Aldabra Atoll’s giant tortoise (Aldabrachelys gigantea) population has grown to a record of about 183,000, according to a new study, making the Seychelles atoll home to the largest wild group of the giant reptiles in the world.
Researchers found the population stayed relatively stable from 2018 to 2021, averaging 183,274 tortoises. It is the highest estimate ever made for the remote Indian Ocean atoll. The study, led by Lara B. Hillyard, was published Sept. 4 in the journal “Ecology and Evolution.”
The numbers point to a striking recovery. Heavy exploitation in the 1800s cut the population to fewer than 1,000 tortoises. A 1978 study put the count at about 150,000, and an estimate published in 1999 placed it near 100,000.
Researchers said earlier surveys likely undercounted because they did not account for tortoises that were present but missed by observers. The new results also challenge the long-held view that the population had hit the atoll’s carrying capacity in the late 1900s.
Surveys help researchers count tortoises hidden in thick scrub
Staff with the Seychelles Islands Foundation, which has managed research on Aldabra since 1979, collected the data. Two observers walked 17 routes across the atoll once each wet season, from November to April. They set out at sunrise and finished by 9 a.m., before the tortoises seek shade.
Scientists estimate over 180,000 giant tortoises live on one Seychelles island
A new study estimates that adult and subadult Aldabra giant tortoises roam freely across Aldabra Atoll, far outnumbering its human population pic.twitter.com/JZMsZzDlb0
— Nature is Amazing ☘️ (@AMAZlNGNATURE) October 10, 2026
Each route was split into 50-meter (164-foot) sections, and observers noted the habitat and how far each tortoise was from the line. A statistical model then estimated how many animals went unseen, especially in thick scrub where they are harder to spot.
The estimate mainly covers adults and older young tortoises. Juveniles tend to hide in dense vegetation and are rarely seen. Tortoises were most numerous in grassland known as “tortoise turf,” followed by exposed surfaces and open mixed scrub.
The 2022 survey produced an even higher figure, but researchers left it out of the average. About half as many sections were covered that year, mostly in areas that historically held more tortoises.
Record giant tortoise population credited to decades of protection, but threats remain
Researchers credited the record giant tortoise population to decades of protection on the atoll, including the removal of feral goats in 2012. They called the result a “remarkable conservation success.”
However, the threats remain. Feral cats and black rats still roam much of Aldabra and eat tortoise eggs and hatchlings. Longer droughts and possible sea level rise could also cut shade and food. Researchers said a planned removal of invasive mammals could help the population grow further.
Aldabra giant tortoises are also used to restore lost wildlife roles on other islands, including Mauritius, Rodrigues and other parts of the Seychelles. Researchers said such projects “remain a viable conservation strategy.”
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