Recent demographic survey of the Radiated Tortoise (Astrochelys radiata) in southwest Madagascar

Abstract

Abstract.—Effective management strategies for at-risk chelonian populations require sound knowledge of population
demographics. Previous research on the Critically Endangered Radiated Tortoise, Astrochelys radiata (Shaw 1802),
revealed a trend of increasing male bias in the sex ratio and a concomitant decrease in the proportion of juveniles in
Lavavolo, southern Madagascar. This location is one of the few remaining sites with a high concentration of this species
and was last studied ten years ago. The aim of our study was to provide an updated snapshot of Radiated Tortoise
demographics at Lavavolo, which, when combined with previous studies, will provide insight into changes over 20
years (1999–2019). Additionally, we provide the first assessment on the impact of soil types (calcareous, ferruginous,
and sandy) on the distribution of individuals across the site. We used mark-recapture data to determine the sex ratio,
age structure, and distribution of size classes. Overall, the study site contained mostly adults (76.87%) with only a
few juveniles (23.13%). Though the proportion of juveniles is low, it has increased over the last ten years. However,
the sex ratio has become increasingly skewed towards males, 1.8:1. We did not find any significant difference in sex
ratios between soil types, but there were significant differences in the distribution of size classes, with the smallest size
class 0–8 cm most prevalent in sandy soil and the juvenile size class 17–24 cm most frequent in ferruginous soil. Most
juvenile size classes were less common than expected on the calcareous soil. In general, populations located on calcareous
soil appear to be more vulnerable, potentially due to the long-term existence of poaching activities in this habitat.