The Aldabra giant tortoise is one of the largest tortoises on Earth and the last surviving giant tortoise of the western Indian Ocean. Once part of a radiation across Madagascar, the Seychelles, and the Mascarene Islands — most driven extinct by human exploitation within a few centuries of contact — the species now survives as a single great population on Aldabra Atoll, numbering on the order of 100,000 animals [Bourn & Coe 1978; Çilingir et al. 2022]. As a dominant grazing megaherbivore it shapes the vegetation of an entire atoll, and it has become a model organism for "rewilding," in which living tortoises serve as ecological stand-ins for their extinct relatives [Hansen et al. 2010; Griffiths et al. 2011].
Biology and Identification
The Aldabra giant tortoise is a massive land tortoise with a high, domed carapace, columnar elephantine limbs, and a long neck used to browse vegetation well above ground level. Large males commonly exceed 1 metre in carapace length and can weigh upwards of 250 kg, substantially larger than females [Bourn & Coe 1978; Walton et al. 2019]. The carapace is brown to greyish and the skin dark; a small bony scale atop the head is a distinguishing feature.
These tortoises are extreme dietary generalists, feeding on grasses, sedges, herbs, leaves, and fallen fruit, and they are central to seed dispersal and nutrient cycling across their habitat [Gibson & Phillipson 1983; Hansen et al. 2010]. Their sustained grazing maintains a distinctive sward of low, cropped vegetation — informally called "tortoise turf" — a community of grazing-tolerant grasses and herbs sustained by intense tortoise herbivory [Gibson & Phillipson 1983]. Because of this landscape-scale influence, the species is frequently described as a keystone megaherbivore, sometimes likened to a "reptilian elephant."
Giant tortoises are notably long-lived, with lifespans commonly cited at roughly a century or more, growing slowly and reproducing over many decades [Çilingir et al. 2022]. They are most active in cooler morning and evening hours, sheltering in shade or shallow pools during the heat of the day to avoid lethal overheating — a behavioural constraint that matters under a warming climate [Falcón & Hansen 2018]. A chromosome-level reference genome now enables assessment of the genetic health of the Aldabra population [Çilingir et al. 2022].
Habitat and Range
The native range of the Aldabra giant tortoise is effectively a single locality: Aldabra Atoll, a raised coral atoll in the outer Seychelles, where the species occupies open grassland, scrub, and mangrove fringe across the atoll's platform and islands [Gibson & Phillipson 1983; Walton et al. 2019]. Tortoise density and habitat use vary markedly across the atoll, tracking vegetation, shade, and access to fresh water [Walton et al. 2019].
Beyond Aldabra, the species exists only as deliberately established or captive populations. Rewilding introductions have placed Aldabra tortoises on islands off Mauritius and Rodrigues and on other Seychelles islands, as ecological replacements for extinct native giant tortoises [Griffiths et al. 2010; Hansen et al. 2010]. These introduced groups are managed conservation interventions rather than natural range.
In accordance with NRWL sensitive-species policy, specific site locations, corridor routes, and seasonal movement details are not disclosed in this article.
Conservation Status
The Aldabra giant tortoise is listed as Vulnerable on the IUCN Red List, under the accepted name Aldabrachelys gigantea (assessed as Geochelone gigantea; synonyms include Dipsochelys dussumieri) [IUCN 1996]. This is an older assessment dating to 1996, flagged by the IUCN as needing updating; the listing rests largely on the species' restriction to a single locality rather than a documented recent decline [IUCN 1996]. The species is included on CITES Appendix II, regulating international commercial trade [CITES 2023].
The Aldabra population is the largest giant-tortoise population in the world, historically on the order of 100,000 or more individuals on the atoll [Bourn & Coe 1978]. While that abundance is reassuring, the entire natural population is concentrated in one low-lying place. A single catastrophic event — a severe disease outbreak, an introduced predator establishing on the atoll, or sea-level inundation of low ground — could affect essentially the whole wild population at once, the central reason the species is treated as at risk despite its large numbers.
Threats
Single-population vulnerability is the defining risk. With the natural range confined to one atoll, the species lacks the geographic redundancy that buffers more widely distributed animals against localized catastrophes [Hansen et al. 2010].
Climate change and sea-level rise pose a structural long-term threat. Aldabra is a low-lying atoll, and rising seas and changing rainfall threaten both the land itself and the freshwater and shade resources tortoises depend on to avoid overheating during hotter, drier periods [Falcón & Hansen 2018].
Historical exploitation devastated giant tortoises across the western Indian Ocean. Provisioning by sailors and settlers wiped out the giant tortoises of the Mascarenes and other Seychelles islands, and Aldabra's own population was heavily harvested before protection — the atoll's isolation is a principal reason its tortoises survived [Hansen et al. 2010; Bourn & Coe 1978].
Introduced species threaten recruitment. Non-native rats and cats prey on eggs and hatchlings, and invasive herbivores compete for forage; biosecurity to keep such species off the atoll is a core management concern.
Limited genetic diversity, a consequence of the species' history and single-population structure, is a further consideration for long-term resilience and for managing assisted colonies [Çilingir et al. 2022].
What Is Being Done
Protection of Aldabra Atoll. Aldabra is a strict nature reserve and a UNESCO World Heritage Site, managed by the Seychelles Islands Foundation (SIF), which maintains a permanent research station, monitors the tortoises and vegetation, and enforces biosecurity and access controls [SIF 2024; UNESCO 2024]. The atoll's remoteness and protected status together safeguard the world's largest giant-tortoise population [UNESCO 2024].
Ecological-replacement and rewilding programs. Living Aldabra tortoises have been introduced to islands where native giant tortoises were driven extinct, restoring lost grazing and seed-dispersal functions. On Île aux Aigrettes off Mauritius they restored dispersal of the large-seeded native ebony Diospyros egrettarum, with gut passage improving seed germination and producing widespread seedling establishment [Griffiths et al. 2011]. On Round Island and in projects on Rodrigues and other Seychelles islands, Aldabra tortoises have been established as functional substitutes for the extinct endemic species [Griffiths et al. 2010; Hansen et al. 2010].
Climate-informed management. Researchers have set out frameworks for rewilding with giant tortoises under climate change, including artificial shade and water to sustain tortoise-mediated functions as conditions warm [Falcón & Hansen 2018].
Genomic and demographic monitoring. A chromosome-level genome and population genetic studies now support assessment of inbreeding and genetic structure within Aldabra and inform genetic management of assisted populations [Çilingir et al. 2022].
How Readers Can Help
Citizen science. Log wildlife observations through platforms such as iNaturalist. Verified records of captive and introduced Aldabra tortoises help researchers track assisted populations.
Support credible institutions. Back organizations with a documented track record on Aldabra and on island rewilding — including the Seychelles Islands Foundation and recognized research programs — rather than operations that treat giant tortoises as a tourist novelty.
Responsible travel and consumer choices. Choose well-regulated, low-impact wildlife experiences, avoid products derived from tortoises or other reptiles, and never participate in the collection or trade of wild tortoises, consistent with CITES protections [CITES 2023].
Climate action. Because sea-level rise threatens a low-lying single-locality species, supporting policies that reduce greenhouse-gas emissions and protect low islands contributes directly to this tortoise's prospects [Falcón & Hansen 2018].
Education outreach. Share accurate, science-based information about giant tortoises as megaherbivores and about the rewilding programs that use them to restore degraded island ecosystems.
References
[Bourn & Coe 1978] Bourn, D. & Coe, M. (1978). The size, structure and distribution of the giant tortoise population of Aldabra. Philosophical Transactions of the Royal Society B, 282(988), 139–175. https://doi.org/10.1098/rstb.1978.0011
[Çilingir et al. 2022] Çilingir, F.G., A'Bear, L., Hansen, D., Davis, L.R., Bunbury, N., Ozgul, A., Croll, D. & Grossen, C. (2022). Chromosome-level genome assembly for the Aldabra giant tortoise enables insights into the genetic health of a threatened population. GigaScience, 11, giac090. https://doi.org/10.1093/gigascience/giac090
[CITES 2023] CITES. (2023). Appendices I, II and III. Convention on International Trade in Endangered Species of Wild Fauna and Flora. https://cites.org/eng/app/appendices.php
[Falcón & Hansen 2018] Falcón, W. & Hansen, D.M. (2018). Island rewilding with giant tortoises in an era of climate change. Philosophical Transactions of the Royal Society B, 373(1761), 20170442. https://doi.org/10.1098/rstb.2017.0442
[Gibson & Phillipson 1983] Gibson, C.W.D. & Phillipson, J. (1983). The vegetation of Aldabra Atoll: preliminary analysis and explanation of the vegetation map. Philosophical Transactions of the Royal Society B, 302(1109), 201–235. https://doi.org/10.1098/rstb.1983.0051
[Griffiths et al. 2010] Griffiths, C.J., Jones, C.G., Hansen, D.M., Puttoo, M., Tatayah, R.V., Müller, C.B. & Harris, S. (2010). The use of extant non-indigenous tortoises as a restoration tool to replace extinct ecosystem engineers. Restoration Ecology, 18(1), 1–7. https://doi.org/10.1111/j.1526-100X.2009.00612.x
[Griffiths et al. 2011] Griffiths, C.J., Hansen, D.M., Jones, C.G., Zuël, N. & Harris, S. (2011). Resurrecting extinct interactions with extant substitutes. Current Biology, 21(9), 762–765. https://doi.org/10.1016/j.cub.2011.03.042
[Hansen et al. 2010] Hansen, D.M., Donlan, C.J., Griffiths, C.J. & Campbell, K.J. (2010). Ecological history and latent conservation potential: large and giant tortoises as a model for taxon substitutions. Ecography, 33(2), 272–284. https://doi.org/10.1111/j.1600-0587.2010.06305.x
[IUCN 1996] Tortoise & Freshwater Turtle Specialist Group. (1996). Geochelone gigantea. The IUCN Red List of Threatened Species 1996: e.T9010A12949962. https://dx.doi.org/10.2305/IUCN.UK.1996.RLTS.T9010A12949962.en
[SIF 2024] Seychelles Islands Foundation. (2024). Aldabra Atoll — management and giant tortoise monitoring. https://www.sif.sc/aldabra (accessed 2024)
[UNESCO 2024] UNESCO World Heritage Centre. (2024). Aldabra Atoll. World Heritage List, ref. 185. https://whc.unesco.org/en/list/185/
[Walton et al. 2019] Walton, R., Baxter, R., Bunbury, N., Hansen, D., Fleischer-Dogley, F., Greenwood, S. & Schaepman-Strub, G. (2019). In the land of giants: habitat use and selection of the Aldabra giant tortoise on Aldabra Atoll. Biodiversity and Conservation, 28, 3183–3198. https://doi.org/10.1007/s10531-019-01813-9