Bottlenose dolphins are “hypersexual creatures”imageBROKER / Alamy Stock Photo
Patricia Brennan has forged a controversial career in studying the twist and turns of the evolution of animal genitalia. A biologist based at Mount Holyoke College in South Hadley, Massachusetts, her latest research suggests that bottlenose dolphins have clitorises that have evolved for pleasure – something she says makes sense given the amount of sex the animals have. Brennan tells New Scientist about her latest discovery and why it’s so important to study female genitalia.
Jessica Hamzelou: Why study dolphin clitorises?
Patricia Brennan: I have been collaborating with a researcher who was studying vaginas in dolphins. Dolphins have very complicated vaginas, which contain many folds. The hypothesis was that these folds were there to exclude salt water during copulation, because it is supposed to be lethal to mammalian sperm. But nobody had actually ever really studied these folds or tried to test the idea.
We haven’t been able to pinpoint exactly why they are that way. But when we dissected the vaginas, I would look at these clitorises and be just amazed. I was like: “Oh my gosh, these are pretty big, well-developed clitorises.” And I thought that might be something interesting to look at.

We know that dolphins have sex all the time. They have sex for social reasons, not just for reproduction. It makes sense that the clitoris would be functional [and give pleasure when stimulated].
Are dolphins really having sex all the time? Are they more sexually active than other animals?
We don’t really know if they are having more sex than other marine mammals. It’s really hard to study sexual behaviour in cetaceans because they’re out there [in the ocean]. But bottlenose dolphins live close to the shore, where scientists can go out on their boats and study them. They see them having sex year-round, even when the females are not receptive, so not ready to get pregnant and have babies.
And not only do they have sex all the time, they have a lot of homosexual sex as well. The females will rub each other’s clitorises with their snouts and their flippers really often. It’s not like every once in a blue moon you’ll see females stimulating each other, it’s actually pretty common. Females also masturbate.
If they’re out there seeking all these sexual experiences, it’s likely that it’s probably feeling good.
Do you see these behaviours in males too?
The males, for sure, have lots of homosexual sex. The males will have anal sex, they’ll insert their penises into each other’s blowholes. Bottlenose dolphins are really hypersexual animals.

How did you go about studying dolphin clitorises?
They came from dolphins that have died from natural causes, mostly from strandings [when a marine mammal is found on shore]. It has taken years to get a sample size that is good enough for a study because it’s rare that an animal becomes stranded and that a scientist finds it before it has decayed too much.
We get a frozen package that has been sent overnight in the mail. They come from all over the US. When they arrive in our lab, we begin thawing the tissues, doing a lot of measurements and performing the dissections.
And for the clitoris in particular, we cut around the entire clitoris structure as much as we can, and then do a couple of different things. We stain some of the tissue so that we can examine it in a micro CT scanner to see the soft tissue. Other samples are dissected, and some undergo histology [detailed analysis under a microscope].
What are you looking for specifically?
We were particularly interested in looking at the erectile tissue. Does it have spaces within it that could fill up with blood? And does it have a blood supply that indicates blood would rush into these spaces on arousal and engorge them? This is what happens in humans.
We also look for the tunica albuginea, a thick band of collagen and elastin that surrounds the erectile tissue. The presence of this tissue suggests that it allows the erectile tissue to distend, but prevents it from bulging out.
We look at the nerves, too. We were able to see that they have very large nerves. Some of these nerves are up to half a millimetre in diameter, which is pretty large. There aren’t many measurements that we could compare them to, we were just shocked by how big they were.

From there, we were also able to see these free nerve endings that are right underneath the skin. These are known to increase sensitivity. We have these in great abundance in our fingertips, and of course in the clitoris and in the human glans penis.
And then we found that the skin itself in the clitoris is about the third of the thickness of the adjacent skin in the genitals. Once we put all of those things together, we were pretty certain that this is functioning in pleasure, just like in humans.
Does this tell us anything new about dolphins or how they form social bonds?
No. I think it makes perfect sense. Dolphins have a lot of sex, females have a lot of sex with each other. Their clitoris is very well developed and it looks like it provides pleasure. It just makes sense. It’s basically providing the morphological evidence that we need to close that case and say, yes, this is what a functional clitoris looks like.
Is this controversial?
There is this hypothesis out there that, because penises and clitorises share the same developmental pathway, the clitoris is just a mini penis. It’s not really designed for anything and it doesn’t necessarily have a function. It’s just there because males have a penis.
There is debate whether even human female orgasms are functional or just a byproduct. It’s one of those things that just refuses to die.

Obviously, as a female scientist who studies sex and sexual reproduction, I have a problem with that idea that there would be no function for a structure because it’s developmentally homologous with the penis.
We can show that this is more than a mini penis; this is actually a fully functional organ that’s serving some kind of purpose. It’s probably evolutionarily a good idea because it makes you seek out sex more often.
Where does the research go from here?
Now we have this set of criteria, we can start looking at other species that normally you wouldn’t keep in a lab and start asking the same questions. We know bonobos have sex all the time, and we know that their clitoris looks very much like a human clitoris. It’s much easier for people to imagine that, of course, if a bonobo is grimacing and vocalising when it’s having sex, it looks like they’re having a pleasurable response, kind of like ours.
Dolphins are very different animals from primates. Now, we’re starting to look at the alpaca clitoris and look for the same kinds of morphological features. We’ve got a whole bunch of vertebrates. In my lab, we work with sharks, snakes, alpacas, dolphins, ducks – just a diverse group of species. And we always find something interesting.
What about male genitalia?
We also have collected plenty of penises and we are interested in looking at their morphology and how they function.
In general, I study genital coevolution. I’m interested in looking at males and females and how they co-evolved together. When it comes to genitalia, you have to look at both sides of the equation to figure out how they work, because mechanically they have to fit together.
The job of genitalia is to facilitate male and female gametes getting together, and you could have a tube going into a cylinder that would achieve that basic goal. But genitalia are not like that. They have all sorts of weird elaborations and other features that are going on. They have spines, bumps, turns, pockets and spirals. We’re trying to understand the evolutionary processes that are influencing their morphology and their function.

We do study both, but what I keep finding is that because people have tended to study the male more than the female, there is a lot about the female that we don’t know. We’re kind of catching up with all the female stuff.
Critics have questioned the importance of your research in the past. What is your response to people who say this kind of work is irrelevant?
It is basic science. It’s not designed to solve a particular problem. When people ask why we study it, they [want to know if] it is going to make any money or heal some disease. The answer is: maybe, we don’t know. We’re trying to describe natural phenomena and understand how they work.
Your question touches on something else. There are people who are uncomfortable with studies of sexual behaviour in general, whether in humans or other animals. That doesn’t mean that sex is unimportant or that we shouldn’t be studying it.
I could be looking at stomach morphology, and that probably wouldn’t bother people. Part of the reason people are bothered is because it’s sex and sexual reproduction. That’s understandable, but I’m a scientist. I ask questions where I think there are interesting questions to be asked.
This interview has been edited for length and clarity.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2021.11.020

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