Roger Downie, Froglife and University of Glasgow
The Reptile Database (September, 2025) lists 366 species of turtles and tortoises (Order Testudines). According to the most recent assessment (Rhodin et al., 2018), an alarming 51.9% of these species are ‘threatened’ with extinction (i.e. are in IUCN categories Critically Endangered, Endangered or Vulnerable).
In this group, the highly terrestrial, herbivorous tortoises have evolved on isolated islands multiple times into giant long-lived species (up to 1.3 m long; 400 kg in mass; some individuals are known to have lived over 200 years). Unfortunately, several characteristics of these animals have made them highly vulnerable to human exploitation: they are slow-moving and easy to catch; often unafraid of humans because of the isolation of their habitats; they can live for months without feeding, making them very convenient as living fresh meat stores on long sea voyages in pre-refrigeration times; and their meat is highly palatable. It is estimated that the Galapagos Islands held about 250,000 giant tortoises in the 16th century, but only 15,000 by the 1970s. This massive decline is not simply the result of over-exploitation. The Galapagos Islands lacked permanent human inhabitants until the 1830s, but once humans settled, they brought other species that competed with the tortoises; goats consumed the vegetation; black rats, which arrived earlier from visiting ships, ate the eggs and hatchlings. Even scientists contributed to the declines: the California Academy of Sciences captured 385 mainly adult specimens for museum collections in 1906.
Currently, two distinct populations of giant tortoises exist: in the Galapagos Islands and in the Seychelles, including Aldabra Atoll. Considerable efforts are now being made to conserve and enhance the extant populations of giant tortoises. There are two main motivations for this (in addition to the ethical impulse to avoid extinctions caused by people):1) giant tortoises are charismatic species which attract tourists, and therefore generate income for their island homes; 2) giant tortoises provide ecological services such as seed dispersal and vegetation control. In this article, I report on some recent conservation-related studies on both populations.

Chelonoidis, the Galapagos Giant Tortoise. The Reptile Database and Wikipedia consider this a single species with historically 15 subspecies, three of them now extinct. But some researchers (see Gray et al., 2022) describe 14 separate species, all threatened with extinction. The saddest case was ‘Lonesome George’, the last remaining individual of the Pinta Island population, who died in captivity in 2012, after a long life and many unsuccessful attempts to provide him with a suitable mate.
Evelyn Jensen (Newcastle University) has led a programme of research into the genetic make-up of the Galapagos species, using new (and rapidly developing) DNA technology to compare the genomes of extant populations with those of museum specimens collected 100 years ago or more.
The uninhabited island of Fernandino held the only population of C. phantasticus. The only sighting had been in 1906, until, surprisingly, a live ‘young’ female, named Fernanda (about 50 years old) was discovered in 2019. DNA analysis has shown that its genome is essentially the same as that of the specimen collected in 1906 (Jensen et al., 2022) and quite distinct from the other Galapagos species. The existence of this individual suggests there must have been parents fairly recently, but surveys in the challenging terrain of the island have found no evidence (it is volcanically the most active of the archipelago). The fate of Fernanda will the same as that of Lonesome George, unless other specimens are discovered.
The species on Floreana, C. niger, was reported extinct by 1840. However, it was recently found that on the largest island, Isabela, which has five distinct species of giant tortoise, each inhabiting the slopes of a different volcano, that some individuals on Wolf mountain had unusual characteristics. Again, a comparison of these individuals’ genomes with those of museum specimens showed that 89 of the Wolf population are Floreana hybrids (ships’ records show that tortoises collected on one island were sometimes released or escaped on to other islands, so a known dispersal route exists). Gray et al.(2022) have tried to sort out which of these hybrids could provide the best breeding stock for an attempted recovery of C. niger. In 2023, the NGO Island Conservation accomplished a successful eradication of the rats on Floreana, and the plan is to release on the island 300 captive-bred tortoises, genetically as close as possible to the original C.niger, in 2026.
The tiny island of Pinzon holds C. duncanensis. A small adult population exists, but they have had no breeding success for about 50 years because of rat predation on eggs and hatchlings. Since the adults are long-lived and can remain reproductively active, it has been possible to collect eggs, incubate them artificially and rear the hatchlings in captivity to a size where rats cease to be a threat (a procedure known as ‘head-starting’). This had been done successfully for decades, but, without removing the rats, it was an unsustainable strategy. Rats were eliminated in 2012 and by 2014, wild-born young were being found. Jensen and colleagues were concerned that genetic variability might have been reduced during the head-starting programme, but comparison with DNA from specimens collected in 1906 showed that no decline invariability.

Aldabrachelys gigantea, the Aldabra (and Seychelles) Giant Tortoise. Another perspective on giant tortoise conservation comes from Sheffield University where Seychellois Alessia Lavigne is studying reproductive failure in the Aldabra Giant Tortoise. This species is native to Aldabra Atoll and a number of the Seychelles islands in the Indian Ocean, with three subspecies currently extant. Evidence suggests that this population diverged from the (now extinct) Madagascar Giant Tortoise 4.5 million years ago. Aldabra Atoll lies 400 km north of Madagascar, with the Seychelles a further1000 km northeast (Aldabra is now part of the Republic of the Seychelles). Conservation has a long history on Aldabra, with Gunther at the Natural History Museum successfully campaigning for it to be declared a nature preserve. However, in the 1960s, the UK government was eyeing up Aldabra as a site for a RAF base. Julian Huxley led the opposition to this idea, with David Stoddart leading a Royal Society study of the atoll’s fauna and flora in the 1970s. The Royal Society gave way to the Seychelles Islands Foundation and, in addition to its nationally protected status, the atoll was declared a UNESCO world heritage site in 1982. The British government transferred its plans for a military base to the Chagos islands, with devasting consequences for the islanders (Sands, 2022), only recently resolved.
Anecdotal evidence suggests that there is currently very little recruitment of new individuals to the Aldabra Giant Tortoise populations, suggesting that, while the populations remain stable, this is the result of the longevity of individuals, not the health of the population. Lavigne suggests that these populations suffer from ‘extinction debt’ where the limited ability of long-lived individuals to adapt to changing environments and produce viable offspring can result in a sudden population collapse. Lavigne et al. (2025) developed a method for assessing whether unhatched turtle eggs are the result of fertilisation failure, or embryonic death at some stage or other. In a comparison of unhatched marine turtle and giant tortoise eggs, they found fertilisation failure was low in the marine turtles but much higher in their sample of Aldabran Giant Tortoise eggs. A follow-up study (Lavigne et al., 2025?) collected data on unhatched eggs from 24 clutches from six separate populations of Aldabra Giant Tortoise. Sixteen clutches showed 100% failure to hatch; the hatching success per clutch ranged from 0-91%. The Aldabra Atoll population showed 46% success, but there was 0% success in four populations. Fertilisation failure was an issue in some populations, but in most, the failure was in embryo development after fertilisation. Sample sizes were small (3-7 clutches from the different populations), but the results are nevertheless alarming. Lavigne and colleagues suggest that the underlying problem is climate change. A recent news item (Watts, 2025) reports that Lavigne has resorted to artificial incubation of a clutch of eggs (from Cousin Island, zero hatching success in the earlier study) with 13 hatchlings resulting from 18 eggs, although no details are yet available on the incubation conditions provided. Although artificial incubation is clearly not a long-term solution to the problem of reproductive failure in giant tortoises, it may provide some much-needed time for good sustainable solutions to be found.
Click to read references
Gray et al.(2022). Temporal monitoring of the Floreana Island Galapagos Giant Tortoise captive breeding programme. Integrative and Comparative Biology 62, 1864-1871.
Jensen et al. (2022). The Galapagos Giant Tortoise C. phantasticus is not extinct. Communications Biology 5, 546.
Lavigne et al. (2025). Understanding early reproductive failure in turtles and tortoises. Animal Conservation 28, 353-364.
Lavigne et al.(2025?). The illusion of stability in long-lived species: poor early-life survival threatens the persistence of Giant Aldabra tortoise populations. (published on-line)
Rhodin et al. (2018). Global conservation status of turtles and tortoises (Order Testudines). Chelonian Conservation and Biology 17, 135-161.
Sands, P. (2022). The last colony: a tale of exile, justice and Britain’s colonial legacy. Weidenfeld and Nicolson, London.
Watts, J. (2025). Thirteen rare giant tortoises hatch thanks to artificial incubation trial. The Guardian 11th October, 2025.



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