Reptiles
Aldabra Giant Tortoise
Aldabrachelys gigantea · also called aldabra giant tortoise
The Aldabra giant tortoise is the last survivor of the giant tortoises that once lived across the western Indian Ocean — every other island population was eaten by passing ships. It hangs on because its home, a jagged coral atoll in the Seychelles, was too hard to land on, and today around 100,000 of them live there: the largest wild population of giant tortoises anywhere on Earth.
Overview
Aldabra giant tortoises live almost entirely on Aldabra Atoll in the Seychelles, with managed populations elsewhere. Big males reach about 1.2 m in shell length and 250 kg; females are smaller. Shells are usually high-domed but sometimes flatter or saddle-backed within the same population. They graze grasses, herbs, and leaves, and heavy grazing has produced a distinctive short “tortoise turf” on the atoll.
The last of the Indian Ocean giants
Giant tortoises once lived on Madagascar, Mauritius, Réunion, Rodrigues, and the granitic Seychelles. From the 1600s, ships took them by the thousands as live meat, and introduced pigs, rats, and goats destroyed their eggs and young. Every one of those populations was wiped out. Aldabra’s remoteness and harsh terrain saved its tortoises, and a 19th-century campaign — backed by scientists including Charles Darwin — got the atoll protected.
A very long life
Aldabra giant tortoises routinely pass 100 years. A close relative named Jonathan, living on St Helena, is estimated at over 190 and is regarded as the oldest known living land animal. The tortoises can also go long stretches without food or fresh water, storing water in the bladder and drinking through the nose from shallow puddles.
A shaper of the atoll
By grazing, trampling paths, toppling saplings, and spreading seeds in their dung, the tortoises shape the entire plant community of Aldabra. They are now introduced to other islands as ecological stand-ins for the extinct tortoises there.
Status
The Aldabra giant tortoise is Vulnerable. The population is large and stable, but its concentration in one low-lying place leaves it exposed to disease, introduced predators of hatchlings, and sea-level rise.
Aldabra Giant Tortoise: common questions
How long do Aldabra giant tortoises live?
Longer than almost any other animal. Many Aldabra giant tortoises live past 100 years, and captive individuals with reasonable records have reached 150 or more. The most famous case is Jonathan, a giant tortoise on the island of St Helena, hatched around the 1830s and therefore estimated to be over 190 years old, which makes him the oldest known living land animal. Jonathan is usually identified as a Seychelles giant tortoise, a close relative of the Aldabra species, but the two are so similar in biology that his age gives a good sense of the potential lifespan. Verifying these ages is difficult because the tortoises outlive the people and records that track them.
Why is the Aldabra giant tortoise the only one of its kind left?
Because its home was too difficult for sailors to exploit. Giant tortoises once lived on many islands of the western Indian Ocean, including Madagascar, Mauritius, Reunion, Rodrigues, and the granitic Seychelles. From the seventeenth century onward, ships took them in enormous numbers as a source of fresh meat that could be kept alive in the hold for months, and introduced pigs, rats, and goats destroyed eggs and young. Every one of those island populations was wiped out. Aldabra Atoll, a ring of jagged raised coral with little fresh water and a treacherous lagoon, was hard to land on and hard to move around, so its tortoises survived, and a nineteenth-century campaign led by scientists including Charles Darwin helped get the atoll protected.
How is the Aldabra giant tortoise different from the Galapagos giant tortoise?
They are not close relatives; they are two separate lineages of tortoise that independently evolved giant size on remote islands. The Aldabra species comes from a single Indian Ocean atoll and exists as one very large population. The Galapagos tortoises are a group of related forms spread across different islands and volcanoes of the Galapagos, several of them endangered or already extinct. Aldabra tortoises tend to have somewhat smaller maximum size than the largest Galapagos tortoises, and their shells vary from high-domed to saddle-backed within the one population, whereas in Galapagos tortoises dome and saddle shapes largely correspond to different island races.
Are Aldabra giant tortoises used in conservation elsewhere?
Yes, in a technique sometimes called rewilding or ecological replacement. Because the extinct tortoises of other Indian Ocean islands played key roles as grazers and seed dispersers, conservationists have introduced Aldabra giant tortoises to some of those islands, such as small islets around Mauritius and Rodrigues, to take over that ecological function. The tortoises crop invasive weeds, spread the seeds of native plants, and help restore a more natural vegetation structure. The Aldabra species is chosen because it is the closest available match to what was lost and because its own numbers are healthy enough to spare founders.