Genus/species: Geochelone gigantea
These are one of the largest land tortoises although the Galapagos tortoise may be larger. Large size may be due to the lack of predation and isolation over many thousands of years. The male’s carapace length may measure four feet (1.22 m) and they may weigh up to 550 pounds (250 kg). The female’s carapace may measure three feet (91 cm) and weigh 350 pounds (159 kg). The males have a concave plastron. They are dark gray to black in color with a highly domed, thick carapace. They have long necks, which helps with food gathering.
Distribution and Habitat
They are found on Aldabra Island, which is northeast of Madagascar in Indian Ocean.
Aldabra Island is a coral atoll rimmed by jagged limestone and small beaches; it encloses a large mangrove-bordered lagoon. The tortoises are found in scrub, mangrove swamp, and coastal dunes. The largest concentration of tortoises is found in the grasslands called platins.
Diet in the Wild
Aldabra tortoises are mainly grazers and browsers that feed on grasses and woody plants. They have also been found feeding on dry grasses and dead leaves. However, they will eat meat when it is available and possibly even eat the carcasses of dead tortoises. They may also eat feces.
They are fed salad and hay six days per week in the winter months. In the summer, they have access to an outdoor enclosure where they graze on grass. Their diet is supplemented with greens and assorted browse. Romaine and carrots are fed during demonstrations.
The breeding season extends from February to May; females carry the eggs for about ten weeks, after which period they are buried in the ground. Clutch size is nine to 25 eggs that are the size of tennis balls. In high-density populations, female Aldabra’s may lay only four to five eggs every few years, whereas in low-density populations they may lay several clutches a year. Incubation is about four months. Females may nest twice in one season. Full growth appears to be reached at about 25 years of age.
They can reach ages of more than 100 years. It is believed that tortoises are the longest lived of all animals although is hard to prove because they have outlived the scientists who were studying them, and proper records were not kept. The Aldabra tortoises on exhibit at the Zoo are about 80 years old. This is a guess because they arrived at the National Zoo as adults. One pair arrived in 1956 and the second in 1976.
They are listed on CITES Appendix II.
The Aldabra tortoise was one of the first species to be protected to ensure its survival. Charles Darwin and other notable conservationists of the late 1800s, along with the then-governor of Mauritius set aside a captive breeding population on Mauritius as well as protecting the Aldabra Atoll.
These tortoises are the remnants of a larger group of tortoises that once lived in the Indian Ocean. There were 18 different species, which, because of hunting by sailors and the predation upon eggs and hatchlings by introduced species such as rats, cats, and pigs, all went extinct except for the Aldabra.
Judging by the apparent decline of the Aldabra tortoise populations during the nineteenth century, one would imagine that today they would be virtually extinct. It is surprising, therefore, to read recent accounts about Aldabra Island, which record an astonishing abundance of tortoises today. Around a water hole it is possible to include 50 tortoises in a single photograph and total population estimates run from 33,000 to 100,000 animals.
The Aldabra tortoise is the largest animal on the atoll.
It fills a similar role to the one occupied by elephants in
Africa and Asia. As with elephants, they are the main consumers
of vegetation and will noticeably alter the habitat during
their search for food. Tortoises have been known to knock
over small trees and shrubs to obtain nutritious leaves. This
makes pathways and clearings within the forestlands for other
animals. Seeds pass through the tortoise's digestive tract
and eventually become food for many other species.