Snake hemipenes are very variable in size, shape, and ornamentation; they are generally considered to be species-specific and to play a major role in male reproductive success. Such variability is extreme among families but observable also at low taxonomic levels. In addition, hemipenal diversity and the degree of ornamentation are influenced by mating strategies: in particular, they are expected to be more pronounced in species that form “mating balls” than in species in which males compete before copulation.

Hemipenal diversity  is thought to evolve rapidly and to create effective mating barriers between closely related clades. It may, therefore, represent a useful tool to study subspecific delimitation, in particular when other morphological and molecular traits are inconclusive. To this end, a paper that has recently been accepted for publication on Acta Herpetologica has studied hemipenal morphology in the two discussed subspecies Hierophis viridiflavus viridiflavus and H. v. carbonarius. For each subspecies, 10 left hemipenes were collected and photographed (18 from museum specimens, 2 from road-killed animals). Morphological observations and statistical analyses did not detect any significant difference in hemipenal morphology between the two subspecies of H. viridiflavus, despite evidence of significant individual variability in spine length. These findings highlight the lack of copulatory barriers between H. v. viridiflavus and H. v. carbonarius. The separation between the two lineages, thus, seems to be maintained by behavioral strategies rather than by the morphology of reproductive organs.. 

The interpretation of the evolutionary role of hemipenal morphology in the two subspecies is complex. Male green whip snake engage in strenuous fights before mating, and their copulatory organs are highly ornamented: even though male mating success primarily depend on intersexual competition, spines and ornamentations may play a secondary but relevant role by prolonging the duration of copulation. The findings of the new study, in concordance with previous investigation of the two clades, suggests that gene flow between subspecies is theoretically possible. Evidence for interbreeding, however, is missing and needs to be investigated, along with the potential role of female genitalia morphology in the maintenance of reproductive isolation.

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