SPIEGEL: Professor Prum, among all the wonders of nature you were most inspired by the sex of ducks. Why?
Prum: For a long time, I have been fascinated by the sex life of birds. But there is probably no other species where the deep sexual conflict between male and female sex is as blatant as in ducks.
SPIEGEL: And so you started studying their genitalia?
Prum: No, it was actually even more simple than that. I had a prospective post-doctoral student who was looking for something to do, and she was interested in studying genitalia. I said to myself: Well, I have never worked on that end of the bird before. As a result, we studied duck sex intensively for six, seven years.
SPIEGEL: What surprised you most?
Prum: Oh, there were many surprises. Not the least that we had all these descriptions of duck genitalia, and when we looked ourselves, we said: There is almost nothing to see. How could this be? That is how we discovered that the genitalia of ducks regress and regrow each year, so that a 10- or 15-centimeter penis in the summer will reduce to less than 1 centimeter in the winter and then grow back the next year.
SPIEGEL: This is part of the sexual conflict you mentioned before?
Prum: Yes, indeed. Mate choice occurs first. In winter the males do these elaborate displays, and the females choose the one they like most. Because, parallel to the evolution of the males' display behavior, the females have evolved preferences for these displays. We call this "coevolution."
SPIEGEL: So far, this sounds quite harmonious.
Prum: Yes, it is. The pairs stay together until the clutch is laid and the females incubate. The conflict part comes next. Because now some of the males pursue an alternative mating strategy, which is to violently enforce copulation. For this they make use of their penis, which is regrown by now. This penis is a very bizarre structure. It is counterclockwise coiled, and erection takes place in less than half a second. Erection, penetration and ejaculation in ducks is one and the same event, and it happens very, very rapidly.
SPIEGEL: How do the females react?
Prum: It's very interesting. Of course, in the short run, females struggle to escape from forced copulations. But in the long run, female ducks coevolve vaginal morphologies for the purpose of preventing forced intromission (or insertion of the penis) -- sort of dead end cul-de-sacs and a series of clockwise spirals that have a chiral (or non-superimposable) mismatch with the shape of the penis. These are literally anti-screw devices.
SPIEGEL: Why all this effort? Wouldn't it be easier to just give in to the aggressor's assault?
Prum: To understand this, you have to consider the evolutionary mechanisms involved: If the female gets the mate she likes, then her offspring will inherit the green head and the quack-quack-quack, all those displays that she likes so much. And since all other females have coevolved to prefer those same traits, her sons will be very successful and she will have lots of grandchildren from him. But if she's fertilized by force, then some random male will father her kids, which means that her offspring are less likely to inherit the attractive traits that she and other females like. That means fewer grandkids. Therefore, evolution will favor any mutation that allows her to get her own choice -- for example by protecting her vagina against forced sex.
SPIEGEL: Are you saying that nature works to protect female rights?
Prum: You can put it like that. Sexual autonomy matters to animals. It's not just a political idea invented by feminists, but an evolved feature of social species.
SPIEGEL: In other words, nature created a sex that is focused on autonomy and another that is focused on violence -- a good and an evil sex?
Prum: You are right: In our world, we do associate violations of autonomy with abuses of power. But this doesn't mean, of course, that there are ethical standards among ducks as there are among humans. Females are not the inherently more ethical sex, but it is just that there is something about female reproduction that limits the potential for the sexual abuse of power.
SPIEGEL: Somehow birds seem to be particularly successful in achieving their sexual autonomy. Among them, female mate choice is more common than among other animals. Why?
Prum: For a very simple reason: Unlike ducks, 97 percent of birds cannot be forcibly fertilized, because the males don't have a penis. Copulation in most birds is achieved by a cloacal kiss, just an apposition (or touching) of orifices. So, to be fertilized, the female has to actively take up the sperm, which means that she retains full control of her sexual choice. By the way, I think this is the essential reason why birds are so beautiful. Since they have the freedom of choice, females exhibit aesthetic preferences. And, as a result of these preferences, males developed amazingly elaborate ornaments.
SPIEGEL: Does that mean beauty arises wherever there is female mate choice?
Prum: Wherever you have mate choice, period -- not necessarily because the females are choosing. There are examples of male mate choice, or mutual mate choice as well. Take puffins for example. They court each other with elaborate displays, and therefore both sexes look the same. They both have the same colorful beaks and the same preferences for these beaks.
SPIEGEL: How about humans? Does our conception of beauty also stem from mate choice?
Prum: I'm convinced it does. Socrates was interested in Eros as the source of art and beauty, but it is important to be aware that our sense of aesthetics was reinvented over the course of human evolution. Because looking at the lives of chimpanzees and gorillas, our nearest relatives, we don't see much evidence for sexual choice. In chimpanzees, males will pursue every sexual opportunity they get and females will acquiesce to every sexual request made to them.
SPIEGEL: And because chimps are sexually indiscriminate they don't have any sense of beauty?
Prum: Yes, I think their lives are basically devoid of the aesthetic.
SPIEGEL: How did this transformation happen -- when did beauty enter our world as humans?
Prum: This question is very difficult to answer. But just posing it already represents substantial progress. If you look in any current textbook of human evolutionary biology you will find that mate choice as a topic is almost entirely absent.
SPIEGEL: Do you think it was more likely to have been the males or the females that introduced mate choice into human evolution?
Prum: Initially, it was the females, for sure. Males didn't need to become choosy until they were actively engaging in the upbringing of their offspring. And that happened much, much later.
SPIEGEL: What do you think were the criteria for female choice then?
Prum: Well, we don't know for sure, of course. But I propose that the main male ornament females were selecting for was social personality itself.
Richard Prum, 55, is a curator at the Peabody Museum of Natural History at Yale University in Connecticut. He has spent decades researching the courtship behavior of birds. He found an inexhaustible variety of methods with which males solicit the pleasure of their female mates. In his new book, "The Evolution of Beauty," the ornithologist seeks to find out how this opulence came about.
SPIEGEL: Which essentially just boils down to the question: Do I like him?
Prum: You name it. And this leads us to aspects of personality that we usually don't imagine as sexual ornaments like humor, empathy or the ability to conceive of someone else's mind.
SPIEGEL: You think it was intrinsic values rather than outward appearances that mattered?
Prum: Let me say one thing about outward appearance. One feature that I think was transformed by female mate choice is the reduction of male canines. It is very notable that the males of other primate species have deadly weapons in their faces that humans lack. And the question is: Under what conditions do males give up their weapons? Well, believe me, in the United States we know how difficult it is to solve this question. But evolutionarily, there is a simple answer: when wearing weapons becomes unsexy.
SPIEGEL: You are suggesting that women were attracted to small teeth?
Prum: Yeah, and I even think that this is where our smile comes from. It is a sexual symbol advertising one's state of de-weaponization.
SPIEGEL: Do you think that our ability to fall in love was another innovation of human evolution?
Prum: Yes, I do think that sexual love has evolved distinctly in humans and doesn't exist in other apes. But we probably share with them the emotion of maternal love for our offspring, and very similar hormonal mechanisms might be engaged there.
SPIEGEL: How about birds? Some pairs seem to engage in very deep, lifelong bonds.
Prum: Well, I can't say for sure. But I find it not unjustified to speculate that some long-term bonds among birds might be similar to what we experience as love. This is the problem with all current attempts to biologically describe love: In evolutionary biology textbooks, you'll see pair bonding analyzed in terms of game theory. Who's got most of the resources, who's cheating whom, what's the right strategy to maximize your benefit, and so on. Which means that, instead of falling in love, we should all go to a lawyer's office and draw up prenuptial agreements. But human mating isn't anything like that. What's missing from that analysis of human reproduction is the aesthetic. And it's clear that if love is anything, it's a deeply emotional and deeply aesthetic experience.
SPIEGEL: But are you sure that the aesthetic experience of birds and the aesthetic experience of humans are the same phenomenon? After all, a peacock hen's sense of beauty is exclusively directed towards the appeal of a peacock tail, whereas we find beauty not only in our partner but also in flowers or landscapes or art.
Prum: I agree, the richness of human aesthetic experience is unparalleled. But still it is amazing how complex and diverse the aesthetic interests of birds can be. Take bowerbirds, for example, which build seduction theaters where they present objects for females. (He pulls a photo out of a pile on his desk.) Here, on this photograph, you can see one of those arenas. Look, what he has exposed: Those are red flowers, that is a bunch of black charcoal. A pile of blueberries is over here and shiny black beetles over there. That green stuff is a rotten log permeated with a spongy green fungus. And you're are telling me that his sense of beauty is limited?
SPIEGEL: But this is an exception.
Prum: I'll give you another example: Many birds learn their song, and some birds even mimic the songs of other birds. There's one species in South America, the Lawrence's thrush, of which there are individuals with repertoires of over 170 species of birds. And they don't imitate the sound of water, they don't imitate insects. They imitate other birds, because their songs are of aesthetic value to them. Or here is another one: A researcher in Sweden discovered a marsh warbler singing a song of a bird from its wintering grounds in Uganda. This warbler introduced aesthetic content from another continent into the acoustic environment of Europe. I can't imagine anything that humans could do that is aesthetically as fascinating as that. And who knows, may be the female that listens to that song will experience nostalgia, remembering the winter holiday in Uganda?
The article you are reading originally appeared in German in issue 23/2017 (June 03, 2017) of DER SPIEGEL.
SPIEGEL: From an evolutionary perspective, how essential was the sense of beauty to modern humans? Is it the driving force that made us humans?
Prum: I wouldn't go that far. But I am convinced that the development of aesthetic preferences was an important prerequisite for human evolution, because it was sexual selection that transformed our aggressive, weaponized maleness into a socially tractable form. Don't forget: The average male primate is an infanticidal psychopath waiting for his moment. Whenever a male baboon wins control over a group of females, the first thing he will do is to murder all the dependent, breast-feeding babies because that gives him a sexual opportunity. Otherwise he would waste a lot of time while she's nursing the babies of another male.
SPIEGEL: And females made them give up this bad habit by choosing more good-natured males?
Prum: Yes. Solving the infanticide problem was the biggest hurdle in human evolution. Infanticide is the single largest source of infant mortality in gorillas and chimpanzees. Approximately 30 percent of all infant deaths are the result of infanticide by males. On the other hand, everything that is special about human biology requires greater investment in longer childhoods -- whether it's complex cognition, language, culture or technology. None of that could possibly have evolved if a large portion of babies are being murdered by sexual violence.
SPIEGEL: How did females solve this problem?
Prum: Just as in ducks. Among the many varieties of new preferences that might arise among females, evolution favored those that enhanced female sexual autonomy and reduced male sexual control. And that is how male weapons were gradually chipped away and male dominance behavior was weakened.
SPIEGEL: And from that point onwards there were no more impediments for conquering the planet?
Prum: One thing is sure: Solving the infanticide problem is one of the main reasons humans have grown to dominate the planet while gorillas and chimpanzees are going extinct in the jungles in Africa.
SPIEGEL: Professor Prum, we thank you for this interview.