Best mates? Female bottlenose dolphins can control the paternity of their offspring Flip Nicklin/Minden Pictures/FLPA

Some female dolphins have evolved a secret weapon in their sexual arms race with males: vaginas that protect them from fertilisation by unwelcome partners.

Penises come in a wide variety of shapes and sizes, especially in dolphins and other cetaceans. That seems to imply a similar diversity in vaginas, but Dara Orbach of Dalhousie University, Canada, says there is “a huge lag” in our understanding of female genitalia.

That is partly because it is tricky to visualise vaginal structure. To overcome this problem, Orbach has created silicone moulds of cetaceans’ vaginas, revealing complex folds and spirals.


“There’s this unparalleled level of vaginal diversity that we had no idea existed before,” Orbach says.

Similarly complex vaginal structures are found in several species of duck. Orbach’s collaborator Patricia Brennan of Mount Holyoke College, Massachusetts, has previously found evidence that duck vaginas have evolved to make it harder for males to force copulation. So Orbach wondered if female cetaceans’ unusual vaginas had also evolved to keep out unwanted sperm.

Orbach, Brennan and their colleagues obtained genitals from marine mammals that had died of natural causes: common and bottlenose dolphins, common porpoises and common seals. They inflated the males’ penises with saline to see how they looked when they were erect, and compared them with the vaginal moulds. They also took CT scans of penises inserted into the corresponding vaginas, to determine whether they fitted in easily and the best positions.

Take back control

The vaginas of the common dolphin and common seal seemed amenable to penetration, suggesting their genitalia had evolved to allow males access. But the common porpoise and bottlenose dolphin had extensive vaginal folds that obstructed penis entry.

Male bottlenose dolphins form alliances two to four strong to keep competitors away from females. When confronted by such an alliance, a female has little choice in who mates with her, and may mate with everyone.

However, vaginal folds could grant her some agency, says Janet Mann of Georgetown University in Washington DC. “She may not choose who she mates with, but might be able to choose which male or, more precisely, which sperm, fertilises her egg.”

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.

“It might appear behaviourally that females are very passive,” says Orbach. “But looking at the reproductive anatomy, we’re learning that they have all sorts of cryptic ways to control paternity.”

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2017.1265