![]() ![]() During copulation, the blunt knob may be obstructed from penetrating the cranial vagina by the largest vaginal fold, as suggested by ex vivo testing 21. Male harbor porpoises have a large blunt knob at the terminal end of the relatively long penis shaft (among the proportionally longest in cetaceans), which connects to a thin distal tip that bends away from the midline 19. Female harbor porpoises ( Phocoena phocoena) have one of the most complex vaginal morphologies and the largest number of vaginal folds described in any cetacean species 16. The penis tip of cetaceans is pliable and may be under voluntary control 19, 20. Both male and female genital morphology in cetaceans is particularly complex. Additionally, among mammals, asymmetric penises are most common in cetartiodactyls 4, which have a fibroelastic penis that maintains a turgid state and is resistant to bending 18. However, mating behaviors are not lateralized in either waterfowl sex.įemale cetartiodactyls (cetaceans and even-toed ungulates) are unusual in possessing vaginal folds, protrusions of the vaginal wall into the vaginal lumen, which may function in sexual selection, among other hypotheses 16, 17. Male and female waterfowl have asymmetric genitalia that spiral in opposite directions and females have evolved behavioral strategies to influence control over insemination 13, 14, 15. Male earwigs with paired penises preferentially use their right penis during copulation, which may be driven by female genital morphology 12. Similarly, male tree swallows tend to copulate from the left, perhaps because the female’s one active oviduct is on the left side 11. For example, lateralized mating behavior occurs in male poeciliid fish, which angle their gonodopodium (intromittent organ) to either the left or right side and are restricted to mating with females that have a genital opening on the same side 8, 9, 10. ![]() Instances in which one or both sexes have asymmetries in both mating behavior and genitalia may be more common than is currently recognized. Questions remain about how asymmetry in one sex influences the behavior or morphology of the other sex and whether asymmetries arise from adaptive (directional evolution) or non-adaptive (genetic drift) mechanisms. While some morphological and behavioral asymmetries related to mating have been identified, the relationship between genital asymmetry and lateralized mating behaviors, as well as their evolutionary significance, has rarely been examined. 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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