Aldabra giant tortoises ( ("Aldabra giant tortoise (Geochelone gigantea)", 2006; Karanth, et al., 2005; Hambler, 1994)) 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.
- Other Geographic Terms
- island endemic
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. (Grubb, 1971)
- Aquatic Biomes
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. ("Aldabra giant tortoise (Geochelone gigantea)", 2006; Grubb, 1971; Hutchins, 2003)
- Sexual Dimorphism
- male larger
- sexes shaped differently
- Range mass
- 160 to 250 kg
- 352.42 to 550.66 lb
- Range length
- 90 to 140 cm
- 35.43 to 55.12 in
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. (Grubb, 1971)
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. (Grubb, 1971)
- 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. (Stearns, 1987; Grubb, 1971; Stearns, 1987)
- Key Reproductive Features
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- 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
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
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. (Stoddart, et al., 2003)
- Range lifespan
- 176 (high) years
- Range lifespan
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. (Grubb, 1971; Bourn, et al., 1999)
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. (Grubb, 1971)
- Communication Channels
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. ("Aldabra giant tortoise (Geochelone gigantea)", 2006; Grubb, 1971; Stoddart, 1968)
- Animal Foods
- terrestrial non-insect arthropods
- Plant Foods
- wood, bark, or stems
- seeds, grains, and nuts
- Known Predators
- humans (Homo sapiens)
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. (Grubb, 1971)
- Ecosystem Impact
- disperses seeds
- creates habitat
- Coenobita rugosus
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. (Karanth, et al., 2005; Bourn, et al., 1999; Klemens, 2000; Weiss and Wilson, 2003)
Economic Importance for Humans: Negative
There are no known adverse affects ofon humans.
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. (Karanth, et al., 2005; Hambler, 1994; Bourn, et al., 1999; Klemens, 2000)
Aldabra giant tortoises were previously known as Geochelone gigantea.
Tanya Dewey (editor), Animal Diversity Web.
Chris Ng (author), University of Maryland, Baltimore County, Kevin Omland (editor, instructor), University of Maryland, Baltimore County.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
uses sound to communicate
- bilateral symmetry
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
flesh of dead animals.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
active at dawn and dusk
- desert or dunes
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
an animal that mainly eats leaves.
A substance that provides both nutrients and energy to a living thing.
an animal that mainly eats fruit
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.
- island endemic
animals that live only on an island or set of islands.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
makes seasonal movements between breeding and wintering grounds
having the capacity to move from one place to another.
- native range
the area in which the animal is naturally found, the region in which it is endemic.
generally wanders from place to place, usually within a well-defined range.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
- scrub forest
scrub forests develop in areas that experience dry seasons.
- seasonal breeding
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- tropical savanna and grassland
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
- temperate grassland
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
uses sight to communicate
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