Giant tortoises live in only two remote island groups: the Galápagos archipelago in the Pacific Ocean, about 600 miles off the coast of Ecuador, and the Aldabra Atoll in the Indian Ocean, part of the Seychelles northeast of Madagascar. These are the last wild populations of giant tortoises on Earth, though rewilding projects are now establishing new colonies on a handful of other islands.
The Galápagos Islands
The Galápagos archipelago is home to multiple species of giant tortoise, each adapted to the specific conditions of its island. Most islands with tortoises host a single species, with two notable exceptions. Santa Cruz Island has two species that arose from separate colonization events and live in different parts of the island. Isabela, the largest island, has five distinct species, one associated with each of its five volcanoes.
Other islands with tortoise populations include San Cristóbal, Pinzón, and Española. Some islands have lost their tortoises entirely: the Floreana tortoise went extinct in the 1800s, and a dome-shelled species on San Cristóbal likely disappeared between the 1880s and 1930s. On Floreana, around 600 tortoises of another species (originally from Española) have been brought in since 2015 to fill the ecological role left behind.
The different environmental conditions across islands drove the evolution of distinct shell shapes and body sizes. Tortoises on drier, low-lying islands tend to have saddleback shells with raised fronts that let them stretch their necks upward to reach cactus pads. Those on lusher, higher islands tend to have rounded dome shells and shorter necks, suited to grazing on ground-level vegetation.
How Galápagos Tortoises Use Their Landscape
Galápagos tortoises don’t simply stay in one spot. On Santa Cruz Island, a portion of the population makes long-distance seasonal migrations up and down the island’s slopes. During the cool dry season from June to December, lowland vegetation dries out and food becomes scarce. Persistent cloud cover in the highlands keeps moisture and plant growth going year-round, so migratory tortoises trek uphill to where the food is, essentially “following the green.”
When the hot wet season arrives from January to May, heavy rains trigger a burst of fresh growth in the lowlands, and the tortoises move back down. This pattern gives migrants access to high-quality food throughout the year, and research shows it improves their body condition and likely their ability to reproduce compared to tortoises that stay put. Not every tortoise migrates, though. The population is a mix of migrants, sedentary individuals, and nomads.
Their daily life is straightforward: feeding on grasses, flowers, and prickly pear cactus, basking in the sun to regulate body temperature, and sleeping up to 16 hours a day.
The Aldabra Atoll
The other wild population of giant tortoises lives roughly 7,000 miles away on Aldabra, a coral atoll in the western Indian Ocean. Aldabra is a dramatically different landscape from the volcanic Galápagos. The atoll is bordered by jagged limestone and small beaches, with a large mangrove-lined lagoon at its center. Tortoises here occupy scrub habitat, mangrove swamp, and coastal dunes.
The largest concentration of Aldabra tortoises is found on open grasslands called platins. These flat, grassy areas provide the bulk of their grazing. The Aldabra tortoise is the largest animal on the atoll, filling a dominant ecological role with no land predators or competitors of comparable size.
Why Only Islands?
Giant tortoises weren’t always confined to remote islands. During the Pleistocene epoch, which ended roughly 12,000 years ago, giant tortoises roamed across North and South America, Europe, Africa, and Asia. Fossils of massive tortoises have been found as far north as what is now southern Texas. As the Pleistocene ended, a combination of climate shifts and the spread of human hunters drove mainland giant tortoises to extinction. The populations that survived were those on isolated islands where humans arrived late or not at all, giving these slow-moving animals a refuge from hunting and habitat destruction.
Even on islands, they weren’t safe forever. Sailors, whalers, and colonists killed huge numbers of Galápagos and Indian Ocean tortoises for food starting in the 1600s. The entire genus of giant tortoises native to the Mascarene Islands near Mauritius was wiped out completely.
New Homes Through Rewilding
Conservation programs are now placing Aldabra tortoises on islands where their extinct relatives once lived, using them as ecological stand-ins. Tortoises have been introduced to Île aux Aigrettes, Round Island, and Rodrigues in Mauritius, where they help restore native ecosystems by dispersing seeds and grazing on vegetation in ways that mimic the role of the original, now-extinct Mascarene tortoises.
In 2018, a rewilding project launched at the Anjajavy Reserve in northwest Madagascar, starting with 12 founder tortoises. Giant tortoises had been absent from Madagascar for centuries, and the goal is to rebuild the ecological relationships those animals once maintained. Scientists have proposed creating an artificial network of connected populations by exchanging juveniles between these various island reintroduction sites, which would improve genetic diversity and long-term survival prospects.
In the Galápagos, conservation efforts focus on protecting nesting sites and monitoring wild populations. Over 90 tortoises across multiple islands now carry satellite tracking devices, and field teams conduct regular expeditions to remote nesting areas on volcanoes like Sierra Negra, where they collect eggs and hatchlings threatened by predators.