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Abstract 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.
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This content will become publicly available on April 1, 2026
Morphological Variation in the Genitalia of the Burmese Python
ABSTRACT Despite the remarkable morphological diversity found in vertebrate genitalia, it has historically been difficult to quantify shape variation of soft tissue structures due to limitations of 3D landmarking methods. New techniques such as automatic landmarking now allow us to examine such structures in detail, and with these methods we quantify the intraspecific variation in the genitalia of Burmese pythons (Python bivittatus). Despite previous assertions that a vaginal pouch is not present in pythons, we find thatP. bivittatushave well developed vaginal pouches, that are morphologically diverse, and change shape over ontogeny. Vaginal pouches and hemipenes are isometric. Hemipenes also vary in shape ontogenetically, but we find no evidence of directional asymmetry in shape or size between adult right and left hemipenes suggesting a lack of laterality. We identify a potentially intersex neonate with hemipenes, testes, and a vaginal pouch. We discuss our results in the context of snake genital evolution and suggest other mechanisms for selection beyond the standard “lock and key” hypothesis. Future work examining genital shape variation of other snake families will provide more insight into the coevolutionary patterns shaping the genitalia diversity across snakes and vertebrates more broadly.
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- Award ID(s):
- 2042260
- PAR ID:
- 10612045
- Publisher / Repository:
- BioOne
- Date Published:
- Journal Name:
- Journal of Morphology
- Volume:
- 286
- Issue:
- 4
- ISSN:
- 0362-2525
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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