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Researchers have recently discovered female dolphins have a secret weapon against fertilization from unwanted partners: complex vaginal structures.

To compare the genitalia, Dara Orbach, a biologist at Dalhousie University in Canada, created silicone molds of the vaginas of four types of marine mammals—common porpoises, common seals, and common and bottlenose dolphins—that had died of natural causes.

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Female dolphins evolved complex vaginas with extensive folds and spirals to control which sperm fertilizes her egg.

She and her team then used penises from deceased male counterparts and inflated them with saline to simulate an erection. Then they inserted the penises into the vagina casts and imaged them using CT scans to determine the best fit and positions for fertilization.

The researchers found that while the vaginas of common dolphins and common seals more easily accepted the penises, common porpoise and bottlenose dolphins had extensive folds that prevented easy entry.

Male bottlenose dolphins form alliances with two to four other dolphins for mating purposes, and in those circumstances, females tend to have little say with whom she mates. It could be one or all, even. But these vaginal folds could give her some control over the situation.

Josh Gabbatiss reporting for New Scientist explains just how she controls the paternity:

Related Jaglavak, Prince of Insects Dogs' Dazzling Sense of Smell Digging Up Panama's Past In all four species studied, the ideal position for successful fertilisation appeared to be the male on top with his penis hooked underneath the female. Any deviation from this tended to result in unsuccessful penetration. That is consistent with Orbach and Mann’s observations of wild porpoises and dolphins mating. This combination of precise positioning and complex vaginal structures means that subtle mid-copulation movements by females could send the penis the wrong way in their vaginas, preventing fertilisation.

Female cetaceans aren’t the only animals with complex vaginal structures. In previous studies, Brennan found evidence that duck vaginas have evolved to make it harder for male ducks to force copulation.

Diversity in penises size and shape is relatively well known in dolphins and other cetaceans, which suggests suggest that there is a similar diversity in vaginas—not just in these species, but others with complex genitalia, too.