27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Distribution, Biology, and Status of the Radiated Tortoise, Geochelone radiata, in Southwestern Madagascar

Thomas E. Leuteritz
George Mason University, Biology, Environmental Science, and Public Policy, Fairfax, VA 22030

Madagascar is one of 25 global "biological hotspots" and a "megadiversity" country. The radiated tortoise or sokatra (Geochelone radiata) is one of four tortoises endemic to Madagascar. The IUCN Red List classifies it as "Vulnerable". The primary threats to the tortoise's survival are collection for human consumption and the pet trade, and habitat loss. There are several published studies on the biology of captive radiated tortoises but only a few reports on field research of radiated tortoises. The project was designed to examine these parameters in the field over a two-year period; specifically during the tortoises' active season (rainy season - November to April). To determine the status and distribution of radiated tortoises, sampling was conducted across the species' entire range. Seven transect sites (Cap Sainte Marie, Lavanono, Ankirikirika, Nisoa-Ambony, Lavavolo, Vohombe, and Lake Tsimanampetsotsa) were sampled during the 1998/99 and 2000 field seasons. Cap Sainte Marie (CSM), a special nature reserve in the extreme southern portion of the Province of Tulear, Madagascar, was chosen as the principal study site and the location of the reproductive study.

Tortoises range from south of Tulear to CSM. East of CSM tortoise populations become severely fragmented. Line transect sampling was conducted across the species range to estimate tortoise density. Density estimates ranged from 27.5 to 5,744 tortoises/km². The estimated mean population size of radiated tortoises in the core of the range is between 12 - 54 million. At my study site at CSM there were between 1,905 - 2,105 tortoises (actual marked individuals 1,438) based on Lincoln-Peterson mark-recapture. Based on mtDNA extracted from blood samples gathered across the range, tortoise populations do not appear to exhibit phylogeographic structure.

This study also examined reproductive behavior and egg production in a natural radiated tortoise population by observing and recording tortoise interactions, and radiographing marked females at regular intervals over two nesting seasons. Individual females at Cap Sainte Marie were polyoestrous, producing from 1 - 3 clutches per season (mean = 1.7+/-1.1, n = 12). The eggs were generally spherical with a brittle calcareous shell and had a mean mass of 39.0+/-5.8 gm (range 28.0 - 55.0 gm, n = 56), a mean length of 4.2+/-0.2 cm (range 3.5 - 4.7 cm, n = 58), and a mean width of 3.8+/-0.2 cm (range 3.4-4.3 cm, n=58). The incubation period was approximately 303 - 309 days. Hatchling success was high and consistent between years; 65.6% of eggs (9 nests, 32 eggs) hatched in 1999, and 66.7% of eggs (3 nests, 9 eggs) hatched in 2000.

A series of general and specific Cap Sainte Marie recommendations are given for radiated tortoise conservation in Madagascar based on this study. One of the most encouraging things about radiated tortoise conservation is that their numbers are quite high, which means now is the time to act before populations start to dwindle and these unique tortoises enter a state of crisis.