Love Bites: Morphological Characterization of Genital Spines in Snakes

Genital spines are present in many groups of animals, including insects, squamates, fish and mammals, and their diverse functions have been established in some species. However, we have yet to characterize the morphology of genital spines in depth. Genital spines are known to influence mating success through several natural and sexual selection mechanisms and examining their morphology may help us to understand their function. Snakes have extraordinary genital spine diversity that has been used in taxonomy for decades. In this study, we characterize the morphological attributes of hemipenial spines in 14 snake species, based on μCT scan segmentations of 39 hemipenes with over 10,000 spines and describe continuous variation in spine morphology, including number, volume, sphericity (shape), and distribution that does not serve to categorize spines into types as has been done in some taxonomic studies, with the exception of the basal hook. We identify areas of the volume/sphericity morphospace that may impose a constraint in spine functional morphology, and find significant variation in ontogenetic changes of the spines that include spine size and number, mineralization rate during ontogeny, and relationship with hemipene shape development. Our work is clearly only the first step to understanding the complex spine morphology in this group, and further work must consider puncture interactions with vaginal tissue as well as hemipene eversion mechanics. This exploratory work gives us a baseline with which to compare the morphology of genital spines in other groups of animals and make better inferences about copulatory function.