The elongated carapace (to 40 cm) is high domed with abruptly descending sides, only a slight cervical indentation, and the posterior marginals upturned and serrated. A rather broad cervical scute is present. Vertebrals are broader than long, and the 5th is expanded. Well-defined growth annuli surround the raised vertebral and pleural areolae. There are usually 11 marginals on each side and a single, undivided supracaudal which is downturned between the posterior marginals. The carapace is dark brown or black; vertebrals and pleurals have yellow or orange areolae from which extend 4 to 12 yellow or orange radiating stripes. Each marginal has 1 to 5 yellow or orange radiations beginning from a light spot at the lower center of the scute and extending upward toward the pleurals. These light stripes may fade with age, but most individuals are strikingly marked. The plastron is well-developed. Its forelobe tapers to the front and is longer but narrower than the hindlobe, which bears a deep anal notch. The plastral formula is: abd > hum > fem > gul > an > pect; the paired gulars are thickened and may protrude beyond the carapacial rim, especially in males. The bridge is wide with a single small axillary and a large inguinal scute. The plastron is yellow with a large black triangle at the outer edge of the humerals, pectorals, abdominals, and femorals; the gulars are usually unpatterned and the anals have black radiations. Black radiations may also occur on the abdominals. The head is moderate in size with a nonprotruding snout and a slightly hooked upper jaw. Its large prefrontal is longitudinally divided and followed by a single large frontal scale; other head scales are small. Crumly (1982b) found that 59% of the adult G. radiata skulls he examined contained an irregular parietal foramen, previously known only from some Gopherus individuals and Geochelone carbonaria. The head is usually yellow with a darkened area on the dorsal surface, and the limbs and tail are yellow. The anterior surface of the foreleg is covered with overlapping scales, only a few of which are large. No enlarged tubercles occur on the thigh, and the tail lacks a large terminal scale.
Males have slightly longer tails, deeper anal notches, and more protruding gular scutes.
Geochelone radiata is endemic on Madagascar where it is restricted to the southwestern Antandroy Territory along the coast between Amboasary and Morambe (Glaw and Vences, 1994). It has been introduced on Mauritius and Reunion.
Geochelone radiata lives in dry, Didierea woodland, stands of euphorbs, and scrub and thorn brush.
Courtship and mating in Florida captives observed by Auffenberg (1978) occurred from spring to early summer. Tactile and olfactory cues seemed more important than visual or auditory cues. Chemical cues are largely from cloacal scents (pheromones?). The male sniffs the female's cloacal region while moving the head vertically. This is often accomplished while walking behind (trailing) the female with head extended. The male then pushes the female's carapace with his extended gulars, and tries to lift or overturn her by hooking his gulars under her shell and pushing upward. He then mounts from the rear and, placing his forefeet on her carapace as a pivot, swings the posterior part of his body downward while simultaneously lifting his hind feet off the ground. This results in his violently striking the lower rear of her carapace with the thickened anal region of his plastron. At this instant the male usually emits rhythmic grunts. The female's cloacal region is probed by the male's tail and intromission follows.
Nestings by semi-captives at the New York Zoological Society's Wildlife Survival Center, St. Catherines Island, Georgia take place in every month, but 90% of the nestings occur between 1 September and 30 April (Behler and Iaderosa, 1991). Only 8% of the eggs laid before 15 October are fertile, while 41% of the eggs laid after 15 October are fertile; overall fertility rate is 32%. Clutch size is 1-9 (mean 5.1), and a female may nest as many as seven times a year, but 5-6 nesting per female is normal. Interclutch interval is 21 days. Some females may stop producing eggs for two or more seasons before resuming oviposition.
Eggs are almost spherical (36-42 x 32-39 mm) with brittle shells; clutch size varies from 2 to 12. Incubation is long, 145-231 days. Hatchlings are brightly marked and have flattened, rounded carapaces (32-40 mm).
Geochelone radiata is basically a herbivorous grazer. In Madagascar, the tortoise eats grasses, introduced Opuntia cacti, fruits, and other succulents; red foods seem to be preferred. At St. Catherines Island, grasses, the vervain (Lippia), and legumes are the favored foods of the grazing adults, but Spanish moss (Tillandsia), poison ivy (Toxicodendron), oak (Quercus), trumpet creeper (Campsis), tree of heaven (Allanthus), and a wide variety of other native and introduced plants are eaten opportunistically (Behler and Iaderosa, 1991).
This species appears most closely related to the other Madagascar endemic, Geochelone yniphora. Auffenberg (1974) thought G. radiata and G. yniphora to be probably conspecific, but this seems hardly the case.
IUCN Red List Status (1996)
Vulnerable (A1acd+2cd, B1+2abc).