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Despite their evolutionary and biomedical importance, studies of the morphology and function of female genitalia have continued to lag behind those of male genitalia. While studying female genitalia can be difficult because of their soft, deformable and internal nature, recent advances in imaging, geometric analyses of shape and mechanical testing have been made, allowing for a much greater understanding of the incredible diversity of form and function of female genitalia. Here, we summarize some of these methods, as well as discuss some big questions in the field that are beginning to be examined now, and will continue to benefit from further work, especially a comparative approach. Topics of further research include examination of the morphology of female genitalia in situ, in-depth anatomical work in many more species, studies of the interplay between natural and sexual selection in influencing features of vaginal morphology, how these diverse functions influence the mechanical properties of tissues, and studies of clitoris morphology and function across amniotes. Many other research topics related to female genitalia remain largely unexplored, and we hope that the papers in this issue will continue to inspire further research on female genitalia.

 

The lower reproductive tract of female mammals has several competing functions including mating, tract health maintenance, and parturition. Diverse vaginal anatomy suggests interactions between natural and sexual selection, yet despite its importance, female copulatory morphology remains under-studied. We undertook a comparative study across the species-rich mammalian order Chiroptera (bats) with a focus on the suborder Yangochiroptera (Vespertilioniformes) to examine how female vaginal features may have coevolved with male penis morphology to minimize mechanical damage to their tissues during copulation. The penis morphology is diverse, presenting great potential for post-copulatory sexual selection and coevolution with the female morphology, but vaginas have not been carefully examined. Here we test the hypotheses that vaginal thickness and collagen density have coevolved with features of the male penis, including the presence of spines and a baculum. We present histological data from females of 24 species from 7 families of bats, and corresponding data on male penis anatomy. We also examine the role of phylogenetic history in the morphological patterns we observe. We found evidence that female vaginal thickness has coevolved with the presence of penile spines, but not with baculum presence or width. Collagen density did not appear to covary with male penile features. Our findings highlight the importance of considering interactions between the sexes in influencing functional reproductive structures and examine how these structures have been under selection in bats.

 

Quantification of genital variation in males and females can inform our understanding of likely copulatory interactions and evolution of genital diversity. 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. 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. Vaginal pouch shape is significantly associated with body size and reproductive status. As females grow larger and become reproductive, the vaginal pouch enlarges, widens and becomes more bifurcated. In reproductive males, the shape of the hemipenes is also significantly associated with body size. As males grow larger, the hemipenes enlarge and widen; their bifurcation becomes more defined and the spines at the base become more prominent. Vaginal pouch and hemipenial centroid size are isometric with respect to body length. The centroid sizes of the hemipenes and vaginal pouch are not significantly different from one another, hence the genitalia match in size. Reproductive females and males covary in the degree of bifurcation and size of their genitalia. 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.

 

Genital evolution can be driven by diverse selective pressures. Across taxa we see evidence of covariation between males and females, as well as divergent genital morphologies between closely related species. Quantitative analyses of morphological changes in coevolving male and female genitalia have not yet been shown in vertebrates. This study uses 2D and 3D geometric morphometrics to quantitatively compare the complex shapes of vaginal pouches and hemipenes across three species of watersnakes (the sister taxa Nerodia fasciata, N. sipedon, and a close relative N. rhombifer) to address the relationship between genital morphology and divergence time in a system where sexual conflict may have driven sexually antagonistic coevolution of genital traits. Our pairwise comparisons of shape differences across species show that the sister species have male and female genitalia that are significantly different from each other, but more similar to each other than to N. rhombifer. We also determine that the main axes of shape variation are the same for males and females, with changes that relate to deeper bilobation of the vaginal pouch and hemipenes. In males, the protrusion of the region of spines at the base of the hemipene trades off with the degree of bilobation, suggesting amelioration of sexual conflict, perhaps driven by changes in the relative size of the entrance of the vaginal pouch that could have made spines less effective.

 

Female genitalia are conspicuously overlooked in comparison to their male counterparts, limiting our understanding of sexual reproduction across vertebrate lineages. This study is the first complete description of the clitoris (hemiclitores) in female snakes. We describe morphological variation in size and shape ( n = 9 species, 4 families) that is potentially comparable to the male intromittent organs in squamate reptiles (hemipenes). Dissection, diffusible iodine contrast-enhanced micro-CT and histology revealed that, unlike lizard hemiclitores, the snake hemiclitores are non-eversible structures. The two individual hemiclitores are separated medially by connective tissue, forming a triangular structure that extends posteriorly. Histology of the hemiclitores in Australian death adders ( Acanthophis antarcticus ) showed erectile tissue and strands/bundles of nerves, but no spines (as is found in male hemipenes). These histological features suggest the snake hemiclitores have functional significance in mating and definitively show that the hemiclitores are not underdeveloped hemipenes or scent glands, which have been erroneously indicated in other studies. Our discovery supports that hemiclitores have been retained across squamates and provides preliminary evidence of differences in this structure among snake species, which can be used to further understand systematics, reproductive evolution and ecology across squamate reptiles.