Aldabra giant tortoise
Among the tortoises, this species is second only in size to the slightly larger species found in the Galápagos Islands. G. gigantea reaches a carapace length of 119 cm and may weigh up to 304 kg. It has a thick, elongated, domed carapace with descending sides, only a slight cervical indentation at best, and posterior marginals that are flared, slightly upturned, and unserrated. A cervical scute is usually present. Vertebrals are broader than long, the 5th is smallest and expanded, and the first is narrower than the 2nd. Growth annuli surround the raised vertebral and pleural areolae, but these are generally low, and the carapace may be worn very smooth in old individuals. Usually 11 marginals lie on each side of the carapace, and the supracaudal is single, undivided, and downturned between the posterior marginals. The carapace is uniformly blackish brown to black. The plastron is relatively short and has a posterior notch. Its forelobe tapers anteriorly and is longer, but much narrower, than the hindlobe. The plastral formula is: abd > hum > fem > pect >< gul >< an; the paired gulars are short and thick and scarcely project beyond the carapacial rim. The bridge is moderate in width (about one-third the carapace length), and bears a single small axillary scute and a larger inguinal. The plastron and bridge are also brownish gray. The head is narrowly pointed and somewhat wedge shaped (when compared to Galápagos tortoises), and with a convex forehead. The snout is nonprojecting, and the upper jaw is either weakly hooked or bi- or tricuspid. The large prefrontal scale is divided longitudinally, its sides are parallel and do not diverge posteriorly; a relatively small frontal scale follows, and other head scales are small. Head, neck, limbs, and tail are gray. The anterior surface of the forelimbs is covered with large, slightly rounded, nonoverlapping scales. No conical tubercles occur on the thighs; a large terminal scale on the tail is present only in very large adults.
The Aldabra tortoise has a karyotype of 52 chromosomes (Benirschke et al., 1976; Bickham and Carr, 1983).
Males are larger and have longer, thicker tails than do the smaller females.
Geochelone gigantea occurs naturally only on the coral-limestone Aldabra Atoll in the Indian Ocean. Population estimates ranged to 150,500 individuals, with most occurring on Grande Terre where the density was about 27/ha (Picard, 5/ha; Malabar, 7/ha; Bourn and Coe, 1978). A census conducted in 1997 indicates that the Aldabra population has declined to some 100,000 animals due to density dependent factors (ERGO, 1997).
During the past three to four decades, feral tortoise populations have been established on Fregate, Curieuse, Cousin, and Assomption Islands and on Farquhar Atoll (Jeanne A. Mortimer, pers. comm.). At Curieuse, in the granitic Seychelles, a tortoise colony was established as a tourist attraction in 1978. Censuses conducted in subsequent years recorded declines in the original population that are apparently attributable to poaching. During three weeks in 1986, 143 (57%) of the original 250 tortoises were encountered (Samour et al., 1987), but only 117 (47%) during twelve weeks in 1990 (Hambler, 1994). A low rate of natural reproduction has occurred, but in 1997 two adult females that has been born on Curieuse were encountered (Jeanne A. Mortimer, pers. comm.). Some 3,000 to 4,000 giant tortoises are kept as pets by Seychellois on the islands of Mahé, Praslin, and La Digue, but little reproduction occurs within the captive herds (Jeanne A. Mortimer, pers. comm.).
Arnold (1976) proposed that giant tortoises may have first reached Aldabra by oceanic drifting from Madagascar, and, since the atoll has apparently been completely submerged twice since the tortoises settled on it, at least three such colonizations may have occurred.
No subspecies are recognized, but some individuals are domed while others are more saddlebacked. Also the tortoises living in some less populated areas are larger; adults from low density population of the same age and sex are twice the size of those from the high density population (Swingland, 1989b). However, Arnold (1979) found the total variation to be less than that found in the Galápagos tortoises.
In 1995(a), Gerlach and Canning announced the possible rediscovery of a Seychelles giant tortoise in a hotel garden in the Seychelles. This was confirmed by Gerlach (1997b, c), who reported it concerned animals of two species that were formerly thought extinct: the flat-shelled Geochelone arnoldi (Bour, 1982a) and the more domed Geochelone hololissa (Günther, 1877).
Geochelone gigantea occurs in a variety of habitats, including grassland, scrub areas, and mangrove swamps. Sheltering trees or bushes are necessary to escape the extreme midday tropical sun, and some tortoises make use of freshwater and/or saline pools and mudholes to cool off. Heavy grazing by tortoises has resulted in a dwarfed vegetation (often called "tortoise turf") over much of the area.
Sexual maturity occurs by the time both sexes have grown to 30 cm carapace length (Peters and Finnie, 1979). Mating takes place from February to May. The male approaches the female and smells her carapace, then attempts to mount with neck outstretched, forefeet on the anterior portion of her carapace, and hind feet on the ground. His tail is then laid next to hers and intromission follows. Most copulatory attempts are unsuccessful. Males emit stereotyped deep-pitched groans or bellows while mounted.
Nests are dug in the dry season from June through September; an April oviposition has been reported in captivity (Peters and Finnie, 1979). Where the soil is deep enough, well-camouflaged flask-shaped cavities about 25 cm deep are excavated. Elsewhere the eggs may be laid in shallow, scraped-out, unfilled depressions; some captives have merely laid their eggs on the surface of the ground (Peters and Finnie, 1979). Patches of grass or scrub in open areas seem to be preferred sites (Bourn, 1977). Nesting occurs at dusk or in the night, allowing the females to avoid overheating.
Females in high-density populations lay fewer eggs (4-6) in a single clutch every few years, while those from low-density populations may lay several clutches (but average about two) of 12-14 eggs each year (Bourn, 1977; Swingland and Coe, 1978; Swingland, 1989b). Eggs are almost spherical (48-51 mm) with brittle shells. Hatching occurs from September and early October to mid-December during the rainy season, some 98 to almost 200 days later.
Hatchlings have carapace lengths of 64-81 mm, and weigh 49-59 g (Peters and Finnie, 1979). Many predators feed on the young tortoises, including crabs, various birds, and feral cats, and this becomes a serious problem when fecundity is so low (Swingland and Coe, 1979; Rainbolt, 1996).
A factor aiding survival is the tortoises' ability as herbivores to both graze and browse. In the drier open areas they are grazers, feeding primarily on sedges, grasses, and small herbs which form the distinctive "tortoise turf" plant community. Usually over half of the tortoises observed eating will be feeding on this turf. So minute and low-growing are these plants that the tortoises inevitably ingest soil as they feed, allowing much erosion. Dry conditions and heavy grazing prevents formation of a continuous plant cover, and in many places dry earth is exposed. Along much of the coastal dunes, Sporobolus grass is dominant to the exclusion of other grasses and sedges and is the favorite food of tortoises there.
In the wooded and scrub areas, tortoises browse on many types of woody plants. A number of species are readily eaten, and some show a conspicuous browse line about a meter above the ground, or as high as the tortoises can stretch their necks. Many woody plants are eaten, but apparently unpalatable species are ignored. Although the branches of the screwpine (Pandanus) are out of reach, they are readily eaten if cut down, as are also dry coconut fronds. Tortoises have considerable influence on regeneration of the browse plants, and the saplings of Ficus and Flacourtia are especially stunted by repeated cropping.
Although primarily vegetarians, the tortoises eat many other foods if the opportunity arises, including faeces and the decaying flesh of dead land crabs and other tortoises. They are probably attracted to these by the stench, and possibly these foods provide additional sources of water. There also seems to be a predilection for red-colored foods.
Drinking water is generally scarce on the atoll, though in places pools are abundant. During the wet season, rain puddles provide drinking water, but when free water is unavailable, tortoises probably gain most water through metabolism.
The name Geochelone gigantea formally is incorrect, as the type description is based on a specimen (now missing) of Geochelone denticulata (Bour, 1982a; Pritchard, 1986). Several sources have used the name elephantina Duméril and Bibron, 1835, but Gerlach and Canning (in press) propose the name Dipsochelys dussumieri (Gray, 1831b); see the Genus Geochelone for a discussion of the generic name.
IUCN Red List Status (1996)
Vulnerable (D2) in Baillie and Groombridge (1996); considered Conservation Dependent the Seychelles Red Data Book (Gerlach, 1997a).