![]() ![]() Variation of the vaginal pouch has been recognized in taxonomic studies, and it has been shown that female genital morphology can vary among families ( Gabe & Saint Girons, 1965 Siegel et al., 2011).īeing able to characterize the factors that are associated with variation in genital morphology is an important step in understanding the evolution of these structures. The portion of the female genitalia that receives the male intromittent organ in snakes has been given several names (cloaca, vagina, oviduct and vaginal pouch Siegel et al., 2012) here, we refer to this portion of the female genitalia as the vaginal pouch. Each hemipenis has a corresponding retractor penis muscle that is attached within the tail below the cloaca ( Cope, 1895). The hemipenes develop and rest within the tail, caudal to the cloaca, and remain inverted until copulation ( Porto et al., 2013). Unlike other amniotes, which possess a single intromittent organ, snakes have two intromittent organs or hemipenes, and they are known to have diverse morphological features ( Dowling & Savage, 1960 Klaczko & Stuart, 2015). Snakes are ideal subjects for studying the morphological evolution of the genitalia. For example, recent work in beetles has demonstrated that female genitalia drive the diversification of male genitalia, highlighting the importance of examining both males and females ( Simmons & Fitzpatrick, 2019). Showalter et al., 2014 Orbach et al., 2020). Quantitative studies of variation of male and female genitalia can help to identify intraspecific patterns that can provide insights into the evolutionary mechanisms underlying morphological changes (e.g. Méndez & Córdoba-Aguilar, 2004 Ah-King et al., 2014), and their co-evolution with male genitalia has not been investigated in many species ( Brennan & Prum, 2015). Female genitalia, in contrast, have been understudied (e.g. Male genitalia are very diverse structures, particularly in squamates, where hemipenial morphology has been used to distinguish among closely related species for decades (e.g. Genital co-evolution, genital morphology, geometric morphometrics, hemipenes, snake reproduction, squamate, vaginal morphology, vaginal pouch INTRODUCTIONĮvaluation of genital co-evolution can elucidate the relative influences of natural selection, sexual selection and sexual conflict in generating the wide array of genital morphological diversity and mating behaviours ( Brennan & Prum, 2015 Orbach et al., 2021). We demonstrate the utility of three-dimensional analysis in studies of the shape of soft tissues and advocate its use in future studies of genitalia. Reproductive females and males covary in the degree of bifurcation and size of their genitalia. The centroid sizes of the hemipenes and vaginal pouch are not significantly different from one another, hence the genitalia match in size. Vaginal pouch and hemipenial centroid size are isometric with respect to body length. As males grow larger, the hemipenes enlarge and widen their bifurcation becomes more defined and the spines at the base become more prominent. In reproductive males, the shape of the hemipenes is also significantly associated with body size. As females grow larger and become reproductive, the vaginal pouch enlarges, widens and becomes more bifurcated. Vaginal pouch shape is significantly associated with body size and reproductive status. ![]() Here, we examine the shape and size of the genitalia of female and male diamondback water snakes, Nerodia rhombifer, using a three-dimensional automated landmark geometric morphometric approach on models of the lumen of the vaginal pouch and inflated hemipenes, applying these techniques for the first time to the genital shape of vertebrates. However, no studies have quantified genital shape variation within a single snake species or examined the shape and size of both the vaginal pouch and hemipenes. Quantification of genital variation in males and females can inform our understanding of likely copulatory interactions and evolution of genital diversity. ![]()
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