Radiated Tortoise

Species Description

The Radiated Tortoise (Astrochelys radiata) is considered by many to be the most beautiful tortoise in the world. It is one of the two large endemic tortoises that inhabit the island of Madagascar, the other being the Ploughshare Tortoise (Astrochelys yniphora). The upper shell, or carapace of the Radiated Tortoise is highly domed with numerous yellow lines radiating from the centers of each scale. It has a large yellow head with Photo: Christina Castellano some black coloration on top and thick, yellow elephantine limbs (Pritchard, 1979).

The Radiated Tortoise can reach lengths of up to 40cm and weigh nearly 12kg. There are very pronounced differences between males and females. Female tortoises have a flattened bottom shell, or plastron and small tail, whereas males exhibit a highly concave plastron and a long, thick tail (Pritchard, 1979).

The common name of this species within its home range is Sokake. The species name radiata refers to the linear, yellow pattern across the scales (Pritchard, 1979).

Taxonomy

Shaw first described the Radiated Tortoise in 1802. The closest living relative to the Radiated Tortoise is the Ploughshare, or Angonoka. It is believed that these were once the same species that occupied dry habitats some 2.5 mya (Caccone et al., 1999). They later diverged when northern and southern refugia came into existence as a result of cooler and wetter climatic conditions. The Angonoka is now confined to a small area in northwest Madagascar and is considered to be the world’s rarest tortoise (Pedrono, 2008).

Distribution

The Radiated Tortoise only exists on the island of Madagascar and shares nearly its entire range with a much smaller species called the Spider Tortoise (Pyxis arachnoides). Both species occur within a narrow band of dry forest about 10-50 km wide along the southern and western coastlines. Its range-wide distribution extends from the area north of Tulear located in the west to the Cap Sainte Marie Special Reserve (CSMSR) positioned at the southern most point of the island, and east almost to the port city of Fort Dauphin (Leuteritz et al., 2005).

Radiated Tortoise populations have decreased drastically in the wild and as a result its distribution has contracted. Over the past two decades, tortoises have been illegally harvested for consumption and the international pet trade. Today, it is nearly extinct from east of the CSMSR to east of Ambondro. It continues to exist in high densities west of the CSMSR to Lac Tsimanampetsosa where the tribes still generally uphold the taboo that forbids people to consume, or harm tortoises. It has been estimated that between 12-54 million tortoises exist within this core region (Leuteritz et al., 2005).

Habitat

The Radiated Tortoise is restricted to the ecoregion known as the Spiny Forest, which extends southwards from the Mangoky River along the west coast to the slopes of the Anosyennes Mountains in the southeast. It is the most arid region in Madagascar with the average annual rainfall being usually less than 500 mm. The climate is dominated by wet and dry seasons with most precipitation occurring between November and April, though January and February are the wettest months of the year. The mean annual minimum and maximum temperature ranges are 15-21°C and 30-33°C, respectively (Donque, 1972).

Vegetation communities can be highly variable throughout the Spiny Forest primarily due to dissimilarities in soil types. Sandy soils support taller and more densely packed plants than calcareous soils that support dwarf species. Plant families characteristic of this region include the Didiereaceae and Euphorbiaceae that exhibit extreme adaptation to the aridity of the region and possess succulent and reduced leaves, thorns, and enlarged trunks and branches. The vegetation along the southwest coastline is usually around 3-6 m tall (Dupuy and Moat, 1996).

The Radiated Tortoise prefers pristine habitats with low thornbush and grass cover, rather than dense thickets of vegetation. It is also occupies areas that have become degraded by livestock grazing, deforestation, burning, and cultivation. It is believed that tortoises rely on a ground-level herb layer for grazing and canopy openings that provide basking sites (Leuteritz et al., 2005).

This species utilizes a mosaic of habitats within its range, however tortoise density can vary significantly among them. For example, it is found in high numbers (10.7 tortoises per ha) in native spiny forest and degraded habitats and low numbers (2.6 tortoises per ha) in gallery forest (Leuteritz et al., 2005).

Movement and Home Range

Published information on the movements and home range of this species is currently unavailable.

Diet

The Radiated Tortoise is primarily herbivorous and feeds on a wide variety of grasses, fruits, and succulent plants. At some locations, the majority of its diet consists of the invasive Opuntia cactus and it has been suggested that this allows them to occupy disturbed areas. In addition, this species feeds opportunistically on animal matter in the form of carrion, animal dung, bone, hair and shells (Leuteritz, 2003).

Although it is likely that they stay hydrated from ingesting plant material, Radiated Tortoises drink freestanding water from pools that form in rock crevices during periods of rain (Leuteritz, 2003).

Reproduction

The Radiated Tortoise may be the most long-lived of all turtles: a maximum age of more than 130 years has been well documented, whereas an account of an individual living 189 years is less reliable (Glaw and Vences, 1994). Both males and females generally exhibit characteristics associated with sexual development at a shell length at, or greater than 26 cm, or between 16 and 20 years of age (Leuteritz and Ravolanaivo, 2005).

Radiated Tortoises have been observed mating throughout the wet season (November- May), but the majority of mating occurs in December. Males often circle, chase and butt shells with the females before attempting to mate. A receptive female will extend her legs to lift her shell off the ground allowing the male to mount her shell and slide his tail beneath her. Loud, frequent thuds and hisses can be heard during copulation (Zovickian, 1973).

The nesting season is mainly between February and April; however, it can extend into the middle of the dry season (August). Nests sites are usually associated with low-growing vegetation. Females excavate a flask- shaped nest cavity about 15cm deep with their hind feet. They often lay one to three clutches per season with clutch size ranging from one to five eggs. The incubation period is lengthy and highly variable as it can range from about 200 to over 300 days in the wild (Leuteritz and Ravolanaivo, 2005).

Threats to Survival

The survival of the Radiated Tortoise is threatened by illegal collection, over-harvesting for local consumption and habitat loss. This species is illegally traded in international pet markets, which has increased substantially within the last two decades. Much of the Spiny Forest has been cleared for agriculture, livestock grazing, and charcoal production. Taboos that once prevented Malagasy tortoises from being disturbed, or collected by local tribes’ people have eroded. The Radiated Tortoise is listed as Critically Endangered by the IUCN (Pedrono, 2008).

Literature Cited

Caccone, A., Amato, G., Gratry, O.C., Behler, J.L., and Powell, J.R. 1999. A molecular phylogeny of four endangered Madagascar tortoises based on mtDNA sequences. Molecular Phylogenetics and Evolution 12:1-9.

Donque, G. 1972. The climatology of Madagascar. In Battistini, R and Richard-Vindard, G. (Eds.). Biogeography and Ecology of Madagascar. Junk (The Hague): pp. 87-144.

Dupuy, D., and Moat, J. 1996. A refined classification of the primary vegetation of Madagascar based on the underlying geology: Using GIS to map its distribution and to assess its conservation status. In Lourenco, W. R. ed. Proceedings of the international Symposium on Biogeography of Madagascar. Editions de l'Orstrom, Paris: pp. 205-218.

Glaw, F., and Vences, M. 1994. A Field Guide to the Amphibians and Reptiles of Madagascar. Moos Druck, Leverkusen and FARBO, Koln.

Leuteritz, T. 2003. Observations on diet and drinking behaviour of radiated tortoises (Geochelone radiata) in Southwest Madagascar. Afr. J. Herpetol. 52:127-130.

Leuteritz, T., and Ravolanaivo, R. 2005. Reproductive ecology and egg production of the radiated tortoise (Geochelone radiata) in southern Madagascar. Afr. Zool. 40: 233 -242.

Leuteritz, T., Lamb, T., and Limberaza, J.C. 2005. Distribution, status, and conservation of radiated tortoises in Madagascar. Biol. Cons. 124:451-461.

Pedrono, M. 2008. The Tortoises and Turtles of Madagascar. Natural History Publications, Borneo.

Pritchard, P.C.H. 1979. Encyclopedia of Turtles. T.F.H. Publications, Inc., Neptune, New Jersey.

Zovickian, W.H. 1973. Captive reproduction of the radiated tortoise. HISS News-Journal. 1:115-118.