Despite having some of the largest sex organs on the planet, we know surprisingly little about the intimate moments of whales and dolphins. It turns out that the sex lives of cetaceans are amazingly complex, involving almost permanently erect penises and vaginas that have evolved to thwart them.

You might think that the logistics of dolphin sex is fairly straightforward, with the males inserting their penis into the females' vagina. But as it turns out, this isn’t necessarily the case. One researcher, Dara Orbach, has made it her mission to understand exactly how the two organs interact, and has discovered some surprising aspects about cetacean copulation.

“While it may seem intuitive that the penis fits well into the vagina during copulation, the biomechanics and details of the anatomical fit can be quite complex and have seldom been explored,” says Orbach, who is presenting her findings at the Experimental Biology 2017 meeting, in a statement. “Whales, dolphins and porpoises have unusual vaginal folds, spirals and recesses that the penis and sperm must navigate through to successfully fertilize the egg.”

To investigate in detail the inner workings of dolphin and porpoise vaginas and penises, Orbach and her colleagues have been collecting the organs from cetaceans that have washed up on US beaches. When they receive a vagina, they then take extensive measurements of the organ, including all the flaps and folds present, before making a cast in silicon.

The penises, however, receive a slightly different treatment. These are physically inflated to their fully erect size using salt water, before being fixed in position using water, methanol, and gas formaldehyde. They then take the vaginas and erect penises and fit them together not unlike a cetacean sex jigsaw. From this, the two organs are then once again fixed in place and imaged in three dimensions.

This slightly bizarre but novel technique allows the researchers to explore exactly how the two organs fit together and interact, and it's in no way as simple as you might expect. The dolphin vagina is full of flaps, folds, and dead ends that they suspect act like a gauntlet for the male dolphin's sperm, as it has to wheedle its way through to find the egg to fertilize.

By studying how the genitals differ from species to species and how the organs interact, they hope to uncover some of the evolutionary driving forces behind copulation. This is particularly interesting in cetaceans, as they have to maneuver themselves in three dimensions, while at the same time prevent sea water from seeping into any openings.

But they also hope that their findings might have conservation applications, particularly in the captive breeding arena.