Aldabra Tortoise
Taxonomy
Order: Chelonia
Family: Testudinidae
Genus/species: Geochelone gigantea
Description
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.
Zoo Diet
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.
Reproduction
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.
Life Span
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.
Status
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.
Fun Facts
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.
