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The Labyrinthine World of Animal Genitals

Dutch evolutionary biologist Menno Schilthuizen plumbs the depths of nature's nether regions.
Mallard duck coupling. Image: Shutterstock

By now, chances are you’ve come across the surprisingly nightmarish world of duck sex. If not, here's the gist: Male ducks, each of whom has a spiny, counterclockwise coil for a penis, are rapists. Over time, female ducks developed a clockwise-coiled vagina that makes it difficult for males to penetrate them fully. This is a prime example of sexually antagonistic coevolution.

Depending on your proclivities, this is either sufficiently disturbing to want to know more about the world of animal genitals, or sufficiently disturbing to happily turn that whole world off.


If you are the former, we are in fortunate times. Rather than furiously flipping through a stack of increasingly obscure science journals, those interested now have an easily digestible text to work with, the charmingly titled, Nature’s Nether Regions, by Dutch evolutionary biologist Menno Schilthuizen.

Menno’s book is a deep dive into the science of genitals, one that comes interspersed with a selection of the finest, and most scientifically-accurate, sex jokes. Humor, he told me, is a great tool “to make people think about a certain subject a little bit longer, in a slightly different way,” but by no means does that signify that Menno is not taking his subject seriously.

In fact, the sheer depth of description he offers as he explains the tape-measure-like genitalia of the male Aleochara tristis beetle, the “labyrinthine plumbing” of a selection of female insects, and “long slug schlongs” (his words, not mine) is enough to prove that his interest goes far beyond the jocular. At times, it even feels like a blackout curtain of detail—a little too much, a little oppressive. But it all serves his grander purpose of explaining why this study is important to our greater understanding of evolution.

Two weeks ago, I called Menno to ask about some lingering questions I had after reading the book. Our chat has been edited and condensed.

MOTHERBOARD: What are the ethics of forcing animals and insects to mate with each other for science? In the course of reading the book, I was struck by the type of experiments scientists had to do when studying animal and insect genitals.


Schilthuizen: Well, I don’t really work myself with animals that are considered to fall under any regulation. For people who do work on mammals and fish and reptiles, the ethics involved in that sort of experimentation is framed in rules that you have to follow as a researcher, which you have to take a course in and sign paperwork, etcetera.

So that doesn’t apply to people like me who work on insects and snails. However, that doesn’t mean that there aren’t any ethics involved. Even if there are no official rules, you still have to think about what you’re doing to an animal in order to find something out about them that’s worth it.

When you’re dealing with their sexuality, there’s an additional layer that you need to think about. It’s always good to try to put yourself in their position [laughs] and see what they might be experiencing. Of course, it’s very hard and, of course, that’s the reason why we have these rules we do apply to vertebrate animals.

But not to invertebrates, which is very artificial. It’s silly to imagine that insects or snails don’t feel pain, pleasure, and discomfort. They survive by virtue of their escaping situations that cause them pain, so it would be silly to think they don’t feel it.

Then it’s a matter of balancing whether you decide to spray freezing spray on a mating pair of snails or not, and the value in doing the experiment even though you realize that you are causing them harm or pain. How important is it and how likely is it that you are going to find something out about them that we find very important to know?

There’s a flagellum in the penis of the male rove beetle. It winds up and unrolls much like a tape measurer, according to Menno.

To what extent do discussions of gender and sexuality from the social sciences affect what you’re doing in your work?

It’s a hard question. It’s often thought or assumed that the views about gender and sexuality in the social sciences are divorced from biology.

Sexual behavior in animals and humans is nowhere near as fixed as you might think. If you ask a biologist what a male is and what a female is, maybe you’d expect a biologist to give a very technical answer. But the truth is that there’s very little consensus about what constitutes a male or a female in biology. It’s partly about whether they produce sperm or eggs, but that in itself does not say very much about how they would behave.

The example appeared two weeks ago of an insect in which the female has organs that resemble a penis. It was a cave insect in which the females had a gynosome with which they penetrate the male and obtain this sperm package he produces. So even though the male still produces sperm and the female still produces eggs, the sex roles seem to be reversed in the sense that the males have the valuable resource and the females have to compete with each other for access to those males.

It makes you realize that in the natural world, being male and female doesn’t have so much to do with the kind of organs you have. It has more to do with how much you invest in offspring. There’s usually some sort of asymmetry there, that one sex invests more time and energy than the other. And usually, what we call the male is the sex that invests less than the other sex. But in these cave insects, it’s the other way around. It’s the male that invests the most because he produces this sperm package that the female needs.


It makes you much more aware of how plastic and how dynamic sex roles are and also how variable they can be within a species. Because for it to evolve, there needs to be variation.

Were you excited to see the news about the female cave insect with a "penis" everywhere?

I wouldn’t say I was excited. When I first heard about it, I knew this was going to be picked up a lot. But, at the same time, I was a bit apprehensive. The science of it is reported in a very anecdotal, fragmentary way. There doesn’t seem to be a proper framework in the reporting about sex in animals for this to be placed in context. Almost all the media reports I saw had the same shape, namely, 'Hey, there’s this weird insect… look what they’re doing and let’s imagine what it would be like if we had to live this way.'

There’s some news story about a discovery about sex organs in animals a few times a year, I guess. And they always take this anecdotal shape. That’s understandable, but it also shows that, with the general public and even with science reporters, there’s no framework that has been popularized… even though for the past twenty years, there has been a framework developed by scientists who work in this field. If you think about it in terms of investment, then it’s much more easy to understand how these things evolve rather than just take the wild factor as the only peg you can use to market this new discovery.

The female zebra finch is into the male zebra finch because he has a fancy crown on his head. But the crown was glued on by a researcher.

You argue in the first section of the book that we’re all sort of fascinated by genitals, but it’s still taboo to discuss them on some level. Is there anything that is itself taboo within this taboo field?

The field is growing. More and more research groups in the world in different universities are beginning to start projects on understanding the evolution of genitalia. But at the same time, if you look at the work that has been done in primates and humans, most of it has been done in previous centuries.

Very few people dare to do experiments with humans or with apes. Part of the reason might be, of course, the ethical rules have become stricter with working with experimental animals or even with humans. But I think also there’s a sense of taboo. Maybe these are just flip sides of the same coin.

If you look at the kidney of a number of mammals, they all look almost identical. And if you look at the penises and vaginas of a number of mammal species, they all look completely different.

What do you think is the most misunderstood thing about genitals?

That they are functional organs, just like a liver or a kidney. I think if you ask people what genitals are for, they would say, 'Well, it’s an organ to transfer sperm if it’s a penis and it’s an organ to receive sperm if it’s a vagina,' or whatever terms you use in the animal that you’re looking at.


It is true, but that doesn’t explain all this diversity. If you look at the kidney of a number of mammals, they all look almost identical. And if you look at the penises and vaginas of a number of mammal species, they all look completely different. So that already tells you that it can’t just be about the very mundane functions. There’s much more going on.

I think people haven’t learned to think about genital organs that way yet and biology is only now beginning to wake up to that.

Labyrinthine: Where a variety of female insects hide their eggs.

Fair enough. Now, what do you think is the most misunderstood thing about the people who study genitals?

Well, I’d say the general response you get from the general public when they hear about you as a serious, publicly-funded scientist studying something as silly as the genitals of some small insect is that you must be pretty perverted to be able to spend your time on something like that. They might think it’s a manifestation of some weird, twisted obsession. Which it isn’t.

On the one hand, just like anybody else, biologists are fascinated by sex, but on top of that, I think especially as biologists, we realize that reproduction and sex play such a key role in evolution that if you really want to catch evolution in its finest hour, as it were, then you need to look at what’s happening to these reproductive organs. That’s where sexual evolution is most powerful. And I think that is the main reason why we do what we do.

Thanks Menno.

All images courtesy of Viking.