Dipsochelys dussumieri

Geographic Range

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 ethiopian
  • introduced introduced
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

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 tropical
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• savanna or grassland savanna or grassland
• scrub forest scrub forest

• Aquatic Biomes
• coastal coastal

• Wetlands
• swamp swamp

Physical Description

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.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• sexes shaped differently

Development

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.

Reproduction

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) 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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous 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

Lifespan/Longevity

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.

Behavior

Aldabra giant tortoises are active mainly over a period of less than four hours in the early morning and late evening. Aldabra giant tortoises engage in various activities including mating attempts, stretching, walking, feeding, sleeping, and defecation during these times. They rest for the remainder of the day. While resting, they lie with the plastron (the ventral part of the shell) on the ground, with the limbs and head slightly retracted. Large males walk with the carapace high above the ground and the limbs fully extended vertically. Aldabra giant tortoises are found both individually and in aggregate herds in areas with plenty of food. When threatened, Aldabra giant tortoises will quickly withdraw their heads and walk away. Tortoises will fully extend their necks while browsing and occasionally rear up against trees. Aldabra giant tortoises have been known to move seasonally between habitats. During dry seasons, tortoises are dispersed evenly throughout the various habitats. When the first rain occurs, tortoises migrate towards the open grasslands and coastal scrub areas.

• Key Behaviors
• terricolous
• crepuscular crepuscular
• motile motile
• nomadic nomadic
• migratory migratory
• daily torpor
• solitary solitary
• social social

Home Range

Home ranges of Aldabra giant tortoises are not documented.

Communication and Perception

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 tactile

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

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.

• Primary Diet
• herbivore herbivore
  • folivore folivore
  • frugivore frugivore

• Animal Foods
• carrion carrion
• insects
• terrestrial non-insect arthropods

• Plant Foods
• leaves
• wood, bark, or stems
• seeds, grains, and nuts
• fruit
• flowers

Predation

Humans hunt tortoises and their eggs for meat. Adult Aldabra giant tortoises have no other natural predators.

Ecosystem Roles

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

Economic Importance for Humans: Positive

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 food
• body parts are source of valuable material
• ecotourism ecotourism
• research and education

Economic Importance for Humans: Negative

There are no known adverse affects of Dipsochelys dussumieri on humans.

Conservation Status

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.

Other Comments

Aldabra giant tortoises were previously known as Geochelone gigantea .

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Chris Ng (author), University of Maryland, Baltimore County, Kevin Omland (editor, instructor), University of Maryland, Baltimore County.

References

Bourn, D., C. Gibson, D. Augeri, C. Wilson, J. Church, S. Hay. 1999. The rise and fall of the Aldabran giant tortoise population. Biological Sciences , 266: 1091-1100.

Grubb, P. 1971. The Growth, Ecology and Population Structure of Giant Tortoises on Aldabra. Biological Sciences , 260/836: 327-371.

Hambler, E. 1994. Giant Tortoise Geochelone Gigantea translocation to Curieuse Island (Seychelles): Success or failure?. Biological Conservation , 69: 293-299.

Hutchins, M. 2003. Tortoises. Pp. 70 in Grzimek's Animal Life Encyclopedia , Vol. 7, 2 Edition. MI: Gale Cengage.

Karanth, K., E. Palkovacs, J. Gerlach, S. Glaberman, J. Hume, A. Caccone, A. Yoder. 2005. Native Seychelles tortoises or Aldabran imports? The importance of radiocarbon dating for ancient DNA studies. Amphibia-Reptilia , 26: 116-121.

Klemens, M. 2000. Turtle Conservation . Washington and London: Smithsonian Institution.

Stearns, B. 1987. Captive husbandry and propagation of the Aldabra giant tortoise Geochelone gigantea: at the institute for Herpetological Research. International Zoo Yearbook , 27/1: 98.

Stoddart, D., D. Cowx, C. Peet, J. Wilson. 2003. Tortoises and tourists in the Western Indian ocean: The curieuse experiment. Biological Conservation , 24/1: 67-80.

Stoddart, D. 1969. Retrospect and Prospect of Aldabra Research. Nature , 221: 1004-1006.

Stoddart, D. 1968. The Aldabra affair. Biological Conservation , 1/1: 63-69.

Weiss, E., S. Wilson. 2003. The Use of Classical and Operant Conditioning in Training Aldabra Tortoises (Geochelone gigantea) for Venipuncture and Other Husbandry Issues. Journal of Applied Animal Welfare Science , 6/1: 33-38.

2006. Aldabra giant tortoise (Geochelone gigantea). Pp. 335-340 in Encyclopedia of Endangered Species , Vol. 2, 1 Edition. Detroit: Thomson Gale.