Directional asymmetry in yellow-bellied sliders (Trachemys scripta scripta) (Schoepff 1792)

Abstract

Bilateral symmetry is not uncommon in animal kingdom, and animals can deviate from expected symmetry and manifest some forms of asymmetries. Fluctuating asymmetry is considered a tool for valuating developmental instability, whereas directional asymmetry is inherited. Here we use the method of geometric morphometrics to analyse left/right asymmetries in the plastron of 96 (33 males and 63 females) yellow-bellied sliders (Trachemys scripta scripta) with the primary aim to infer and explain morphological asymmetries between sexes in this species. In all individuals analysed we found both fluctuating asymmetry and directional asymmetry for size and shape variation components, and sexual dimorphism for the latter. Fluctuating asymmetry did account a low contribution of the total asymmetric variation, while directional asymmetry assumed the prominent contribution. This may be an adaptive response, which is present in both sexes. We suggest that natural selection for lateralised locomotor efficiency, and not sexual selection (i.e. genetic quality), must be the principal factor for directional asymmetry in this species. The results are consistent with reports from other turtle species using the same technique.