Sexual dimorphism and geographic variation in the morphology of a small southern African tortoise Psammobates oculifer

Abstract

We studied morphological traits of Psammobates oculifer over its range to evaluate patterns in sexual size dimorphism (SSD) and geographic variation. Females were larger than males for 40 of the 44 characters measured and the growth trajectories of female parameters, scaled to body size (ANCOVA and MANCOVA on carapace length), most often followed juvenile patterns. For males, either the slopes or elevations of morphometric parameters were lower than in females. These divergent growth trajectories resulted in shape differences with female shells being higher and wider than the shells of males. Males matured at a smaller age and size, and had wider hind feet and larger shell openings relative to females; the latter being due to a shorter and narrower plastron, a shorter bridge length, and a bigger anal gap and cranial space. These male characteristics possibly enhance mobility for mate searching and combat, as well as courtship and mating behaviour. Small sample sizes for females in two of the three regions restricted geographic evaluations to males, for which shell shape, front foot width and hind leg length of the eastern group differed from the others. We interpreted these variations as the consequence of differences in the substratum and vegetation openness over the range of the species.