Mammal sex is simple: The penis fits into the vagina. Right? You might think so—unless you’ve seen a dolphin’s vagina.

At the 2017 annual meeting of the American Association of Anatomists, biologist Dara Orbach of Dalhousie University in Canada explained just why she and her colleagues recently used a CT scanner to image simulated sex between the reproductive organs of dead dolphins.

It may seem obvious that a dolphin penis would fit into a dolphin vagina, but, since dolphins can’t talk about their sex lives, scientists couldn’t be sure. "Whales, dolphins, and porpoises have unusual vaginal folds, spirals, and recesses that the penis and sperm must navigate through to successfully fertilize the egg,” Orbach said in a press statement.

Dara Orbach, Dalhousie University

Orbach and Patricia Brennan, of Mount Holyoke College, used real reproductive tracts they got from dolphins that died naturally. They re-inflated the penises mechanically so that they could stick them inside the dead dolphin vaginas and see exactly how they fit together. ("My collaborator, Dr. Diane Kelly, designed a system that pumped pressurized saline into the penis so that we could adjust the pressure and the rate of inflation," Orbach elaborated to mental_floss in an email.)

They also created silicone molds of the genitals to study how the mammal’s penis and vagina structures might have co-evolved, especially considering that marine mammals have some interesting challenges to deal with during sex—for one thing, they have to be swimming, and for another, no one wants salt water in their bicornate uterus. As you can see, the dolphin penis navigates those twists and turns pretty well, despite the challenges.

This research isn’t just to satisfy scientific curiosity about how dolphins manage to get it on, though that's great, too. Understanding how actual dolphin sex looks could potentially help dolphin breeding programs figure out how to optimize artificial insemination processes in favor of conception.

Dolphins aren't the only ones with complicated reproductive tracts, though. Brennan has previously studied the sex lives of ducks, who have co-evolved genitals even more complicated than the ones dolphins are packing.