Variation of Genital Morphology in the Red-sided Garter Snake (Thamnophis sirtalis parietalis)

Abstract

Despite being one of the most studied snake species, little is known about genital morphology in red-sided garter snakes. This knowledge gap is influenced by sociocultural factors and limited access to techniques for analyzing soft tissues. In this study, we used 3D geometric morphometrics to quantify shape variation in female and male genitalia. We examined how genital shape varies with body size, female living status, and male mating strategy. We also analyzed hemipene symmetry and genital allometry. Our findings show shape variation in both sexes, but it is not explained by body size. Vaginal pouches from live females were narrower and smaller than those from deceased ones, suggesting that there is active muscular contraction during copulation and that interpretation of postmortem data should consider this difference. Male mating strategy did not influence hemipene shape, supporting the idea that female mimicry is a flexible behavior, not a distinct morph. We found no directional asymmetry in hemipenes, indicating no consistent preference for one side. Finally, vaginal pouches have isometric allometry while hemipenes exhibit negative allometry, suggesting copulatory fit despite size differences between the sexes.