The 13-Fold Labyrinth
Harbor porpoises have up to 13 vaginal folds that may allow females to control which male's sperm reaches her eggs.
While bottlenose dolphins have a single complex spiral-shaped vaginal fold, harbor porpoises take genital complexity to another level: up to 13 separate folds in their vaginas.
These labyrinthine structures aren't just anatomical curiosities — they may give females control over paternity.
Researchers believe that with a slight body shift during mating, a female can direct the male's penis into a fold instead of toward her cervix. This means she can effectively reject sperm from unwanted males while appearing to mate normally.
This is a form of cryptic female choice — females selecting mates after copulation has already occurred.
Why would this evolve? In harbor porpoise society: - Males often pursue females aggressively - Females may not be able to reject matings outright - But they can still control whose offspring they carry
Scientists study cetacean vaginal anatomy by creating silicone molds of the reproductive tract. These 3D casts reveal incredibly complex internal structures that vary dramatically between species.
Bottlenose dolphins have spiraling folds, while harbor porpoises have multiple discrete barriers. Each species' anatomy reflects its unique mating dynamics and evolutionary history.
Female cetaceans appear to have far more reproductive control than their outwardly "passive" mating behavior suggests.