Asymmetries in genital morphology are known in several animal taxa 3, 4, 5, and although lateralization in courtship behavior is found across animal taxa 6, it was unknown in mammals until recently 7. Left- or right-bias in morphology and behavior in otherwise bilaterally symmetrical animals manifests in diverse biological phenomena such as mating, foraging, predation, predator defense, and communication 1, 2. We suggest that the reproductive anatomy of both sexes and their lateral mating behavior coevolved. The asymmetric shapes of the vaginal lumen and penis tip were both left-canted with similar angular bends that mirrored one another and correspond with the left lateral mating approach. This asymmetry resulted from complex 3D spirals and vaginal folds with deep recesses, which may curtail the depth or direction of penile penetration and/or semen movement. We found that the vaginas varied individually in shape and that the vaginas demonstrated both significant directional and fluctuating asymmetry. To understand if this unusual lateralized behavior may have coevolved with genital morphology, we quantified the shape of female and male harbor porpoise reproductive tracts using 2D geometric morphometrics and 3D models of the vaginal lumen and inflated distal penis. Males attempt copulation by approaching a female exclusively on her left side. While lateralized mating behaviors are taxonomically widespread, among mammals they are only known in the harbor porpoise ( Phocoena phocoena). Asymmetric genitalia and lateralized mating behaviors occur in several taxa, yet whether asymmetric morphology in one sex correlates or coevolves with lateralized mating behavior in the other sex remains largely unexplored.
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