The Aldabra giant tortoise (Aldabrachelys gigantea)
is native to the Aldabra Atoll in the Seychelles archipelago group. The giant
tortoise shares a common ancestry with both the Madagascar radiated tortoise
and the extinct Pinta Island tortoise from the Galapagos Islands. In the past,
the giant tortoise was also found on Mahé, Silhouette, Glorieuse, Coëtivy, and
other islands of the Seychelles. Today it has been reintroduced to two islands
in the Seychelles. A subspecies is found on Madagascar, but this has now been
assigned to a separate genus−the Madagascar flat-shelled tortoise (Astrochelys
yniphora). The largest known specimen of this species weighed 249 kilograms
(550 lb) and measured 1.3 metres long (4 ft 3 in). Shells of this species are
smooth. High-domed individuals are somewhat variable in appearance, with
different populations having distinct characteristics. However, human poaching
and the destruction of their natural habitat have made these gentle giants
highly vulnerable. As one of the few surviving species of the Indian Ocean
giant tortoises, efforts have been made to protect the future of the Aldabra
giant tortoises.
Aldabra giant tortoise endemic and unique to the Aldabra
Atoll in the Seychelles
The Aldabra
Atoll in the Seychelles is located about 1,000 miles off the east coast of
Africa and consists of 20 coral islands. The island group has been designated a
UNESCO World Heritage Site because it is home to many rare species, including
giant tortoises and giant land snails. More so, the Aldabra giant tortoise
differs from other species of giant tortoise on earth. In fact, DNA testing has
revealed that three species of giant tortoise on earth: Isla Santa Cruz,
Aldabra Island, and Galapagos Island, are actually separate subspecies; they
are now all known as A. g. abingdoni (Galapagos), A. g. dofleini (Santa Cruz),
and A. g gigantea (Aldabra).
Aldabra giant tortoise’s shared ancestry
The Aldabra giant tortoise shares a common ancestry
with both the Madagascar radiated tortoise and the extinct Pinta Island
tortoise from the Galapagos Islands, the volcanic archipelago in the Pacific
Ocean. It is believed that these species are all descended from a single
ancestor that arrived on the islands hundreds of thousands of years ago.
However, these species, such as the Madagascar flat-shelled tortoise (Astrochelys yniphora), are now assigned
to a separate genus. Yet, the Aldabra giant tortoises are closely related to Geochelone gigantea and Geochelone platynota, which have been
considered part of their own family since then.
Reduction of habitat and ongoing conservation efforts
In the past, the giant tortoise was also found on
Mahé, Silhouette, Glorieuse, Coëtivy, and other islands of the Seychelles.
Also, this species was found in Madagascar; however, by 1960, only the Aldabra
remained. Today they are extinct in all other locations except the Seychelles.
The Aldabra giant tortoises have on several occasions been
bred successfully in captivity. However, their offspring have not been
successful in breeding back in the wild. This is likely due to hybridization
issues between these two very closely related species. The Galapagos tortoises
face similar problems when trying to breed outside of their native habitat;
although they seem to be somewhat more successful at it than their cousins!
The introduction to two islands of the Seychelles
In recent years, the Aldabra giant tortoise has been
reintroduced to Cousin Island and Astove Island in the Seychelles group. In
July 2015, seven male tortoises arrived on Cousin Island as part of a joint
effort between Nature Seychelles, the Durrell Wildlife Conservation Trust (UK),
and the Government of Seychelles. The first females arrived on Cousin in early
2018, with a total of 22 females being transferred by 2020.
In October 2016, eighty-five juvenile Aldabra giant
tortoises were collected from Ile Ste Marie and transported back to their
ancestral home on Astove Island, where they will live in an environment similar
to the one their ancestors inhabited centuries ago on Cosmoledo Atoll. This was
part of the effort made by Durrell Wildlife Conservation Trust (UK) and
Mauritius Wildlife Foundation (Mauritius).
The uniqueness of these giant tortoises
The Aldabra giant tortoise is the largest living
species of tortoise. It has an average shell length of 1.5 meters (4.9 feet),
with some specimens reaching more than 2 meters (6.6 feet). The shells are
smooth and high-domed, with a strong keel on the lower side that runs from the
hinge to about halfway along its length. Here it divides into two ridges that
continue for another third of its length before ending in a small point at each
end of the shell. The carapace is usually almost flat on top, but it may have a
slight slope in mature individuals. The carapace is made up mainly of calcium
carbonate crystals, which form under pressure as part of its normal growth
process. Some people who live near Aldabra report hearing these large animals “cracking
their shells” by rubbing them together! This phenomenon can also be heard when
handling captive AGBTs, who often enjoy rubbing against one another during
feeding time or while exploring new areas within their enclosure(s).
The largest known specimen of this species weighed 249
kilograms (550 lb) and measured 1.3 metres long (4 ft 3 in). The tortoise was
found on the Seychelles island of Mahé, but it is believed to have come from
Aldabra. It was found by a group of hunters who hunted it for food.
The Aldabra giant tortoise has immense sexual
dimorphism (distinct differences between males and females). The male can grow
up to 1 m long; females reach lengths up to 1.2 m only after many years. Males
weigh about 200 kg, but females average around 150 kg when fully grown - much
less than males due to their shorter length but greater width.
Habits
and diet leading to regional differences
The shape of the shell is one of the most noticeable
features that can be used to identify tortoises and turtles in the wild. The
shell shape of an Aldabra giant tortoise is influenced by its habitat and diet.
A tortoiseshell will change as it grows older, becoming more domed as it ages.
The genetics of a tortoise can also influence its shell shape; however,
scientists have not found evidence that this occurs in Aldabra giant tortoises.
Nevertheless, differences exist in the appearance of
individual Aldabra giant tortoises. They are somewhat variable, with different
populations having distinct characteristics. The patterning on the carapace is
often brighter and more defined than that of other species of giant tortoise;
but some individuals have a carapace with minimal markings.
Variations between individuals seem to be caused by a
combination of genetic differences and environmental factors. For example,
heat-producing algae are more abundant in some areas than others; this may
affect how quickly an individual grows or develops patterning on its shell.
For example, those on Curieuse Island have
particularly dark shells and red eyes; those on Astove Atoll are slightly
smaller, and those on Cosmoledo Atoll are among the largest individuals known.
Nonetheless, the Aldabra giant tortoise is a unique species found only in the
Indian Ocean. It is one of the most magnificent land animals roaming through
its home on Aldabra, an island in the Seychelles archipelago.
The bottom line
The Aldabra giant tortoise is one of the most
endangered reptiles in the world. The species has been listed since 1982 by the
International Union for Conservation of Nature as critically endangered; having
been listed before 1982 on the IUCN’s Red List as critically endangered. It is
also worth visiting Aldabra Island to view and interact with these gentle
giants. We recommend adding this destination to your bucket list as part of
your week-long trip with Beyond the Reef, or as a day trip from Mahé. Contact
us for more details.
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