Hemipenial morphology does not provide insight on mating barriers between the two main lineages of Hierophis viridiflavus (Lacépède, 1789)

Abstract

Copulatory organs are a key trait in reproductive compatibility and sexual isolation. The role of male genitalia in boosting mating success is well known and is often the outcome of behavioural and biological constraints, although no clear and common interpretation about their evolution appears broadly applicable. In snakes, hemipenial morphology has often been described under the perspective of sexual selection, taking into consideration both behavioural and morphological traits of both sexes. We investigated hemipenial morphology and ornamentation in the two subspecies of Hierophis viridiflavus, a male-male combating colubrid, and compared it to the sister species H. gemonensis, to assess intraspecific variation in size of genitalia and ornamentation richness. The male intromitted organ of this species is unilobed and bulbous, with rich ornamentations consisting of basal spines and apical calyculations. We detected no statistically significant difference in hemipenial size, basal spine count, and spine length between the two subspecies, suggesting that no copulatory barrier is present between the two clades. Although hemipenial morphology and anatomy do not seem suitable to assess intraspecific variability as shown in this study, they are often highly variable at the family or genus level, suggesting that the evolution of male genitalia is linked to phylogenetic relationships and that hemipenial divergence might be correlated to lineage splitting despite not being necessarily the main cause. Future studies should be aimed at investigating hemipenial morphology and anatomy across species under the perspective of comparative phylogeny and reproductive behaviours to address thoroughly the constraints of hemipenial evolution and development.