Humans hunt tortoises and their eggs for meat. Adult Aldabra giant tortoises have no other natural predators.
Known Predators:
Aldabra giant tortoises are the largest living terrestrial species of tortoise (Testudinidae). They are dark gray to black in color and have a high, thick, domed carapace, a very long neck (to aid in branch feeding), and short, thick legs. The limbs and head are covered in bony scales. Mature males have an average carapace length of 120 cm and can weigh up to 250 kg. They have longer and lower carapaces which widen near the rear and longer, thicker tails. Females are smaller than males, with an average carapace length of 90 cm and weight of 160 kg.
Range mass: 160 to 250 kg.
Range length: 90 to 140 cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: male larger; sexes shaped differently
Lifespan in the wild is unknown in Aldabra giant tortoises. The estimated lifespan is over 100 years, possibly up to 150 years. These tortoises tend to outlive researchers and no sufficient records have been kept. However, a positive correlation between age and carapace size has been reported. One zoo estimates the age of their tortoise to be 176 years old.
Range lifespan
Status: captivity: 176 (high) years.
Aldabra giant tortoises are terrestrial and occur in a wide variety of habitats, including scrub forests, mangrove swamps, and coastal dunes and beaches, each with their respective vegetation. The largest populations of tortoises are found on grasslands called "platins." Due to prolonged periods of heavy grazing, a habitat known as “tortoise turf”, consisting of a variety of grasses, has developed in certain areas.
Habitat Regions: tropical ; terrestrial
Terrestrial Biomes: desert or dune ; savanna or grassland ; scrub forest
Aquatic Biomes: coastal
Wetlands: swamp
Aldabra giant tortoises (Dipsochelys dussumieri) are endemic to the Aldabra Atoll of the Seychelles, an archipelago nation in the western Indian Ocean about 930 miles east of Africa and northeast of Madagascar. Populations have also been introduced to Mauritius, Réunion, and islands in central Seychelles.
Biogeographic Regions: ethiopian (Introduced , Native )
Other Geographic Terms: island endemic
Aldabra giant tortoises were previously known as Geochelone gigantea.
Aldabra giant tortoises are limited in their social communication. The only non-sexual social behavior observed is ‘nosing’. One tortoise will approach another, lie down, and rub its nose on the latter’s head or neck. This lasts for several minutes. There currently does not exist an explanation for this behavior.
Communication Channels: tactile
Perception Channels: visual ; tactile ; acoustic ; chemical
Aldabra giant tortoises are one of the few surviving species of Indian Ocean giant tortoises. They are considered vulnerable due to years of human poaching and encroachment. Translocation of these tortoises has been unsuccessful, partly due to inadequate attention to human-related habitat interactions. Human poaching has significantly jeopardized chances of establishing populations that will survive far into the future.
Public concern has increased dramatically since the 1960's, and efforts at conservation are currently underway. The Seychelles Islands Foundation (SIF) under the Seychelles National Parks and Conservancy Act has been managing Aldabran affairs. Aldabra was also designated a World Heritage Site by UNESCO in 1982.
US Federal List: no special status
CITES: appendix ii
IUCN Red List of Threatened Species: vulnerable
In young tortoises, the carapace is black and shiny. As they grow, tissue is added concentrically and is marked by radial striations, or growth rings. As growth continues, the head protrudes outward, while the limbs grow larger and stockier.
Sexual maturity is determined by size rather than by age; most individuals begin to reproduce when they reach approximately half their full-grown size, usually around 25 years of age. The growth of Aldabra giant tortoises is likely discontinuous and episodic, and research suggests that growth rate slows with increased age.
There are no known adverse affects of Dipsochelys dussumieri on humans.
Humans have hunted Aldabra giant tortoises and their eggs for food. They have also been bred in captivity as tourist attractions. Aldabra giant tortoises have also been the subjects of numerous research experiments including population studies, classical and operant conditioning, and conservation practices.
Positive Impacts: food ; body parts are source of valuable material; ecotourism ; research and education
Because of their heavy grazing, Aldabra giant tortoises have created a habitat known as tortoise turf. Tortoise turf is a combination of a variety of grasses and herbs and serves as the natural habitat for several smaller species. Tortoises also clear pathways in the forest for smaller animals. Seeds pass through their digestive tracts and are dispersed through their feces. Coenobita rugosus (a species of land hermit crab) is dependent on tortoise feces for food.
Ecosystem Impact: disperses seeds; creates habitat
Commensal/Parasitic Species:
Aldabra giant tortoises are primarily herbivores and feed mainly on vegetation such as grasses, leaves, woody plant stems, a variety of herbs and sedges. They are flexible and opportunistic and will sometimes supplement their diets with small invertebrates or carrion, even carrion of its own species. In captivity, Aldabra giant tortoises have been known to enjoy fruits, such as apples, pears, tomatoes and bananas, carrots, peas, beans, almonds, and compressed vegetable pellets. Little fresh water is available in their natural habitat and they must obtain most of their water from food sources.
Animal Foods: carrion ; insects; terrestrial non-insect arthropods
Plant Foods: leaves; wood, bark, or stems; seeds, grains, and nuts; fruit; flowers
Primary Diet: herbivore (Folivore , Frugivore )
Mating attempts only occur when tortoises are active, during early morning or late evening. Mounting results from casual encounters, but it is possible that males undergo a period of heightened sexual activity during the mating season. The initiation of the mating attempt begins with a male approaching a female and climbing onto her back with his neck fully extended. Once in this position, he pushes off his forefeet and thrusts forward in four-second intervals. The male emits a loud moan or grunt with each thrust and up to 44 thrusts are performed. Sometimes the male appears to bite at the female’s head. Often the female will respond to the male’s mount by walking away or propping herself up on her forelegs, forcing her rear into the ground and dislodging the male. Males appear to be promiscuous in their selection of prospective partners, not all of which are necessarily female. One selective criteria is the relative size of the partner; males with a carapace length of 50 cm or more generally will only select smaller mates between 45 and 65 cm in length. Most mating attempts are not successful.
Mating System: polygynandrous (promiscuous)
The breeding season of Aldabra giant tortoises occurs from February to May. Females lay between 4 and 14 rubbery eggs in a shallow, dry nest, of which less than one half are fertile. The average clutch size increases in captivity, where females lay about 9 to 25 eggs. The incubation period is largely dependent upon temperature: in warm temperatures, incubation lasts 110 days, but in cooler temperatures, hatchlings emerge after about 250 days of incubation, between October and December. Females often produce a second clutch within the same breeding season, especially in healthy, uncrowded populations.
Breeding interval: Aldabra giant tortoises breed once or twice yearly.
Breeding season: Aldabra giant tortoises breed from February to May.
Range number of offspring: 4 to 25.
Range gestation period: 110 to 250 months.
Average gestation period: 243 months.
Average age at sexual or reproductive maturity (female): 25 years.
Average age at sexual or reproductive maturity (male): 25 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; oviparous
Like other turtles, Aldabra giant tortoises do not care for their young after females have deposited the eggs in a safe nest. The young hatch and dig out of the nest on their own.
Parental Investment: no parental involvement; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female)
The Commission has conserved the specific name Testudo gigantea Schweigger, 1812 (family TESTUDINIDAE) in its customary usage for the giant land tortoise found on Aldabra Atoll in the western Indian Ocean, by affirmation of the neotype designation of 2006 and suppression of T. dussumieri Gray, 1831.
The Aldabra giant tortoise (Aldabrachelys gigantea) is a species of tortoise in the family Testudinidae. The species is endemic to the Seychelles, with the nominate subspecies, A. g. gigantea native to Aldabra atoll. It is one of the largest tortoises in the world.[7] Historically, giant tortoises were found on many of the western Indian Ocean islands, as well as Madagascar, and the fossil record indicates giant tortoises once occurred on every continent and many islands with the exception of Australia and Antarctica.[8]
Many of the Indian Ocean species were thought to be driven to extinction by over-exploitation by European sailors, and they were all seemingly extinct by 1840 with the exception of the Aldabran giant tortoise on the island atoll of Aldabra.[9] Although some remnant individuals of A. g. hololissa and A. g. arnoldi may remain in captivity,[9] in recent times, these have all been reduced as subspecies of A. gigantea.[6]
The carapace of A. gigantea is a brown or tan in color with a high, domed shape. The species has stocky, heavily scaled legs to support its heavy body. The neck of the Aldabra giant tortoise is very long, even for its great size, which helps the animal to exploit tree branches up to a meter from the ground as a food source. Similar in size to the famous Galápagos giant tortoise, its carapace averages 122 cm (48 in) in length with an average weight of 250 kg (550 lb).[10]
Females are generally smaller than males, with average specimens measuring 91 cm (36 in) in carapace length and weighing 159 kg (351 lb).[11] Medium-sized specimens in captivity were reported as 70 to 110 kg (150 to 240 lb) in body mass.[12] Another study found body masses of up to 132 kg (291 lb) most commonplace.[13]
This species is widely referred to as Aldabrachelys gigantea, but in recent times, attempts were made to use the name Dipsochelys as Dipsochelys dussumieri. After a debate that lasted two years with many submissions, the ICZN eventually decided to conserve the name Testudo gigantea over this recently used name (ICZN 2013).[14] This also affected the genus name for the species, establishing Aldabrachelys gigantea as nomen protectum.
Four subspecies are currently recognized.[6] A trinomial authority in parentheses indicates that the subspecies was originally described in a genus other than Aldabrachelys:
The subspecific name, daudinii, is in honor of French zoologist François Marie Daudin.[16]
Genetic evidence suggests that A. gigantea is most closely related to the extinct giant tortoise Aldabrachelys abrupta from Madagascar, from which it is estimaged to have diverged from approximately 4.5 million years ago.[17]
The main population of the Aldabra giant tortoise resides on the islands of the Aldabra Atoll in the Seychelles. The atoll has been protected from human influence and is home to some 100,000 giant tortoises, the world's largest population of the animal.[18] Smaller populations of A. gigantea in the Seychelles exist on Frégate Island and in the Sainte Anne Marine National Park (e.g. Moyenne Island),[19], where they are a popular tourist attraction.
Another isolated population of the species resides on the island of Changuu, near Zanzibar. Other captive populations exist in conservation parks in Mauritius and Rodrigues. The tortoises exploit many different kinds of habitat, including grasslands, low scrub, mangrove swamps, and coastal dunes.
A peculiar habitat has coevolved due to the grazing pressures of the tortoises: "tortoise turf", a comingling of 20+ species of grasses and herbs. Many of these distinct plants are naturally dwarfed and grow their seeds not from the tops of the plants, but closer to the ground to avoid the tortoises' close-cropping jaws.
As the largest animal in its environment, the Aldabra tortoise performs a role similar to that of the elephant. Their vigorous search for food fells trees and creates pathways used by other animals.
Primarily herbivores, Aldabra giant tortoises eat grasses, leaves, woody plant stems, and fruit. They occasionally indulge in small invertebrates and carrion, even eating the bodies of other dead tortoises. In captivity, Aldabra giant tortoises are known to consume fruits such as apples and bananas, as well as compressed vegetable pellets. In 2020, a female Aldabra giant tortoise on Fregate Island was observed hunting and eating a juvenile lesser noddy, indicating that the species was in the process of learning to catch birds. [20][21]
Little fresh water is available for drinking in the tortoises' natural habitat, so they obtain most of their moisture from their food.
The Aldabra giant tortoise has two main varieties of shells, related to their habitat. Specimens living in habitats with food available primarily on the ground have more dome-shaped shells with the front extending downward over the neck. Those living in an environment with food available higher above the ground have more flattened top shells with the front raised to allow the neck to extend upward freely.
The Aldabra giant tortoise is a herbivorous animal, spending much of its time browsing for food in its surrounding well-vegetated environment. The Aldabra giant tortoise is known to be found in places that are commonly known as "tortoise turf". Tortoise turf is composed of:[22][23]
Aldabra tortoises are found both individually and in herds, which tend to gather mostly on open grasslands. They are most active in the mornings, when they spend time grazing and browsing for food. They dig wallows, hide under shade trees or in small caves, as well as submerge themselves in pools to keep cool during the heat of the day.
Large tortoises are among the longest-lived animals. Some individual Aldabra giant tortoises are thought to be over 200 years of age, but this is difficult to verify because they tend to outlive their human observers. Adwaita was reputedly one of four brought by British seamen from the Seychelles Islands as gifts to Robert Clive of the British East India Company in the 18th century, and came to Calcutta Zoo in 1875. At his death in March 2006 at the Kolkata (formerly Calcutta) Zoo in India, Adwaita is reputed to have reached the longest ever measured lifespan of 255 years (birth year 1750).[24]
As of 2022, Jonathan, a Seychelles giant tortoise, is thought to be the oldest living giant tortoise at the age of 191 years.[25] Esmeralda, an Aldabra giant tortoise, is second at the age of 179 years, since the death of Harriet, a Galapagos giant tortoise, at 175. An Aldabra giant tortoise living on Changuu off Zanzibar is reportedly 197 years old.
Mating takes place between February and May, and in July-September[26] females lay between 9 and 25 hard-shelled eggs in a 30 cm deep nest. Usually, less than half of the eggs are fertile. Females can produce multiple clutches of eggs in a year. After incubating for about eight months, the tiny, independent young hatch between October and December.[27]
In captivity, oviposition dates vary. Tulsa Zoo[28] maintains a small herd of Aldabra tortoises and they have reproduced several times since 1999. One female typically lays eggs in November and again in January, providing the weather is warm enough to go outside for laying. The zoo also incubates their eggs artificially, keeping two separate incubators at 27 °C (81 °F) and 30 °C (86 °F). On average, the eggs kept at the latter temperature hatch in 107 days.[26]
The Aldabra giant tortoise has an unusually long history of organized conservation. Albert Günther of the British Museum, who later moved to the Natural History Museum of London, enlisting Charles Darwin and other famous scientists to help him, worked with the government of Mauritius to establish a preserve at the end of the 19th century. The related, but distinct, species of giant tortoise from the Seychelles islands, Seychelles giant tortoise A. g. hololissa and Arnold's giant tortoise A. g. arnoldi, were the subject of a captive-breeding and reintroduction program by the Nature Protection Trust of Seychelles.[29][30]
A reference genome and low-coverage sequencing analyses has looked at revealing within- and among-island genetic differentiation within the Aldabra population, as well as assigning likely origins for zoo-housed individuals. This has managed to differentiate between individuals sampled on Malabar and Grande Terre and resolve the exact origin of zoo-housed individuals.[31]
The Aldabra giant tortoise (Aldabrachelys gigantea) is a species of tortoise in the family Testudinidae. The species is endemic to the Seychelles, with the nominate subspecies, A. g. gigantea native to Aldabra atoll. It is one of the largest tortoises in the world. Historically, giant tortoises were found on many of the western Indian Ocean islands, as well as Madagascar, and the fossil record indicates giant tortoises once occurred on every continent and many islands with the exception of Australia and Antarctica.
Many of the Indian Ocean species were thought to be driven to extinction by over-exploitation by European sailors, and they were all seemingly extinct by 1840 with the exception of the Aldabran giant tortoise on the island atoll of Aldabra. Although some remnant individuals of A. g. hololissa and A. g. arnoldi may remain in captivity, in recent times, these have all been reduced as subspecies of A. gigantea.
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