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Seychelles giant tortoise

The Seychelles giant tortoise (Aldabrachelys hololissa, synonym Dipsochelys hololissa) is a taxon of large-bodied Testudinidae endemic to the granitic islands of the Seychelles archipelago, distinguished by its robust build, grayish-brown domed carapace, and capacity for extreme longevity exceeding 190 years in verified individuals such as Jonathan.^[1]^[2] Adult males can attain carapace lengths up to 138 cm and exhibit sexual dimorphism with longer tails and concave plastrons, while females are smaller at around 95 cm; these reptiles primarily graze on grasses, herbs, and low vegetation in scrub, woodland, and grassland habitats.^[1] Once abundant across islands like Mahé and Silhouette, populations were eradicated in the wild by the mid-19th century through intensive exploitation for meat and oil, compounded by habitat clearance for agriculture and invasive species impacts.^[1] Taxonomic classification remains contentious, with debate over whether A. hololissa constitutes a distinct species, subspecies, or variant within the broader Aldabrachelys gigantea complex, particularly given morphological similarities to Aldabra Atoll forms but potential genetic divergence reflected in saddle-like shell tendencies in some historical specimens.^[1] Classified as Extinct in the Wild by tortoise specialists, with a global captive population of approximately 37 adults as of 2011 assessments, conservation has relied on breeding programs initiated in the late 20th century, yielding juveniles for reintroduction to predator-free islands such as Cousine and Cerf.^[1]^[3] These efforts underscore the tortoise's ecological significance as a keystone herbivore that shapes vegetation structure, promotes seed dispersal, and restores island biodiversity dynamics disrupted by historical extinctions.^[1] Recent advancements, including artificial incubation trials producing viable hatchlings, signal potential for bolstering self-sustaining wild populations amid ongoing threats like inbreeding and stochastic events in small founder groups.^[4]

Taxonomy and classification

Subspecies status and morphological distinctions

The Seychelles giant tortoise encompasses two recognized subspecies: the domed Aldabrachelys gigantea hololissa (Günther, 1877) and the saddle-backed A. g. arnoldi (Schlegel, 1877), both historically native to the granitic Seychelles islands.^[1] Their subspecies status remains debated, as morphological distinctions persist in captive populations but genetic analyses have yet to confirm divergence from the Aldabra atoll's A. g. gigantea; detailed genomic studies are required to resolve whether they represent adaptive morphotypes or true phylogenetic lineages.^[5]^[6] A. g. hololissa is characterized by a broad, highly domed carapace adapted for grazing, with reduced ossification of the skeleton, a flatter anterior carapace margin, and absence of a dorsal groove, setting it apart from both Aldabra tortoises (narrower build, greater ossification) and saddle-backed forms.^[1] In contrast, A. g. arnoldi displays a narrower saddle-backed shell with a pronounced dorsal groove, elevated anterior margins, distinct markings, and size variations, reflecting adaptations potentially linked to different foraging or habitat pressures in Seychelles environments.^[7] Captive rearing of Seychelles-Aldabra hybrids and pure lines reveals three persistent morphotypes—domed broad (hololissa-like), domed narrow (gigantea-like), and saddle-backed (arnoldi-like)—each exhibiting biphasic growth: rapid initial expansion to 20–30 cm carapace length, followed by morphotype-specific patterns thereafter, underscoring heritable morphological fidelity despite interbreeding potential.^[6] Seychelles tortoises generally possess smaller, narrower heads and a nuchal scute on the carapace, further delineating them from Aldabra populations where such features are less consistent.^[8]

Genetic and taxonomic debates

Genetic analyses have cast doubt on the taxonomic distinctness of the Seychelles giant tortoise (Aldabrachelys hololissa), traditionally distinguished from the Aldabra giant tortoise (A. gigantea) by morphological traits such as a more domed and broader carapace. A 2003 study by Palkovacs et al. examined mitochondrial DNA (mtDNA) control region sequences and eight microsatellite loci from 55 captive tortoises morphologically assigned to Seychelles morphotypes (D. hololissa, D. arnoldi) and wild Aldabra specimens, revealing no mtDNA variation (a single shared haplotype across all samples) and insignificant genetic structuring (F_ST = 0.015, P > 0.05).^[9] Bayesian clustering supported a single population (K = 1), indicating that purported Seychelles lineages lack the genetic divergence expected from long-term isolation and are likely derived from Aldabran stock introduced historically to Seychelles islands.^[9] This genetic evidence contrasts with morphological revalidations, such as Gerlach and Canning's 1998 assessment based on carapace osteology and shape, which proposed A. hololissa as a separate species or subspecies.^[1] Subsequent synonymizations, including Fritz and Havaš (2007), have treated Seychelles forms as conspecific with A. gigantea, attributing shell differences to environmental influences like diet and growth rather than fixed genetic traits.^[1] Austin et al. (2003) corroborated the lack of significant genetic differentiation, suggesting native Seychelles giant tortoises went extinct by the mid-19th century, with survivors representing hybridized or imported Aldabran populations.^[1] Ongoing debates persist due to limited sample sizes and the persistence of distinct morphotypes in captive breeding programs, where Seychelles-like domes develop despite uniform rearing conditions, hinting at underlying genetic factors not captured by early mtDNA or microsatellite assays.^[10] As of 2011, only 37 adults were identified as A. hololissa (predominantly males), complicating resolution, with calls for expanded genomic studies to assess nuclear differentiation and hybridization extent.^[1] These findings underscore how historical translocations have obscured pure lineages, prioritizing genetic data over morphology for taxonomic decisions in light of low diversity signaling recent common ancestry rather than ancient divergence.^[9]

Physical characteristics

Size, morphology, and morphotypes

The Seychelles giant tortoise possesses a robust build with a domed carapace that is dorsally flattened and wider posteriorly than anteriorly, featuring serrated marginal scutes and typically 11 pairs of marginals, with the supracaudal scute undivided and slightly upturned.^[1] The shell color is grayish-brown in adults, while hatchlings are pale gray, darkening to blackish-brown within a month.^[1] The plastron is grayish-brown, relatively short (71–91% of carapace length), and usually lacks a pronounced anal notch, with the humero-pectoral suture angled laterally.^[1] The skeleton shows reduced ossification, remaining unfused even in aged individuals, and the skull features broad postorbitals and an hourglass-shaped processus.^[1]^[11] Adult males achieve straight carapace lengths up to 138 cm, with females reaching up to 95 cm; body weights for adults are not precisely documented for pure hololissa lineages but align with those of related Aldabra populations, where males exceed 250 kg.^[1]^[12] Juveniles grow rapidly, attaining 15–20 kg by five years of age, transitioning from a rounded juvenile shell to the flattened-dome adult form by one year.^[1] Morphotypic variation occurs within populations associated with Seychelles stock, including the hololissa form characterized by a broad, consistently domed shell suited to grazing, and rarer variants like the arnoldi morphotype, which develops a central carapace constriction leading to a partial saddleback shape.^[13]^[11] These differences emerge post-hatching, with geometric morphometric studies of captive juveniles revealing distinct growth patterns: hololissa maintains a consensus dome until 31–40 cm carapace length before increasing dorsal height, while other morphotypes diverge earlier in shell profile.^[6] Such variations, once attributed to separate species, persist in captive herds despite genetic evidence suggesting shared ancestry with Aldabra tortoises, indicating potential developmental or epigenetic influences on morphology.^[9]^[14]

Diet and foraging behavior

The Seychelles giant tortoise (Aldabrachelys gigantea hololissa) maintains a primarily herbivorous diet dominated by fallen leaves, which comprise approximately 80% of intake, supplemented by grass at about 19%.^[1] Additional vegetation includes leaves from trees and shrubs, woody plant stems, herbs, sedges, fruits, and flowers, reflecting adaptation as both grazers on low-growing plants and browsers reaching foliage up to 1 meter high via neck extension.^[15] In field observations of released individuals, tortoises utilized 18 of 26 available plant species, exhibiting selective foraging on 9 species, with males preferring 6 and females 7 (2 overlapping).^[16] Diet composition shows minimal sex-based differences overall, though juveniles emphasize herbs and rock mosaic vegetation more than adults; seasonal shifts occur, with greater reliance on available greens during wet periods.^[17] Foraging occurs diurnally, peaking in early morning and late afternoon, with average daily active time of 257 minutes and a feeding rate of 3.86 grams of plant material per minute of activity.^[16]^[1] Individuals roam freely in habitats like forests and beaches, grazing opportunistically without fixed territories.^[15] Opportunistic omnivory supplements the herbivorous baseline, including carrion, bones, snail shells, and small invertebrates; rare documented instances involve active predation on bird chicks, such as terns in nesting colonies, suggesting behavioral flexibility under specific ecological conditions rather than routine carnivory.^[18]^[12]

Life history and ecology

Lifespan, growth, and reproduction

Seychelles giant tortoises (Aldabrachelys gigantea hololissa) exhibit exceptional longevity, with individuals capable of surviving more than 150 years in the wild and captivity.^[19] ^[2] Verified specimens, such as the tortoise named Jonathan, have reached ages exceeding 190 years, demonstrating the species' potential for extended lifespan under protected conditions.^[19] Growth rates vary with environmental factors, but juveniles experience rapid development, achieving curved carapace lengths of 70-75 cm within the first 20-25 years.^[20] Thereafter, growth decelerates episodically, approaching an asymptote between 20 and 30 years of age, though tortoises continue incremental expansion throughout life if resources permit.^[20] ^[21] Sexual maturity typically occurs between 20 and 30 years, coinciding with substantial body mass accumulation.^[21] Breeding season spans February to May in both wild and captive populations, during which males compete aggressively for females through vocalizations, biting, and mounting.^[22] ^[21] Females oviposit clutches of 4 to 25 flexible-shelled eggs in shallow, dry nests excavated in sandy or friable soil, often producing multiple clutches per season.^[22] ^[21] Incubation lasts 4 to 8 months, influenced by temperature and substrate moisture, but natural hatching success remains below 50% due to predation, desiccation, and fungal infections.^[23] Recent conservation efforts, including artificial incubation on islands like Cousin, have improved hatchling survival rates to over 70% in controlled settings.^[4]

Habitat preferences and behavioral ecology

The Seychelles giant tortoise, primarily represented by populations of Aldabrachelys gigantea from Aldabra Atoll, occupies diverse terrestrial habitats including grasslands, open mixed scrub, mangrove swamps, and coastal dunes.^[22] ^[21] At the landscape scale, individuals exhibit a strong preference for grassland or "tortoise turf" habitats across seasons, followed by open mixed scrub, which together comprise only about 19% of Aldabra's land area but support the majority of the population.^[24] ^[25] Within established home ranges, habitat selection appears more random, suggesting opportunistic use once territories are settled.^[25] These preferences align with the species' need for open areas facilitating foraging and thermoregulation, while avoiding dense vegetation that impedes movement.^[26] Behaviorally, Seychelles giant tortoises are primarily herbivorous grazers and browsers, consuming lowland grasses, leaves, fruits, and occasionally carrion or invertebrates, with foraging activity peaking during cooler morning and late afternoon periods to minimize heat stress in semi-arid conditions.^[3] ^[27] As ectotherms, they exhibit conformer-regulator patterns in body temperature, basking in direct sunlight during cooler times and retreating to shade or burrows when ambient temperatures exceed 35°C, which helps maintain core temperatures around 28–32°C.^[22] ^[27] Activity levels fluctuate diurnally and seasonally, with reduced movement during dry periods when vegetation productivity declines, particularly in preferred grasslands.^[26] Social interactions are limited, primarily involving non-sexual "nosing" behaviors for recognition, though rare opportunistic predation on bird chicks has been documented, indicating flexibility beyond strict herbivory.^[21] ^[18] In reintroduced populations on granitic Seychelles islands like Silhouette, tortoises adapt to more forested environments but show similar behavioral patterns, including habitat modification through trampling that creates pathways and enhances understory openness, functioning as ecosystem engineers.^[28] Individual personalities, characterized by traits like boldness or aggression, remain consistent across contexts and sexes, influencing space use and interaction rates.^[29] Resting behavior displays subtle lateralization, with preferences for positioning limbs asymmetrically, potentially aiding vigilance or comfort.^[30] These traits underscore a behavioral ecology adapted to island insularity, with low population densities (up to 40 individuals per hectare in optimal habitats) and minimal intraspecific competition.^[24]

Historical distribution and population dynamics

Pre-human and early human impacts

Prior to human arrival, Seychelles giant tortoises (Aldabrachelys gigantea hololissa), endemic to the granitic Seychelles islands such as Mahé, Silhouette, and Praslin, occupied a wide range across these inner islands, with populations likely numbering in the tens of thousands based on ecological modeling of habitat capacity and fossil evidence of similar Indian Ocean tortoises.^[31]^[32] These tortoises functioned as ecosystem engineers, grazing on vegetation and dispersing seeds, which shaped the islands' lowland forests and grasslands in the absence of large mammalian herbivores or significant native predators beyond birds of prey.^[33] Isolation on predator-free oceanic islands allowed for their evolution into large-bodied forms, with minimal natural population pressures evident from subfossil records dating back millennia showing stable distributions.^[32] European discovery of the granitic Seychelles in 1609 by English explorer John Jourdain initiated sporadic exploitation, as passing sailors hunted tortoises for their meat and oil, stacking live individuals in ship holds for extended voyages due to their low metabolic needs.^[34] This overharvest targeted accessible coastal populations, reducing densities on smaller islands first, though systematic records indicate initial impacts were localized until French settlement in 1770 on Mahé and nearby islands escalated demand for fresh provisions.^[31] Early colonists cleared forests for plantations, fragmenting habitats and exposing tortoises to increased predation by introduced rats (Rattus spp.) on eggs and juveniles, as well as competition from goats and pigs released for sustenance.^[33] By the early 19th century, accounts from visitors noted tortoises as scarce on main islands, with catastrophic declines—estimated at over 90% from pre-settlement levels—attributable primarily to direct hunting rather than disease or climatic factors, as corroborated by historical logs and subfossil site analyses.^[31]^[33]

Extinction events and population bottlenecks

The Seychelles giant tortoise (Aldabrachelys gigantea hololissa, syn. Dipsochelys hololissa) experienced near-total extirpation from its native granitic Seychelles islands, including Mahé, Silhouette, and Praslin, primarily due to overexploitation by European sailors and settlers between the late 17th and mid-19th centuries. These actors harvested tortoises en masse for meat, oil, and ship provisions, exploiting the reptiles' abundance and ease of collection in accessible lowlands; historical accounts document thousands shipped annually from Seychelles ports, with populations crashing as easily caught individuals were depleted first.^[1]^[9] Introduced predators such as rats, pigs, and cats further exacerbated declines by preying on eggs and juveniles, while habitat clearance for plantations reduced foraging areas, compounding direct mortality.^[10] By 1840, wild populations on the granitic Seychelles were effectively extinct, marking the culmination of this anthropogenic extinction event, though small numbers may have persisted into the 1850s on remote islets before vanishing.^[9] Unlike the Aldabra atoll population, which endured through isolation, Seychelles lineages did not recover naturally; genetic surveys of purported survivors reveal no distinct mitochondrial DNA haplotypes or nuclear structuring indicative of long-term isolation, but rather signatures of recent founder events from Aldabra imports, implying native hololissa did not bottleneck and persist but were wholly replaced.^[9] Microsatellite analyses across 55 specimens confirm invariant mtDNA control regions and minimal variation, consistent with a secondary, very recent bottleneck post-introduction rather than survival of endemic stock.^[9] Captive populations, numbering around 37 adults by 2011 (with 28 in zoos, 1 free-ranging on Cerf Island, and 8 on Cousine Island), represent an ongoing bottleneck exacerbated by a severe male bias (27 males to 1 female) and reliance on a single breeding female, elevating risks of inbreeding depression and stochastic loss.^[1] This low effective population size, stemming from haphazard 19th-20th century collections of the last remnants, has preserved morphological traits like a more domed carapace but at the cost of genetic health, with reintroduction efforts since 2002 (e.g., 2 males to Cousine in 2002, 6 adults and 40 juveniles in 2011) dependent on bolstering diversity to avert functional extinction.^[1] Populations on Silhouette Island, rediscovered in the 1990s, exhibit hybrid or Aldabra-derived genetics without evidence of pure hololissa bottleneck survival, underscoring the irrecoverable loss of original diversity.^[9]

Rediscovery and modern conservation

Identification of remnant populations

Following the presumed extinction of Seychelles giant tortoises on granitic islands by the 1840s due to overhunting and habitat alteration, no wild populations were known to persist.^[1] In the mid-1990s, biologist Justin Gerlach conducted a global survey of captive giant tortoises purportedly originating from Seychelles, examining morphological features such as carapace doming, scute configuration, and plastron shape against historical specimens and descriptions.^[10] This effort identified a small number of individuals—primarily elderly males—matching the characteristics of Dipsochelys hololissa, the broad-domed Seychelles form, in collections including zoos in Europe and private holdings. By 1997, Gerlach and collaborators formally announced the rediscovery of D. hololissa, with confirmed remnants numbering fewer than 20 pure individuals, distinguished from the more abundant Aldabra giant tortoise (Aldabrachelys gigantea gigantea) imports that had dominated Seychelles collections since the 19th century.^[10] A morphologically distinct saddle-backed variant, Dipsochelys arnoldi, was similarly recognized from a handful of specimens exhibiting narrower, raised carapaces akin to early accounts from islands like Silhouette.^[35] These identifications relied on comparative osteology and external morphology, as historical type specimens provided benchmarks for native granitic forms extinct in the wild.^[36] Genetic analyses, including a 2003 study employing mitochondrial DNA and nuclear microsatellites on purported hololissa samples, revealed affinities to Aldabra lineages, raising questions about purity and suggesting possible historical hybridization or mislabeling.^[37] Gerlach countered that morphology in captive-bred progeny maintained distinctions, attributing genetic similarities to incomplete sampling or shared ancestry, and advocated preservation of these morphotypes to safeguard putative endemic diversity.^[10] No verified wild remnants were located during these surveys; all identified populations were captive, prompting immediate isolation and breeding initiatives by the Nature Protection Trust of Seychelles to avert total loss.^[38]

Reintroduction and breeding programs

Captive breeding programs for Seychelles giant tortoises have focused on preserving remnant populations of morphologically distinct forms, such as Dipsochelys hololissa, using surviving individuals identified in zoos and private collections. These efforts, initiated in the late 20th century, involve maintaining small founder groups—typically comprising 2–6 individuals per morphotype—in controlled environments with emphasis on genetic diversity and headstarting juveniles before potential release. For instance, programs have prioritized outdoor enclosures mimicking natural habitats to encourage reproduction, though success has been limited by low numbers and historical bottlenecks.^[39]^[39] Reintroduction initiatives primarily utilize Aldabrachelys gigantea from Aldabra Atoll as ecological proxies to restore vegetation dynamics and seed dispersal on Seychelles' granitic islands, where native giant tortoise lineages were extirpated by the 19th century. On Curieuse Island, an experimental translocation of 69 adult A. gigantea occurred between 1978 and 1982, resulting in a self-sustaining population now exceeding 300 free-roaming individuals that exhibit natural foraging and nesting behaviors.^[40]^[41] Similarly, North Island received initial reintroductions in the early 2000s, reinforced by seven quarantined adults in February 2008, contributing to habitat restoration amid broader biodiversity recovery efforts.^[42]^[42] On Silhouette Island, a dedicated breeding program at Grande Barbe produced multiple generations of A. gigantea (including arnoldi morphs) by 2006, after which adults were released into secure areas, with ongoing monitoring confirming establishment and juvenile recruitment. Cousin Island supports a monitored population since 2011, where introduced tortoises have transitioned to wild breeding, evidenced by discoveries of hatchlings weighing approximately 500 grams and measuring 15 cm, indicating successful reproduction without supplementation.^[43]^[44]^[45] Recent advancements include the first successful artificial incubation of A. gigantea eggs in Seychelles, reported in 2025, yielding thriving hatchlings from five tortoise species with 75% fertilization rates in undeveloped eggs, enhancing headstarting viability for future releases. Organizations like Nature Seychelles and the Seychelles Islands Foundation oversee these programs, emphasizing predator exclusion and habitat suitability to mitigate risks such as poaching and invasive species.^[4]^[4] Despite progress, critiques highlight the proxy use of Aldabra stock potentially overlooking genetic distinctions from extinct Seychelles forms, though empirical evidence shows ecological benefits in vegetation control and soil aeration.^[39]

Current status, threats, and management

Population estimates and IUCN assessment

The Seychelles giant tortoise (Aldabrachelys hololissa), a subspecies endemic to the granitic islands of Seychelles, is extinct in the wild, with no self-sustaining populations persisting after extirpation in the mid-19th century due to human exploitation and habitat alteration. As of assessments in 2011, the global population consists of approximately 37 known adults, of which 28 are held in captivity under the Nature Protection Trust of Seychelles, exhibiting a critically imbalanced sex ratio of 27 males to 1 female, which hampers breeding viability.^[1] No substantial increases have been documented since, with genetic analyses confirming ongoing risks of inbreeding depression in these remnant lineages.^[32] Limited reintroduction efforts have established small founder groups on protected private islands. On Cousine Island, eight adults were reintroduced starting in 2002, supplemented by 40 head-started juveniles released in 2011, monitored as free-ranging but not yet forming a viable wild population.^[1] Similarly, single individuals have been placed on sites like Cerf Island, but survival and reproduction remain precarious without broader habitat restoration and genetic supplementation from captive stock. These efforts represent the entirety of "wild" individuals, totaling fewer than 50 tortoises outside formal captivity. The IUCN Red List has not formally assessed A. hololissa separately from the broader Aldabrachelys gigantea complex, which is classified as Vulnerable overall due primarily to the stable Aldabra Atoll population exceeding 100,000 individuals. However, specialist evaluations by the IUCN Tortoise and Freshwater Turtle Specialist Group propose A. hololissa for Extinct in the Wild (EW) status, reflecting the absence of natural recruitment and dependence on human intervention for persistence.^[1] This provisional categorization underscores the subspecies' de facto extinction in native ecosystems, with conservation reliant on captive breeding to avert total loss, though male-biased demographics and limited genetic diversity pose ongoing threats to long-term viability.^[32]

Ongoing threats including human development

Human development, particularly through tourism infrastructure and resort expansions, endangers Seychelles giant tortoise habitats on peripheral islands within the Aldabra group. A Qatari-funded luxury resort project on Assomption Island includes airport upgrades to accommodate larger aircraft, leading to risks of habitat fragmentation, sand dune destruction, pollution from construction and operations, increased vehicular traffic, and heightened potential for invasive species introduction via human activity.^[46] These developments threaten spillover effects to Aldabra Atoll, which harbors an estimated 150,000 Aldabrachelys gigantea individuals, the species' primary stronghold.^[46] Conservation organizations, including the Indian Ocean Tortoise Alliance and Seychelles Islands Foundation, have urged project suspension pending comprehensive biosecurity assessments, citing jeopardy to the atoll's UNESCO World Heritage status designated in 1982.^[46] In response, UNESCO issued a formal request in 2024 to Seychelles authorities for details on ecological safeguards.^[46] Beyond direct development, ongoing habitat loss stems from associated activities like unregulated tourism foot traffic and coastal alterations, which degrade foraging grounds and nesting areas on introduced populations in the granitic Seychelles.^[22] Illegal poaching and collection for the exotic pet trade persist as threats, particularly on smaller, accessible islands where enforcement is limited, with tortoises captured and exported despite CITES Appendix II protections.^[3] Compounding these pressures, climate change drives habitat shifts through intensified droughts and rising sea levels, reducing available vegetation and low-lying refugia on Aldabra; models project further contraction of suitable tortoise habitats by mid-century.^[47]^[26] Invasive alien species, including rats and goats, continue to compete for resources and damage native vegetation, while potential oil spills from increasing maritime traffic in the region pose acute contamination risks to atoll ecosystems.^[48] Recent analyses also highlight low juvenile survival rates, potentially warranting an IUCN uplisting from Vulnerable, as environmental stressors amplify demographic vulnerabilities.^[49]

Conservation achievements and effectiveness critiques

Conservation programs for the Seychelles giant tortoise, primarily involving the Aldabra subspecies Aldabrachelys gigantea, have achieved notable successes through habitat protection and reintroduction efforts. The safeguarding of Aldabra Atoll, home to an estimated 100,000 to 152,000 individuals, has prevented further declines and served as a source for translocations, demonstrating effective management in a UNESCO World Heritage site.^[50] Reintroductions to outer Seychelles islands, such as 50 adults to Aride Island in June 2024 by the Island Conservation Society, aim to restore ecological roles like seed dispersal and vegetation control.^[51] On North Island, translocated tortoises exhibited high survival and reproduction, growing the population beyond 100 by 2016.^[42] Recent advancements include the first successful artificial incubation trial in 2025, yielding 13 viable hatchlings from wild-laid eggs, potentially boosting recruitment for threatened populations.^[4] Breeding and headstarting initiatives have also supported subspecies like A. g. hololissa, with captive programs increasing numbers from remnant zoo-held individuals rediscovered in the 1990s. These efforts have restored tortoise densities on islands like Frégate and Moyenne, where populations now contribute to ecosystem restoration by suppressing invasive vegetation and promoting native plant regeneration.^[52] Such rewilding projects highlight the species' utility in reversing degradation, with tortoises acting as ecosystem engineers in granitic Seychelles habitats.^[53] Critiques of these programs center on limited long-term effectiveness, particularly juvenile survival and genetic viability. A 2025 study analyzing clutch data from six Seychelles islands found poor early-life survival rates, with high nest predation and low hatching success threatening population persistence despite adult protections.^[49] Reintroduction outcomes vary; for instance, of 250 tortoises released on Curieuse Island between 1978 and 1982, only 117 remained by 1990, hampered by poaching and negligible recruitment to age five due to feral predators.^[40] In degraded environments, a seven-year post-reintroduction assessment revealed minimal vegetation recovery, questioning the scalability of tortoise-driven restoration amid ongoing invasives and climate stressors.^[54] Genetic analyses of reintroduced groups indicate risks from small founder populations, potentially reducing adaptive potential; efforts to achieve viable effective population sizes remain ongoing but challenged by incomplete monitoring.^[55] While adult translocations succeed numerically, critics argue that without addressing nest failures—estimated at 75% egg loss in undeveloped clutches—and persistent human pressures, programs may fail to yield self-sustaining wild populations, echoing broader concerns in island rewilding where short-term gains mask underlying demographic bottlenecks.^[4]^[3]

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[28]
A survey of the Aldabra giant tortoise population introduced on ...
From evidence gathered in the field it appears that the tortoises have adapted well to the environment, are causing little damage and represent a major tourist ...
[29]
Investigating the Behavior and Personality Structure of the Aldabra ...
Feb 10, 2022 · Overall, the study revealed that the tortoises possessed personalities that were independent of sex or scenario. The study also revealed that ...
[30]
The “right” side of sleeping: laterality in resting behaviour of Aldabra ...
The aim of this study was to investigate lateralization in Aldabra giant tortoises (Aldabrachelys gigantea), focusing on asymmetrical positioning of the limbs ...
[31]
Giant Tortoise Distribution and Abundance in the Seychelles Islands
Jul 1, 2013 · Recent taxonomic revisions are unanimous in recognizing the giant tortoises of Seychelles and Madagascar as a distinct genus (formerly included ...Missing: debates | Show results with:debates
[32]
Ancient DNA elucidates the lost world of western Indian Ocean giant ...
Jan 11, 2023 · Ancient DNA elucidates the lost world of western Indian Ocean giant tortoises and reveals a new extinct species from Madagascar.Missing: distinction | Show results with:distinction
[33]
(PDF) Giant Tortoise Distribution and Abundance in the Seychelles ...
Aug 7, 2025 · The giant tortoises (Aldabrachelys or Dipsochelys) of the Indian Ocean Islands have been in decline since the first human settlement of the islands.Missing: debates | Show results with:debates
[34]
Historical Records of Indian Ocean Giant Tortoise Populations - jstor
considered extinct. THE GRANITIC SEYCHELLES. The granitic Seychelles were first discovered in 1609, when John Jourdain arrived at North. Island, near Silhouette ...<|control11|><|separator|>
[35]
Breeding supposedly extinct Seychelles Giant tortoises
The identification and rediscovery of Dipsochelys/Aldabrachelys hololissa and D./A. arnoldi as species formerly believed to be extinct is controversial and ...<|separator|>
[36]
[PDF] Chelonian Conservation and Biology, 1998, 3(1)
Aldabrachelys gigantea (Schweigger, 1812), Loveridge and. Williams, 1957:259 ... Extinct throughout much of the presumed historic range; some 150,000 survive on ...
[37]
Are the native giant tortoises from the Seychelles really extinct? A ...
Our results indicate that the morphotypes suspected to represent Seychelles species do not show levels of variation and genetic structuring consistent with long ...Missing: debates | Show results with:debates
[38]
[PDF] 5.7 Giant tortoises: - Saint Helena Island Info
Dr Justin Gerlach has provisionally identified Jonathan as a member of one of them, the Seychelles Giant Tortoise, Dipsochelys hololissa. As such he is a ...
[39]
BREEDING AND REINTRODUCTION OF SEYCHELLES CHELONIA
The captive breeding programme for the Seychelles giant tortoises comprises almost all the known survivors of the two species: 2 female and 4 male D. hololissa, ...
[40]
Giant tortoise Geochelone gigantea translocation to curieuse island ...
Taxonomy, history and geography of Seychelles land tortoises and fresh-water turtles ... The conservation biology of tortoises, IUCN, Gland (1989), pp. 105-110.
[41]
About 300 free roaming Aldabra giant tortoises (Aldabrachelys ...
Feb 3, 2025 · 300 free roaming Aldabra giant tortoises (Aldabrachelys gigantea) live in Curieuse Marine National Park, Curieuse Island, Seychelles.
[42]
Conservation - North Island
The conservation program has continued to expand. A significant translocation in March 2025 in partnership with IOTA and MACCE further increased the tortoise ...
[43]
Giant tortoise reintroduction - Island Biodiversity
In that year we released the adults from the breeding programme at Grande Barbe on Silhouette island. The report of the release can be dowloaded.Missing: programs | Show results with:programs
[44]
Counting the giants of Cousin Island - Nature Seychelles
We have monitored the tortoise population on Cousin since 2011. We check on population size, breeding habits, and any potential impacts a larger population ...
[45]
Cousin gives torti youth a chance - Nature Seychelles
The discovery of the young torti shows that the great reptiles are breeding onCousin in a 'wild state', which is great news for conservation.
[46]
Giant tortoises in Seychelles face threat from luxury hotel development
Sep 11, 2024 · Giant tortoises in Seychelles face threat from luxury hotel development ... ICS acknowledges there are significant threats with the project that ...
[47]
Chromosome-level genome assembly for the Aldabra giant tortoise ...
Oct 12, 2022 · The species is endemic to Aldabra Atoll in Seychelles and is listed as Vulnerable on the International Union for Conservation of Nature Red List ...
[48]
Aldabra Atoll - UNESCO World Heritage Centre
Due to difficulties of access and the atoll's isolation, Aldabra has been protected from human influence and thus retains some 152,000 giant tortoises, the ...Missing: IUCN | Show results with:IUCN
[49]
[PDF] Poor early-life survival threatens the persistence of Giant Aldabra ...
May 11, 2025 · Clutch data from 24 different Aldabra tortoise clutches from six Seychelles island was used to calculate the number of clutches experiencing.
[50]
[PDF] Aldabra Atoll - 2020 Conservation Outlook Assessment
Dec 1, 2020 · Ocean acidification thus poses a potentially high threat to many forms of marine life. Overall assessment of threats. High Threat. The list of ...<|separator|>
[51]
Seychelles' conservationists re-home 50 Aldabra giant tortoises on ...
Jun 21, 2024 · The Island Conservation Society (ICS), a non-profit environmental organisation in Seychelles, has successfully re-homed 50 Aldabra giant tortoises on Aride ...
[52]
Protecting Aldabra's Giant Tortoises - The Orianne Society
Feb 16, 2025 · These colossal reptiles can grow up to four feet in length and weigh nearly 800 pounds. Their sheer size is striking—standing beside the first ...
[53]
Island rewilding with giant tortoises in an era of climate change
Oct 22, 2018 · We here present and discuss a conceptual framework that can serve as a roadmap for the study and application of tortoise rewilding in an uncertain future.Missing: threats | Show results with:threats
[54]
Rewilding in cold blood: Restoring functionality in degraded ...
Recently, however, a study found that the long-term effects (after 7 years) of reintroducing tortoises into a severely degraded environment have had a low ...
[55]
https://onlinelibrary.wiley.com/doi/10.1002/ece3.72005