The Radiated Tortoise
David T. Kirkpatrick, Ph.D
Turtles and tortoises come in an almost overwhelming variety of sizes, shapes, and colors. Among the most distinctive and beautiful are those tortoises which have a "star" or "radiating" pattern on their carapace, and many people believe that of these, the rare Radiated Tortoise, Geochelone radiata, is the most distinguished. Including the Radiated tortoise, seven species of tortoise have similar markings: the Geometric tortoise [Psammobates geometricus], the Serrated Star tortoise [P. oculifera], the Tent tortoise [P. tentorius], the Star tortoise [Geochelone elegans], the Burmese Star tortoise [G. platynota], the Malagasy Spider tortoise [Pyxis arachnoides], and the Radiated tortoise itself. [Incidentally, the Radiated tortoise is sometimes placed in the genus Asterochelys.] In all of these tortoises, the basic "star" pattern consists of a series of light lines or regions emerging from a central area of each black or dark brown scute on the carapace, although in some species this is reversed with the background lighter and the rays darker. Sometimes the same radiating pattern is also present on the plastron. However, the Radiated tortoise has the most striking pattern of any of the "star" tortoises, with a yellow or orange center to each black scute, from which extend four to twelve yellow or orange lines of varying thicknesses.
The Radiated tortoise has the basic "tortoise" body shape: a high-domed carapace, blunt head, and elephantine feet. Its legs and feet are yellow, as is its head, except for a variably sized black patch on top. Some animals can grow to almost 16 inches (40 centimeters) in straight length, making the Radiated tortoise the largest of the "starred" tortoises. This alone makes them easily distinguishable from other "starred" tortoises, but there are further differences. Unlike some tortoises, such as the Star tortoise, the center of each scute is not raised up in the Radiated tortoise. This gives the carapace of the Radiated tortoise a smooth appearance rather than a lumpy, pyramidal shape. Finally, their star pattern seems to be more finely detailed and intricate than the normal pattern of other species. There are also differences between male and female Radiated tortoises, but these are relatively minor. Males generally have a longer tail, while the notch in the plastron beneath the tail is less noticeable in females than in males.
Native only to the southern portion of the island of Madagascar, where they are known as "Sokakes", Radiated tortoises live in dry regions of brush, thorn bushes and woodlands. In the wild they are herbivorous, feeding on grasses and succulents, including the Opuntia cactus, which was introduced to Madagascar and spread quickly. In captivity they will accept a number of fruits and vegetables, including sweet potatoes, carrots, apples, bananas, alfalfa sprouts, and melons. Ernst and Barbour in their book "Turtles of the World" report that they are partial to red foods.
Radiated tortoises are among the world's rarer tortoises, although they are not as rare as the closely related Madagascar tortoise Geochelone yniphora, or "Angonoka", of which there may be only a few dozen surviving in the wild. This species, which lacks the distinctive "star" pattern, is confined to a much smaller area of Madagascar than the Radiated tortoise. Both species are being affected by loss of habitat and unfortunately the Radiated tortoise is also used as a food source occasionally. Both tortoises are listed in Appendix 1 of the Convention on International Trade in Endangered Species (CITES), prohibiting the import or export of either species under most conditions. Thus very few animals are ever exported, limiting the number of animals that are available to captive-breeding programs.
Breeding of the Radiated tortoise has occurred in captivity, and the courtship and mating process has been described in detail. Males will begin to attempt to mate when they are about a foot long, and females when they are a few inches longer. Males usually do not succeed in mating until they are over 13 inches in length, due to the disparity in sizes between the young male and mature females. When mating does take place, the male begins the procedure by following and circling the female, bobbing his head up and down and sniffing at the female's hind legs. He may attempt to lift her up with the front edge of his shell, presumably to keep her from moving away. If the female remains stationary the male will mount her from the rear, positioning his tail under and against her tail. He then strikes the lower anal region of his plastron against the rear of the female's carapace by pumping his rear legs in and out. In addition to the noise of their shells striking together, the male will also hiss or grunt at this time. All in all, it can be a fairly noisy procedure!
When the female is ready to lay her eggs, she begins digging a nest. Using her back legs, she excavates a flask-shaped hole that is about 6 to 8 inches deep. In this she will deposit from 3 to 12 nearly spherical brittle-shelled eggs. She then covers up the nest and moves away. 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.
A number of Radiated tortoises have been imported into the United States, some prior to the establishment of the CITES regulations and others since then in special shipments. Robert Baudy, for example, imported a relatively large number of animals in 1969 for captive breeding purposes. The first recorded Radiated tortoise breedings in the United States were by the Gladys Porter Zoo in Brownsville, Texas and by Dr. William Zovickian, in 1973. The New York Zoological Society's collection, then in Gainesville, Florida, also recorded births that year. Since then, the number of births and animals has steadily increased.
In the early 1980's the American Association of Zoological Parks and Aquariums (AAZPA) initiated a program designed to encourage the breeding of endangered species of animals in captivity. Known as the Species Survival Plan (SSP), this program monitors captive breeding of endangered species to ensure maximum growth and diversity of the captive stocks. A Species Survival Plan was begun for Radiated tortoises in 1985, with 13 institutions and 1 private individual contributing information on 179 tortoises. A studbook was established, with the aim of insuring the maximum amount of genetic diversity through selective breeding. This means that when a tortoise's offspring and therefore his genes become too well-represented in the captive population, it is no longer permitted to breed. Some of these animals are loaned or donated to zoos which do not have a breeding program but wish to have an animal for display purposes. Through careful record-keeping and management the genetic variability and hopefully the overall health of the captive population is maintained. Since its beginning the number of involved institutions and individuals has more than doubled, and the Radiated tortoise Species Survival Plan seems to be off to a very good start.
Breeding of the Radiated tortoise in captivity has been remarkably successful. Since those first few births in 1973, the number of captive-born animals has steadily increased. For example, in the 1990 edition of Frank Slavens' Inventory of Reptiles and Amphibians in Captivity, over fifty births were recorded. The Gladys Porter Zoo currently maintains a colony of 60 animals, while the New York Zoological Society's colony, now being kept on St. Catherine's Island in Georgia, contains 88 members. Currently, Slavens' Inventory lists 343 Radiated tortoises in captivity.
Luckily for the Radiated tortoise, the captive breeding program seems to be a success. Endangered in its native habitat, captive breeding has kept the tortoise from the brink of extinction. However, captive breeding programs cannot and should not be used to justify continued elimination of habitat for endangered species such as the Radiated tortoise. We are all indebted to the institutions and individuals who have worked so hard to make this program a success, for without their efforts one of the most beautiful tortoise species, Geochelone radiata, the Radiated tortoise, might not be around today for the world to admire.
References for further information:
Ernst, C.H. and R.W. Barbour. 1989. Turtles of the World. Smithsonian Institution Press, Washington, DC. pp 251-253.
Holmstrom, W. 1989. The Radiated Tortoise Species Survival Plan. Proc. 12th Annual Herpetological Symposium on Captive Propagation and Husbandry, pp 47-51.
Hudson, R. 1983. The Species Survival Plan and its application to reptiles. Proc. 7th Annual Herpetological Symposium on Captive Propagation and Husbandry, pp 1-9.
Obst, F.J. 1988. Turtles, Tortoises, and Terrapins. St. Martin's Press, New York. pp 67, 183-184.
Pritchard, P. 1979. Encyclopedia of Turtles. TFH Publications, Neptune, N.J. pp 275-286.
Slavens, F. and K. Slavens. 1990. Reptiles and Amphibians in Captivity: Breeding, Longevity and Inventory. Slaveware Publishing, Seattle, Washington.
This article copyright 1992 by David T. Kirkpatrick. Originally published in Reptile & Amphibian Magazine, March/April 1992, pages 18-24.
Reprinting of this article for non-profit purposes is permitted provided that it is unaltered and appropriate attribution, including copyright information, is included. Please notify the author of any reprinting.