Measuring Fluctuating Asymmetry in Plastron Scutes of Yellow-bellied Sliders: the Importance of Gender, Size and Body Location

Abstract

The use of fluctuating asymmetry (FA) estimates in animal research is increasing, but most studies thus far have focused on birds, amphibians and insects. Turtles have bilateral shell scutes that can be easily measured with modern imaging techniques and, therefore, should serve as ideal candidates for FA research, although identifying the most appropriate characters to measure must first be determined. With this issue in mind we undertook the current project to assess levels of plastron scute asymmetry in a common freshwater turtle species, specifically examining the level of variation among scutes and the effects of age (size) and gender on our estimates of scute FA. We photographed 86 museum specimens of adult yellow-bellied sliders, Trachemys scripta (32 males, 54 females) and used image analysis software to measure their carapace lengths and absolute differences in left-right surface areas of plastron scutes. We found that scutes varied in the degree of FA, but the magnitude of the variation tended to be higher among males and there was higher FA in general in the forward-most plastron scutes. We also found that scute asymmetry increased with carapace size, indicating that turtle shells become increasingly asymmetrical with age. This may be the result of adjacent scutes growing against each other over time, leading to random flux in symmetrical growth, or from age-related bioaccumulation of pollutants, which could interfere with normal symmetrical shell production.