Despite the popularity of the lock-and-key hypothesis in explaining genital evolution, research has disproportionately focused on male reproductive morphology. While male genitalia are typically described in three dimensions, female genital anatomy is often reconstructed from dissected specimens, resulting in flat, two-dimensional representations that fail to capture the intricate dynamics occurring during copulation

To address this gap, a recent study provided a fine-scale, three-dimensional analysis of the female genital tract in 16 adult snakes across seven Old World species from the families Boidae, Colubridae, and Viperidae. Specimens were obtained from museum collections and roadkill. The researchers introduced a novel, straightforward method: injecting the cloaca with two-component dental silicone, which, once solidified and removed, yielded detailed casts of internal reproductive structures.

This innovative use of silicone allowed for the creation of precise 3D models of the female genital tract, offering an unprecedented view of its morphology. The method proved particularly effective for capturing the form of hollow structures—such as the reproductive system—making it highly suitable for broader application in snake anatomical studies, especially given the current lack of data on female genitalia in reptiles.

The study revealed notable species-specific differences in female genital morphology, alongside minimal intraspecific variation, supporting the idea that these structures are finely tuned at the species level. Importantly, anatomical alignment between male and female genitalia was near-perfect in species with divided hemipenes and moderately bifurcated vaginal pouches. Conversely, species with simpler hemipenes and more pronounced bifurcation exhibited poorer alignment, raising questions about how effective semen delivery is achieved in these cases.

The findings suggest that snakes may fall into distinct groups based on their copulatory mechanics, challenging the one-size-fits-all assumption of the lock-and-key model. The authors emphasize the need for further research to uncover the diversity of reproductive strategies and genital interactions across snake species.

Find out more by reading the full paper.

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