It's hard to study how dolphins and other cetaceans have sex in the wild. Think about it: They live out in the ocean, often far from shore, where scientists can't easily camp out and observe them at the critical moment.
"There are very few studies of the mating behaviour of cetaceans because of these challenges," Dara Orbach, postdoctoral fellow at Dalhousie University, told me over the phone. "I've done several of them, so I can tell you."
Orbach, an expert in the morphology and biomechanics of cetacean genitalia, figured out an unusual way to study dolphin sex: She and her team obtained the reproductive tracts of three species of cetacean (bottlenose dolphins, common dolphins, and harbour porpoises) and harbour seals. Then they pumped the dead animals' penises full of saline solution to make them erect, placed the penises inside corresponding vaginas, and took CT scans to get an in-depth look at what's happening when these animals have sex.
"We pumped saline from a pressurized keg into these penises to fill them with fluid and simulate an erection," she told me. The animals' rigid penises were fixed in a formaldehyde-based solution so they wouldn't lose their shape before being inserted into the vaginas. The research, from Orbach and Diane Kelly, Mauricio Solano, and Patricia Brennan, is published in Proceedings of the Royal Society B: Biological Sciences.
The hardest part, Orbach explained, was getting the samples. These were adult animals, all found in the US, that died of natural causes: Their parts were frozen within 24 hours of death and shipped to Orbach's lab. The samples had to be in impeccable condition for the research to work, Orbach explained. "I get very sneaky packages from FedEx." The study was done at Mount Holyoke College, where Orbach has a joint appointment.
While you'd think that the penis goes inside the vagina, and that's really all there is to it, the biomechanics of cetacean sex are actually fairly complex.
"[L]ittle is known about which features of male and female genitalia interact and how these interactions affect fertility," the new paper says. Whales, dolphins, and porpoises are known to have "unusual vaginal folds, spirals, and recesses," Orbach said in a statement earlier this year. The size of cetaceans' organs is also "pretty variable," she told me: A harbour porpoise has a penis the "length of my forearm," she explained, whereas the common dolphin's is about the size of a human hand. (This research used one female and one male of each species.)
In the study, she collected female parts including the vaginal opening, the clitoris, cervix, and ovaries; from the males, "we took from the penis tip and the entire shaft through to the pelvic bone," and associated musculature, she said.
The researchers made casts of the animals' vaginas with silicone, then inserted the inflated penises into the actual flesh vagina. The penis and vagina were sewn together in simulated copulation, and the entire setup was preserved with a formaldehyde solution.
Scientists used a CT scan to peer through the tissues. They also made 3D models of the penises and vaginas that they could manipulate and study on a computer.
According to Orbach, the degree of coevolution that these cetaceans show "is astounding." In the common dolphins and harbour seals, the vagina does not "present physical barriers to obstruct the penis," the paper says, but they found something different in the bottlenose dolphin and the harbour porpoise. There, vaginal folds seem to prevent the penis from penetrating too deeply in some cases—perhaps indicating something like the sexual arms race between male and female ducks, which has driven the coevolution of extremely complex genitalia.
Pumping dolphin penises full of saline to simulate cetacean sex in the lab might sound like a lark, but understanding how these animals reproduce could have important implications for species conservation. "To some extent, it could be applied to artificial insemination and conservation. But a lot of it is basic science," Orbach said—scientists trying to understand the intricacies of the world around us, including how dolphins have sex.
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