Population Structure and Sexual Dimorphism in a Southern Population of Tent Tortoises (Psammobates tentorius tentorius) of the Karoo, South Africa

Abstract

This study examined population ecology parameters for a southern population of the tent tortoise (Psammobates tentorius tentorius) in the Karoo. We found a relatively low density of tortoises (49 tortoises/km 2 ) skewed to adults with a 1:1 sex ratio. The tortoises' contrasting dark and light carapacial patterns create crypsis with the disruptive shadows cast at the base of vegetation (mostly shrubs) where the tortoises seek refuge. The largest female was 14.7 cm, which makes her approximately 42 yrs old. The largest male was 11.6 cm, which makes him approximately 42 yrs old. We can distinguish males and females at carapace lengths of approximately 8-10 cm, when the tortoises are approximately 8-10 yrs old. Psammobates t. tentorius is a sexually size-dimorphic species in which adult females are significantly larger than are males. Selection for body shape and size appears attributable to fitness between the sexes (e.g., egg production and size in females, and opportunities to mate by males). We characterize this first group of nominate tent tortoises for future comparisons with other population. The study provides critical baseline information for the long-term management and conservation of this species, and adds to our understanding of the fauna of the Succulent Karoo.