Intraspecific and interspecific variation of female genitalia in two species of watersnake

The morphological differences in female genitalia within and between species are little studied and poorly understood, yet understanding patterns of variation in female genitalia can provide insights into mechanisms of genital evolution. The present study aimed to explore the patterns of intraspecific and interspecific variation in female genitalia in two sister taxa of watersnake (Nerodia sipedon and Nerodia fasciata) that have similar genital shape. We used a geometric morphometric (GM) approach to study variation in shape of the vagina between and within two sister species. We examined genital shape in female watersnakes ranging from small, sexually immature females to large reproductive females that had recently given birth. We found that shape variation of genitalia is strongly correlated with body size, where larger but not smaller females have a bifurcation in the vagina. However, we also found significant shape variation in the structure of the vagina between the two species, where N. fasciata has narrower genitalia with more prominent bifurcation, whereas N. sipedon has wider genitalia with less marked bifurcation. Using GM allowed us to detect significant differences in genital shape that were not apparent upon visual examination alone, suggesting that shape variation in female genitalia may be greater than previously assumed. Additional study of morphological differences in male reproductive organs for these species would help to determine whether there has been genital co‐evolution, and potentially mechanical reproductive isolation, in these two closely‐related and occasionally sympatric species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 183–191.