Evolutionary morphology of genital spines informed by puncture mechanics.

Genital spines are widespread in animals with internal fertilization, and their morphology is diverse. Many hypotheses have been proposed to explain their presence, yet their function has not been tested in most systems. We do not yet have a framework to characterize their morphology and potential function, even to determine if they puncture the vaginal tract (as is often assumed). Here, we examine the morphospace and puncturing ability of genital spines by testing different 3D-printed spines from a diverse sample of μCT-scanned snake hemipenes, which are among the most morphologically variable intromittent genitalia in vertebrates. We performed serial compression tests into polydimethylsiloxane silicone polymer at increments of 10° until puncture was impossible. We found that the range of successful puncture angles differed based on spine curvature and tip sharpness, suggesting that some spines are built to puncture at almost every angle of approach, while others are extremely unlikely to puncture vaginal tissues at all. These results provide a framework for continued study and characterization of the shape and performance of genital spines in other animal groups and help to refine functional hypotheses in the context of copulatory interactions.