Asymmetric and Spiraled Genitalia Coevolve with Unique Lateralized Mating Behavior

Area of Science:

• Marine Biology
• Zoology
• Evolutionary Biology

Background:

• Asymmetric genitalia and lateralized mating behaviors are observed in various species.
• Lateralized mating behaviors are known in mammals only in the harbor porpoise (Phocoena phocoena), with males exclusively approaching females from the left.
• The potential coevolution between genital morphology and lateralized mating behavior remains understudied.

Purpose of the Study:

• To investigate the correlation and potential coevolution between genital asymmetry and lateralized mating behavior in harbor porpoises.
• To quantify the three-dimensional (3D) morphology of male and female reproductive tracts in Phocoena phocoena.
• To determine if the asymmetry in genital shape corresponds with the observed left-sided mating approach.

Main Methods:

• Utilized 2D geometric morphometrics and 3D modeling to analyze the shapes of female vaginal lumens and male distal penises.
• Quantified individual variations and directional/fluctuating asymmetry in reproductive tract morphology.
• Correlated anatomical findings with the known lateralized mating behavior of the species.

Main Results:

• Harbor porpoise vaginas displayed significant individual variation, directional, and fluctuating asymmetry due to complex 3D spirals and folds.
• These anatomical features may influence penile penetration depth and semen movement.
• Both the vaginal lumen and penis tip showed a left-canted asymmetry with similar angular bends, mirroring each other.
• The observed genital asymmetry corresponds with the males' exclusive left-lateral mating approach.

Conclusions:

• The study provides evidence for the coevolution of reproductive anatomy and lateralized mating behavior in harbor porpoises.
• The asymmetric genital morphology in both sexes appears to be functionally linked to the species' unique left-sided mating strategy.
• This finding offers insights into the evolutionary pressures shaping sexual traits and behaviors in mammals.