Astrochelys radiata
Radiated Tortoise
Class: Reptilia. Order: Testudines. Family: Testudinidae.
Other names: Sokakes (Malagasy).
Physical Description: The radiated tortoise is considered to be
one of the most beautiful tortoises due to the coloration of its
carapace, which is brilliantly marked with yellow lines radiating
from the center of each dark plate (scute) of the shell, which reflects the name of the tortoise. They can
grow to a length of 16 inches and weigh up to 35 lbs. They have a highly-domed carapace, blunt head, and
elephantine feet. Radiated tortoises have yellow limbs and a black patch on the top of their head.
One of the most unique features of turtles and tortoises is their protective shell. Because of the shell,
flight, running, and climbing are not evolutionary options! But some adaptive radiation occurs within the
order. Turtles and tortoises inhabit marshes, lakes, rivers and the open sea, as well as forests, grasslands
and deserts. The top shell is called the carapace and the bottom shell the plastron. The carapace develops
from a layer of skin and the backbone and ribs are fused to the carapace. The plastron is developed from
the bones of the shoulder girdle. The shell is so successful that it is the cornerstone of turtle design and
lends to the lineage’s longevity while at the same time limiting species diversity.
Diet in the Wild: Grasses, vegetation and succulents, especially certain species of cactus.
Diet at the Zoo: Tortoise biscuits, mustard greens, collard greens, escarole, romaine, sweet potato, carrot and
occasional fruit.
Habitat & Range: Radiated tortoises are found only in the extreme south and southwestern part of the
island of Madagascar, where they inhabit areas of dry brush, thorn forests, and dry woodlands. They have
also been introduced to the island of Reunion.
Life Span: Up to 40-50 years.
Perils in the wild: Human activity has pushed the radiated tortoise to the point of being “Critically
Endangered.” Threats include significant habitat loss, collection for the pet trade, and poaching for food
and their shells for medicinal markets in China.
Physical Adaptations:
Sharp beak-like mouth aids in grasping and tearing vegetation and other food sources.
The Radiated tortoise has the basic "tortoise" body shape: a high-domed carapace, blunt head, and
elephantine feet.
Behavioral Adaptations:
Diurnal
Radiated tortoises are solitary except during breeding.
Radiated tortoises will graze the same area and keep the vegetation trimmed low, as they prefer
fresh growth to older vegetation.
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Reproduction and Development:
Once about 12 inches long, the males are old enough to mate. They begin by following a female,
smelling, and nudging her. It can be a noisy affair, as the males grunt loudly as they pursue and
mate with the females. Males may attempt to lift females up with the front edge of their shell,
presumably to keep females from moving away.
When the female is ready to lay her eggs, she begins digging a nest with her back legs, excavating a
flask-shaped hole that is about 6 to 8 inches deep. She then will deposit 3-12 nearly spherical,
brittle-shelled eggs, cover the nest, and leave. In captivity the length of incubation of Radiated
tortoise eggs has proven to be somewhat variable, but overall it is rather long in duration - from 145
to 231 days.
When the juvenile Radiated tortoises finally hatch, they range in size from 32 to 40 mm in length.
Their carapaces are brightly colored, with a white or an off-white shade to the light regions, in
contrast to the more yellow shade of the adults. The fine webbed pattern is present in the juvenile
tortoise upon hatching. The carapace itself is somewhat flattened when the tortoise first hatches,
but it soon achieves the high-domed appearance of the adult tortoise.
Additional Information:
After a heavy rain, the tortoises become much more active. In fact, one traditional Mahafaly story
states: Tortoises bring the rains—God sends rains, not for the humans, who can look after
themselves, but for the animals, like the tortoises, which need the rain and God’s help. When there
are no more animals, there will be no more rain.
The tortoise’s shell is supplied with blood vessels and nerves and can feel when being touched.
The radiated tortoise is an endemic species. An endemic species is a species that is found in only a
small geographical area. Many island species are considered to be endemic, and due to their small,
limited range, can be a conservation challenge.
Conservation Status: (IUCN Status)
Status is assessed as “Critically Endangered” by the IUCN. A taxon is Critically Endangered when the best
available evidence indicates that it meets any of the criteria for Critically Endangered by the IUCN. It is
therefore considered to be facing an extremely high risk of extinction in the wild.
Conservation Efforts:
No estimates of wild populations are available, but their numbers are declining, and many authorities see
the potential for a rapid decline to extinction in the wild. In the North American studbook, 332 specimens
are listed as participating in captive breeding programs such as the SSP (Species Survival Plan). Captive
breeding has shown great promise. However, captive management is not the answer to the problems in
their wild range and the many other unusual species endemic to Madagascar.
Glossary: List of definitions of the most important recurrent technical terms used in the text.
Carapace - The top shell of a turtle. The carapace develops from a layer of skin and the backbone and ribs
are fused to the carapace.
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Critically Endangered- A taxon is Critically Endangered when the best available evidence indicates that it
meets any of the criteria for Critically Endangered, and it is therefore considered to be facing an extremely
high risk of extinction in the wild.
Endemic- A species or taxonomic group that is only found only in one particular country or geographic area.
Plastron - The lower shell of a turtle, developed from the bones of the shoulder girdle.
Scute - Any enlarged scale on a reptile; sometimes called a “plate”.
SSP- Species Survival Plans are captive management plans implemented by the AZA in order to maintain
genetic diversity by managing captive populations in order to ensure genetic diversity through breeding
programs.
Sources:
Cincinnati Zoo & Botanical Gardens
http://turtlesale.com/gallery/displayimage.php?album=5&pos=68
http://www.unc.edu/~dtkirkpa/stuff/radiated.html
Hickman, C.P. Roberts, L.S. 1994. Biology of Animals
Halliday, T. Adler, K. 1986. The Encyclopedia of Reptiles and Amphibians.
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