil and no permanent rivers or lakes.

Because it is so remote and hard to live on, Aldabra was never heavily settled. It has been a UNESCO World Heritage Site since 1982 and is managed by the Seychelles Islands Foundation, which runs a small research station on Picard. Visitors are few and need permits. That isolation is the main reason the Aldabra giant tortoise survived when every other giant tortoise in the Indian Ocean was wiped out by sailors in the 1700s and 1800s.

How many tortoises live on Aldabra?

The atoll’s tortoise population is usually estimated at around 100,000 animals, although counts vary between surveys and over time. Numbers are not spread evenly. Most tortoises live on Grande Terre, the largest island, while Malabar and Picard hold smaller populations.

Density has a striking effect on the animals themselves. In the southeast of Grande Terre, where tortoises are packed closely on open grassland, food and shade are limited and adults stay relatively small and breed less. Where tortoises are fewer and food more plentiful, they grow larger. Population size also rises and falls with rainfall, because a run of dry years reduces the plants they eat.

What habitats do Aldabra tortoises use?

On Aldabra the tortoises use almost every land habitat on the atoll:

• Open grassland and tortoise turf: short lawns of more than 20 grasses and herbs kept closely cropped by the grazing tortoises themselves.
• Scrub and low woodland: dense thickets, including the tough coastal shrub Pemphis acidula, where tortoises browse leaves and find shade.
• Mangrove edges: the fringes of the mangrove forests around the lagoon.
• Coastal dunes and beaches: sandy areas near the shore, where some tortoises feed on plants that grow just above the tide line.

Shade is one of the most important resources. Midday temperatures on the open coral are high, and tortoises spend the hottest hours under trees, inside small caves and rock overhangs, or lying in muddy pools. Fresh water is scarce, so they drink from rain pools when available and otherwise get most of their water from plants.

Introduced and reintroduced populations

Aldabra tortoises have been moved by people for centuries, and several islands now hold free-roaming herds or protected groups.

Thousands more live in zoos and private parks in Europe, North America and elsewhere. Jonathan, the famous old tortoise on the South Atlantic island of Saint Helena, belongs to the closely related Seychelles form; his story is told in Who is Jonathan the tortoise?

Rewilding: tortoises as replacement grazers

Mauritius and Rodrigues once had their own giant tortoises in the genus Cylindraspis, but all were hunted to extinction by the early 1800s. Without them, native plants lost a key grazer and seed disperser, and introduced plants spread.

Since 2000, conservationists have released Aldabra tortoises onto Île aux Aigrettes, a small nature reserve off the coast of Mauritius, as stand-ins for the lost species. Studies there found that the tortoises eat the fruit of a rare native ebony tree and spread its seeds through their droppings, helping new trees grow. A similar project began on Round Island in 2007. The idea, called taxon substitution, uses a living relative to restore the job an extinct animal once did.

How did tortoises reach Aldabra in the first place?

Aldabra has been submerged by the sea several times in its geological history, most recently around 125,000 years ago, so its tortoises must have arrived from elsewhere afterward. Genetic studies point to ancestors from Madagascar. Giant tortoises float well and can survive weeks without food or fresh water, so they probably drifted across the sea on ocean currents. Can tortoises swim? explains how this works and describes a real case of an Aldabra tortoise washed up in East Africa. The Galápagos giant tortoise reached its islands in a similar way from South America.