Quirks and Quarks·QUIRKS & QUARKS

Inflated penises and silicone vaginas. It's not what you think

Scientists have mapped the complex genitalia of dolphins.
Two dolphins mate underwater. (Ecohotel - Wikimedia commons)

Given how much we know about dolphins, it's hard to believe that until now, their basic reproductive anatomy has largely been a mystery. This is due to the complexity of dolphin genitalia, as well as the challenges of studying animals that live and reproduce in the water.

A team of scientists spent considerable time and effort to satisfy their curiosity about exactly how male and female dolphin private parts fit together.

Canadian scientist Dara Orbach a post-doctoral fellow in biology at Dalhousie University in Halifax, and a research associate at Mount Holyoke College in South Hadley, Massachussetts worked alongside Patricia Brennan of Mount Holyoke College, Diane Kelly of the University of Massachusetts, Amherst, and Mauricio Solano of Tufts University, to reveal the intimate details. 

The following interview has been edited and condensed for clarity. 

Bob McDonald: So how did you go about studying a female dolphin's reproductive tract?

Dara Orbach: So I'm very fortunate that it's not a very popular field, for whatever reason! I'm waiting opportunistically on dolphins and whales and porpoises to die of natural causes. I have permits for that when these animals do die, various networks along the U.S. coastline dissect out the reproductive tracts and send them to me.

BM: Okay. So what did you find when you began looking at them?

DO: As I started opening these reproductive tracts I found this amazing level of vaginal diversity. We found there's variation in the number, present, shape and positioning of these vaginal folds across different species.

BM: What do these folds look like?

DO: They're basically parts of the vaginal wall. They can look like a spiral shape, they can look like a small indentation. They can look like a giant flap.

BM: Sounds like it's a bit of a complicated path that the sperm has to follow.

DO: That's exactly what we're finding. And this level of vaginal diversity hasn't been documented really in a mammal before. The closest analogy would be in ducks which also have a bunch of blind recesses and spirals in their vaginas.

A CT scan shows how the penis of the common bottlenose dolphin (red) fits inside the vagina of a female bottlenose dolphin. (Dara Orbach, Dalhousie University)

BM: So once you saw the vaginal folds in the female, how do you figure out how the penis fits into that?

DO: So we have a twofold approach to that. One thing we do is when the vaginas arrive is we're putting liquid into them which then becomes solid silicone, and when we pull them out they take on the shape or the lumen of the inside of the vagina. 

Then we also found a way to inflate penises using pressurized air. So we pump air from a nitrogen tank into a keg full of saline and then put that into the penis so that we can control the flow rate of fluids moving through it. We then took the penis which was fully inflated and fixed it in formalin, so it would maintain rigidity and then we put it inside the female to approximate, or to simulate, copulation. Then we stick them together and stain them in iodine and then do C.T. scans on them.

BM: So once you put all of these parts together, how did you solve the mystery of what these folds are doing and what role they have?

DO: We're still trying to understand exactly what the function of these folds are. Previous hypotheses have suggested they're an adaptation to mating in the water but we wouldn't expect to see such variation across species if that was their only function. Also, these folds are found relatively close to the cervix and that wouldn't really be expected if they're an adaptation to keep saltwater out. You'd expect to find that the folds right by the vaginal opening.

So we think they play a role in sexual selection. The idea that the more complicated these reproductive tracts are the more difficult it is for males to penetrate the female. So from the male perspective he wants to penetrate as deep as possible so that his sperm has the least possible distance to travel to reach the eggs.

BM: But why would the female be making it difficult for the male?

DO: Dolphins are a highly multimate group of animals, so many males mate with many females. In my study of the animals in the wild, a female can mate with 15 different males in 15 minutes. It doesn't seem to be a particularly consensual situation.

And then also keep in mind that the female can only have one offspring at a time. She's pregnant for 11 months. She might be lactating for another two years and then a year or two later have another offspring. So she's investing an awful lot in terms of having these offspring and it makes sense for her to have some sort of mechanism to control who is going to be the father.

BM: Other than reproduction could there be another purpose for these vaginal folds? Could it be for pleasure?

DO: That is a possibility as well. We are now currently looking at the cellular level of these different folds and trying to understand how they contract and how neurons and sensory neurons are enervated.