Reproductive organs throughout the animal kingdom are just about the craziest thing that's ever been described in scientific literature. Did you know, for example, that female ducks have complex vaginas that coil in the opposite direction of males' equally complex penises, allowing them to thwart attempts at forced copulation? Or that the penises of particularly lucky species of crane fly function as vibrators? Or, for that matter, that the human female clitoris is about the size of a medium zucchini -- and sports more nerve endings than the penis?
Evolutionary biology, it turns out, is a lot kinkier than you might have imagined when you first learned about Darwin's theories. And genitals -- which come in a mind-boggling array of shapes and sizes -- play a much more significant role in evolution than scientists, up until very recently, were aware of.
"From time immemorial, we have taken the mechanics of sexual intercourse for granted," writes Dutch evolutionary biologist and ecologist Menno Schilthuizen. His book, "Nature's Nether Regions," changes that, taking a closer look between the legs (or, in the case of the Australian velvet worm, on the head) to explore what the sex lives of various creatures can teach us about reproduction, diversity and human sexuality.
Schilthuizen spoke with Salon about the emerging field of genitalia research, and about the many mysteries we've yet to crack, from bumblebee sex to the female orgasm. This interview has been lightly edited for length and clarity.
I actually missed my stop on the train this morning because I was engrossed in the chapter about duck sex. That was a first. So you have me convinced, but I guess the first thing I should ask you is, why genitals? Why write an entire book about this?
First, nobody had ever tried to popularize the whole field. I thought there were enough people working on it, enough data, enough examples and enough theory to make it possible to spin it together into a more or less coherent story. Second, I think it’s very strange that nobody has written a book about it yet, because it’s a subject that allows you to show what evolution can do, in a sort of quadrupled way compared to the regular features that are always used to show you evolution.
It’s also, as you write, a very new science. Why did it take biologists so long to catch on that this was something important that they should be studying?
The whole field of sexual selection is quite young. Even work on why bird plumage is so colorful in males, or in females if the sex roles are reversed, only really started in the 1960s. So almost a century after Darwin, people really started studying it and doing experiments, trying to see if they can really prove that all of these features were naturally selected for and if it worked the way that Darwin has said it did. So if you consider that the start of proper sexual selection research, then it’s only about 25 years later that people started looking at genitalia.
All of the information was there: People who were studying insects had been drawing and dissecting and measuring their genitalia for a century already, using them as a useful way to separate species that were otherwise very similar. But that information never really passed the divide between museum biologists and experimental, or field biologists. I think that biology is so subdivided into different subfields that we don’t talk to each other.
You also write about this prudish attitude we’ve had about the subject -- it’s something we still see today. You bring up “duckpenisgate,” where the right-wing media jumped on the fact that money was being spent on this kind of research.
Yeah, it’s a bit of minefield also. There are various conflicting ways in which people react to proper, serious scientists studying genitalia of very obscure animals, or even less obscure animals. So there’s sometimes the response that it’s just too silly for words and we shouldn’t spend any money on it. Or that it’s sort of perverted in a certain way, that scientists who study these things, they’re just little dirty old (or young) scientists, men and women. And they’re not really objective or not really neutral in the way that they approach the subject.
And at the same time you see that once this kind of research is mentioned to the broader public it immediately attracts a lot of attention. Because people automatically translate it to their own sex lives; they start imagining what it would be like, I guess. There’s this whole sea of bizarre shapes and behaviors that you could imagine, once you read about it. We’re animals too. We have genitalia, we have sex, so there’s immediately some sort of connection that either brings about fascination or disgust.
Of course, you have to be careful to separate human intercourse from what bugs or snails are doing. But what are some of the major things that we can learn about human sexuality from knowing how other species go about it?
First of all, the fact that it evolves. I think that's already for many people a revelation because it’s something that we are so used to. The shapes, behaviors and all those sexual functions and physiology is so natural to us and so familiar that it’s very hard to imagine that in the past, these things were different. That fact that we differ so much in all those aspects from our closest relatives means that people who lived over half a million years ago probably had somewhat different sex lives, sexual feelings, sexual organs, sexual physiology. That in itself is very important to realize.
It also means that for something to evolve, you need variation. So on the one hand, we are used to variation -- when you’re talking about sex, we know there lots of variations in shape, in behavior, in preferences and in function, but we still tend to think that’s variation around the norm. That there is some sort of norm, whereas there is only a norm in a statistical sense, and this variation is the raw material that evolution uses for the changes to take shape. So it means that we probably always have varied in aspects relating to our sexuality, and that’s probably always going to be that way. What is common now is probably going to be rare in the future and vice versa.
Can you give some specific examples of those sorts of things?
Well, I talk a lot in the book about female orgasm. We know there’s a huge degree of variation there, in the sense that many women hardly ever have an orgasm. Other women have orgasms much more easily or much more frequently. Some women have vaginal orgasms, others not or rarely. That’s something that, of course, is being talked about a lot and it’s often medicalized. It’s talked about, again, as if there is a norm and the variation around that norm is by definition abnormal and treatments are necessary.
Whereas if you consider that it’s possible -- there’s very little proper evidence yet, but it’s at least possible -- that human females “use" orgasm as a way of selecting sperm, then it’s actually quite likely that there should be variation in this. It becomes a sort of choosiness. And we know that choosiness in behavior, and also in physiology, varies a lot within species. When it has been studied in birds or other animals that are used a lot for sexual selection research, you see that some females are much more choosy than others. The variation in orgasm frequency could be cast in these terms as well, rather than seeing it as something that is or isn’t a degree of abnormality.
Female orgasm is one of the things I wanted to ask you about. To me what was really cool is the way that theory about women "using" orgasm to "choose" which sperm fertilizes their eggs is that it kind of challenges the cultural perception that females play a more passive role in reproduction. I was wondering if you see other ways in which this kind of research challenges cultural norms or our sexual stereotypes.
In humans, when you look at the information that we have, it always comes from the 1970s. Well a little bit from the 1990s also. But it really seems to have stopped, even though research funding in general and the numbers of people doing research has always been increasing over the last few decades. The amount of data that we have on something that is so fundamental, and so important and that so many people are interested in -- it’s actually mind-boggling to me that nobody is studying this. And I’m sure one of the reasons for that must be cultural. It’s just something that you don’t do. That’s also why I refer to some of the research that has been done for this work as courageous. There’s an enormous barrier between proposing an idea and actually carrying it out, for a researcher to ask a perfectly reasonable scientific question about a species that is very important and interesting to us, namely ourselves, and then take the next step and actually get people to participate in experiments to study the function of female orgasm. There are only a very few people who have done this. And there hasn’t really been anything on the past 20 years, which may have to do with some newfound prudishness in society in general which sort of filters down to science as well.
Would you say that our understanding of other species is greater than what we know about human sex and sexuality?
Yes, it is. I think in many ways we know more about the fruit fly than about our own sex lives. Which is crazy. On the one hand, it’s not crazy, because you can’t do the same thing with a human as you can with a fruit fly.
You mean you can’t freeze them in action then dissect them under a microscope?
Yes, and there’s lots of other things you can’t do. But there are also lots of things that you can do with humans that you can't do with fruit flies. For starters, the fact that we’re much larger makes it possible to study details that are much harder to study in small insects. But yet, it’s certainly true, I guess partly because of those cultural barriers. We’ve been uninhibited in studying fruit fly sex, but we have been inhibited in studying human sex.
One of the craziest things that you describe in the book, in my opinion, is the crane fly with the vibrating penis. You describe it as a way for the male to "titillate" the female. Is it going too far to read sexual pleasure into what’s happening with other species? Is that the wrong way of thinking about it or can that also help us understand sex?
I think it can, it’s just that if you imagine that pleasure is physiology, neurotransmitters, nerves, etc., then it’s perfectly legitimate to assume that a similar physiological response could also be called pleasure in a crane fly. If you take a more psychological or spiritual approach to pleasure then it suddenly becomes much, much harder to imagine it. But to me I think the parallels are such that you could easily compare pleasure in humans to a similar sensation in insects. Surely it will be a less complex sensation, because they have simpler nervous systems, but in principle it’s the same thing.
What do you think is the best framework for understanding sex in general? Is it through this evolutionary idea of furthering of the species, or through the competitiveness between the species? Is it through pleasure?
Well that’s one of the things that makes this field so interesting, the fact that it can go in all directions. Whatever furthers your reproductive output as an individual will be selected. And it depends, I think, on which species you are and what sort of habitat you live in and how easy it is to meet other insects of the same species and of the opposite sex. How much you have to be afraid of predators who might take advantage of the fact that you’re distracted by sexual partners, etc. All these things conspire to drive sexual evolution in a myriad different ways. You have to realize that these are all snapshots of evolution; the sexual behavior of a species can change over time. So maybe at the moment competition between males is important, but as the species becomes rare evolution could go into a different direction. So it’s very had to say sex is about this or it’s about this. It’s basically about anything that will improve your chances as a mate in a population.
But even within a single species, there can be multiple strategies. You see that in many species also, that a certain morphology, or shape, is being selected for males (or for females), and at the same time another form is also being selected, so you get evolution of what they call polymorphism: multiple shapes, or multiple strategies which coexist and often survive by virtue of the fact that somebody else in the population is using the other strategy. It’s very variable and it’s very hard to pin it down to a single take-home message.
So far as your own research is concerned, and for the field in general, what are some of the really big questions that you would like to see answered?
Like I said, I think humans are certainly something that we should do more experimental research on. Besides that, in general, I think the female genitalia have also been neglected in nonhuman animals. In insects, and in mammals too. I'm not sure -- some people like to say that’s also a cultural thing, but I think there’s also a very important practical reason: especially for museum biologists, they study dried and preserved specimens, and things that stick out are simply more easy to study and measure than things which have to do with dissection and figuring out how all these invaginated folds work. So I think that’s also a very important reason why this is, but it’s certainly something that has caused some blinders in the field.
And those may have been very important because as you may have noticed, Bill Eberhard's theory is that genitalia evolved most strongly in males and that the female genitalia are much more uniform between species, and that they don’t evolve so fast. But then you often see in studies where people really have taken the trouble to look at female genitalia, that they find much more diversity than they thought there was initially. So the duck study is a good example where initially everybody said those female genitalia are all the same, and then when they started looking properly, they found out there was a sort of coevolution going on in the females. Which means that it’s not about females passively selecting whatever diversity the males offer them, in terms of genital shape, but that there is an evolutionary arms race going on there. To really see how often that is the case, we need to know more about those female genitalia.
The bumblebees that I mentioned in the beginning of the book are also a good example. It’s really very mysterious: You have all this male variation in the genitalia, and there doesn’t seem to be any variation in the female genitalia. But the mystery is that bumblebees only mate once as far as we know, so there’s no way for a female to select among multiple males. So either what we know about bumblebee sex is wrong, and they do mate multiple times, or there’s something else weird going on. But the key may lie in the exact shape of the female genitalia.
So I think across the board, in all animals, including humans, it’s the female side that has been neglected. It's remedied slowly, but it’s certainly something we should focus on -- to try to tread the unbeaten path a bit.
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