Evaluating bilateral asymmetry of the adult western pond turtle (Emys marmorata)

Deviations from symmetrical growth in bilateral characters of an organism are referred to as bilateral asymmetries and can be found in three forms: 1) anti-symmetry (AS), 2) fluctuating asymmetry (FA), and 3) directional asymmetry (DA). In this study turtle shell ventral morphological characters (plastron scutes) were measured to determine if bilateral differences were present. We compared turtles from populations with minor human modifications, Big Chico Creek Ecological Reserve (BCCER) and Butte Creek Ecological Preserve (BCEP) to a site with major human modification, Chico Wastewater Pollution Control Plant (CWPCP). Over the course of three collection seasons (2010-2012) a total of 231 adult E. marmorata were measured and photographed. With modern imaging software asymmetry values were determined from scute pair differences (right-left). Shell and body measurements (SBMs) were also recorded for each turtle. Significant differences in SBMs were found between sites and sexes. Individuals from CWPCP were generally larger in all SBMs than BCCER and BCEP individuals. Male pond turtles were generally larger in all SBMs than females. Significant differences in FA values were found in femoral scutes between sites. FA values were generally larger in individuals from CWPCP. Significant differences in DA values of gular, humeral, pectoral, and femoral scutes were found among sites and between sexes. Males had significantly larger DA values in the pectoral and abdominal scutes and these differences were more extreme in the two populations with male-biased sex ratios. This suggests that DA could be related to male-to-male competition for mates.