Meeting Abstract
Consistent lateralized mating behaviors have only been reported in one species of mammals. Male harbor porpoises (Phocoena phocoena) sexually approach females exclusively on the female’s left side while hooking their lengthy penis around the female into her vaginal opening. We assessed the morphological symmetry and shape of post-mortem reproductive tracts of male and female harbor porpoises to understand the evolution of this unusual lateralized behavior. Two-dimensional geometric morphometrics of the vagina and three-dimensional models of the vaginal lumen and inflated penis tip were used to characterize and quantify genital shapes and assess the influences of asymmetry on overall genital shape. In 2D, there was substantial individual variation in vaginal shape that was not correlated with total body length. The vaginas exhibited significant fluctuation and directional asymmetry, suggesting the asymmetry is functional. The vaginal lumens were highly asymmetric, which was driven by complex 3D spirals and vaginal folds with deep recesses. These vaginal folds appear to physically obstruct the penis and curtail the depth of penetration. The asymmetric shapes of the penis tip of free-swimming harbor porpoises, excised penis tip, and vaginal lumen were remarkably similar. We suggest that the left-sided sexual approach of males enables the penis to deeply penetrate the vagina. We demonstrate that the reproductive anatomy of males and females and their lateral mating behavior have coevolved in harbor porpoises.
