Around 180,000 giant tortoises live on one Seychelles atoll
Researchers estimate roughly 180,000 adult and subadult giant tortoises on Aldabra, with methods that account for animals hidden from view.
Even a giant tortoise can disappear from a headcount. On Aldabra Atoll in Seychelles, researchers estimate roughly 180,000 adult and subadult giant tortoises, accounting for animals that surveyors could not see. The University of Exeter announced the findings on October 7, 2026. [1] The paper was first published in Ecology and Evolution on September 4. [2]
An atoll shaped by its biggest grazers
Aldabra is a ring of four main coral-limestone islands around a shallow lagoon, with smaller islands inside and a reef outside. Its rugged ground supports a patchwork of habitats. UNESCO describes an unusually intact island ecosystem where isolation has limited human interference, allowing researchers to observe ecological relationships with relatively little human disturbance. [3]
The tortoises help create that landscape. Their grazing is so important that a habitat called tortoise turf depends on them. Seychelles Islands Foundation, which manages Aldabra, studies their movements, seed dispersal and influence on vegetation alongside its regular wildlife monitoring. Counting these animals therefore helps conservationists understand the condition of the wider terrestrial ecosystem, as well as the fortunes of one conspicuous species. [4]
How to count an animal you cannot see
SIF staff collected observations along survey routes during 2018–2022, recording each sighted tortoise’s perpendicular distance from the route. Researchers used Bayesian hierarchical distance sampling to distinguish abundance from the chance of detection. Their model allowed visibility to vary between habitats and estimated unseen animals. The main analysis used wet-season observations; tortoises sheltering in dense cover during dry periods can be less available to count. [2]
That matters when comparing the result with the often-repeated late-1990s estimate of 100,000. The Exeter team warns that the higher figure cannot be read as proof that the population nearly doubled. Earlier methods could miss animals concealed by shade or vegetation. A change in how reliably researchers detect tortoises changes the estimate, even before any genuine population change is considered. [1]
What the estimate can tell us
The population appeared relatively stable during the study’s 2018–2021 period. The researchers excluded 2022 from the headline mean because fewer areas were surveyed and coverage favoured places with historically high counts. The result also depends on assumptions, including that existing routes represent wider habitat types and that animals directly on the survey line are detected. It provides an informed estimate rather than an exact census of every tortoise. [2]
Aldabra’s conservation history gives the finding context. Commercial exploitation once severely depleted its tortoises. SIF traces their recovery to protection beginning in the 1960s and its own stewardship from 1979. Continued fieldwork is part of that protection: the foundation monitors wildlife and environmental conditions over time, looking for changes that would otherwise be difficult to recognise from occasional visits. [4]
The next challenge is keeping them there
The announcement also highlights unfinished work. Longer droughts could reduce forage and shade, while rising seas threaten low-lying habitat. Cats and black rats still eat eggs and hatchlings; removing those predators could improve the tortoises’ prospects. [1]
UNESCO notes that the same remoteness which helped protect Aldabra creates formidable practical challenges for its management. Invasive species, climate change and potential oil spills still require attention. [3] The useful achievement here is a clearer starting point for future comparisons: a better way to notice when this remarkable population needs help.
Story context and image credits
Study first published September 4, 2026; publicized October 7. Originally published on OddityDrop Instagram October 9, 2026, at 16:04:26 UTC. Expanded website article added October 13, 2026.
View the full photo-credit recordView the original Instagram publicationSources & notes
- UNESCO World Heritage Centre: Aldabra Atollwhc.unesco.org
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