How sexual selection shaped birds and human brains

Have you ever seen a lek? Matt Ridley has. In fact, the prolific science writer somewhat bashfully admits to having viewed “umpteenth” leks in his lifetime.

After googling “lek” to make sure it’s not … what you think it is, you may be tempted to watch a video of one. If you do, you’ll witness something extraordinary. You’ll see dozens of beautifully ornamented male birds strutting their stuff in a field as the more modest females amble around. The ladies are sizing up the lads for potential suitorship, and only a select few will be chosen to receive the spoils: the opportunity to reproduce.

To the lay eye, a lek is a primitive mating game. But as Ridley vividly relates in his new book, Birds, Sex & Beauty: The Extraordinary Implications of Charles Darwin’s Strangest Idea, there’s majesty and mystery in the ritual. 

In its opening pages, Ridley transports readers to April’s picturesque Pennine Hills of Northern England to watch, feel, and hear a lek of the region’s black grouse. But a literary vacation is only one of Ridley’s aims. The other is to illustrate and explore, he writes, “an idea so powerful, so weird, so wonderful, that I still feel unsure whether I have gone far enough in accepting all its implications.”

That idea is sexual selection. You may already know of its more popular sibling: natural selection, the primary driving force of evolution. Natural selection is the idea that organisms more adapted to their environment will survive and reproduce; meanwhile, sexual selection contends that certain traits will evolve and endure simply because they help animals reproduce. The classic example is the male peacock’s flamboyant tail. Does it help the bird survive? Not really. But does it help it get laid? Absolutely.

In advance of Birds, Sex, and Beauty’s release, Ridley kindly chatted with Big Think about how sexual selection ties into beauty itself, the long and simmering scientific debate over the theory, and how sexual selection may have shaped the human mind. 

Big Think: What are some of the most memorable bird mating displays you’ve witnessed?

Ridley: The ones that blew me away most were the bowerbirds of Australia. They’ve outsourced the flamboyance from their feathers onto the decorations they put on their bowers. These birds build sophisticated structures with objects arranged by color and size, art installations basically. It’s truly amazing to find a bird doing that.

Big Think: How is it possible that we get these behaviors?

Ridley: I think it is sexual selection. One of the things I catalog in the book is the two centuries of explaining this (behavior) without sexual selection, without Darwin’s particular version of female choice. They said, “Well, it might be male competition,” or “It might be because the female needs to recognize the male of her species.” Brown birds seem to manage that perfectly well, so that doesn’t make sense. 

The key feature that we need to explain is how random it is. Species that get colorful use different colors, emphasize different parts of the plumage, and grow different shaped feathers. If it was all about saying, “Look, I’m strong,” you’d get a more consistent pattern. Instead, you get these unbelievably inventive ways of making an animal look beautiful and ridiculous. This force is so creative, and you don’t associate that with evolution. 

The whole point of “survival of the fittest” is that you’ve got to be tough and serious and boring and careful and cautious. Whereas, this process says, “No, let’s have fun. Let’s be bright red. Let’s make a bird disappear into a weird shape. Let’s make it collect art. Let’s grow a very long tail.” There’s something here finding new ways of doing things — which is why I think sexual selection is probably a hugely underestimated aspect of evolution.

I’ve started using two phrases to drive home the difference: “survival of the fittest” versus “seduction of the hottest.” Survival of the fittest gives you consistent, predictable, and to some extent directional results. Yes, the variation is random, but the selection is not. You end up with strong muscles or well-shaped wings or strong digestive systems. You can see why they’re designed the way they are. But sexual selection can come up with a peacock’s tail. You’re making a mistake if you say, “The precise pattern on the peacock’s tail has a reason, has a meaning, has a purpose.” Of course, that can’t be because it’s so random. It’s so arbitrary.

Big Think: The big theme in your book is sexual selection, but a supporting theme is beauty. How did birds become so keen on beauty compared to other animal groups?

Ridley: (When) Darwin wondered why birds have a taste for the beautiful, he was getting at something that many of his contemporaries just didn’t understand: What’s beautiful to them is also beautiful to us. Is that a coincidence or not? 

Birds and mammals are 400 million years apart in terms of evolution, and so it’s unlikely that the same taste for the beautiful existed in that common ancestor. You look at other mammals, and you don’t see the same taste for the beautiful. I was looking at pictures today of elephant seals mating. They’re truly hideous.

As Darwin wrote in a letter in 1860, “The sight of a feather in a peacock’s tail when I gaze at it makes me feel sick.” He’s referring to the fact that he’s cracked the problem of how you build an eye through natural selection to his own satisfaction. He hasn’t cracked the problem of how you come up with something useless but beautiful. His critics jumped on this. They asked why evolution decided that one hummingbird should have a red throat and one a blue throat. Show me the survival value of that. 

Darwin had to come up with another explanation, and he came up with the sexual selection argument, which I think is a good one.

Big Think: In the book, you describe how Alfred Russell Wallace, arguably the co-discoverer of evolution by means of natural selection, and Darwin got on pretty well. That friendliness didn’t extend to the idea of sexual selection, though. Why were Wallace and others so adamantly opposed to accepting beauty as a factor in evolution?

Ridley: To some extent, it must be a degree of jealousy on Wallace’s part that Darwin got all the credit for evolution. I think Wallace behaved beautifully throughout his life. He was the one who wrote (the theory) down and sent it for publication, but because he was off in the East Indies, Darwin got to choose how it was presented, and his paper was presented first. That’s quite good of Wallace to take that on the chin.

Big Think: So was challenging sexual selection his way of giving Darwin his comeuppance, then?

Ridley: A bit. But Darwin gets unreasonable at this point, too. Wallace does get the last word, however, because Darwin dies, and Wallace goes on writing. He wrote Darwinism (1889), and in it he basically trashes sexual selection by mate choice.

I’m not quite clear about what role Victorian sensibilities played in all this. Darwin doesn’t seem to have any problem saying that females will choose beautiful males. Other Victorians didn’t like saying things like that. Maybe they were nervous about what their wives would think. Maybe they genuinely believed that it’s men who choose wives and not wives who choose husbands. Maybe they were just worried that if they said this kind of thing it might lead to women becoming more sexually forward.

There’s undoubtedly a real problem that this science was being developed when culture had become very puritanical.

Big Think: After this saga, Wallace’s view of sexual selection dominated for almost a century. Then we get to Robert Trivers. Can you describe his simple, yet profound idea that ultimately helped cement sexual selection as a valid scientific theory?

Ridley: Robert Trivers is a truly brilliant man. Also a very eccentric man. The particular theory that he brought to the sexual selection argument — which is blindingly obvious really but nobody thought of it before — is parental investment. Whichever sex puts the most effort into bringing up the kids is going to be competed for by the opposite sex. You’ll get a flamboyant display and aggression in the sex that isn’t doing all the work. Another way of putting it is that they each put an equal amount of energy in; one puts it into fighting and the other into nurturing. 

That often occurs to me when I watch my black grouse because the males are spending months exhausting themselves displaying and fighting. The females are spending weeks exhausting themselves laying eggs and looking after chicks.

Big Think: Yale University Ornithologist Richard Prum told you that “The reason birds are so beautiful is because they don’t have penises.” Can you briefly explain what he meant by that simultaneously insightful, hilarious, and jaw-dropping assertion?

Ridley: About 97% of birds don’t have penises. The ones that do quite often engage in quite violent sex in which females can be harassed, raped, and even killed by ardent males. Ducks, in particular, do this, and they have large, explosive penises.

What most birds do for sex is something called “cloacal kissing.” Basically, two open-ended tubes are put next to each other, and sperm is shot through from one to the other. The point Richard Prum was making is that it is up to the female whether or not the male mates. She has to present her cloaca to him. If he had a penis, that wouldn’t be to the same degree. Therefore, what she can do with this newfound power is choose beauty as opposed to strength or fitness or something else. 

Big Think: On that note, what do a peacock’s tale and an Argentinian Lake Duck’s penis have in common? What can we learn from that?

Ridley: A peacock’s tail is a weirdly exaggerated sexual ornament designed to seduce a female. The bigger and better your tail display, the more likely you are to get a female to mate with you. An Argentinian lake duck was discovered to have a penis longer than its body, which nobody even suspected because, most of the time, it’s wrapped up inside. But during mating, it shoots out in a sort of hydraulic fashion. 

Clearly, there has been selection for a very long, very explosive penis in the Argentine lake duck — not because the females admire it, but because it leads to reproductive success. As does the peacock’s tail.

Big Think: What does this all mean for us? You write that sexual selection has been largely ignored by scientists studying human evolution, especially of the human mind. Why do you think this is?

Ridley: I think the starting point is to note that humans are quite a lot like birds. We sing a lot, which most other mammals don’t. We have a lot of color vision. We like dressing up. We seem to have an aesthetic sense. So Darwin was on to something when he said birds might be a better analogy for what we’re doing sexually than other mammals.

If that’s the case, then what’s the human version of the peacock’s tail? What is the flamboyant ornament that males or females are showing off? The obvious answer — one that Geoffrey Miller explored in a good book titled The Mating Mind — is that the human brain itself grew large as a seduction device rather than as a survival device. That is to say, the main thing that gave you an advantage is that you were able to impress members of the other sex.

When you think about things like wit and humor and song and poetry and art, none of these things seem to help you survive. They certainly wouldn’t have been much use in the African savannah for hunter-gatherers, but they help you get mates. 

Now, these traits may have another function which is more like natural selection in that they help you survive in a group and encourage others to like you. There’s a social brain hypothesis that is well-developed. But the sexual brain hypothesis has never really been explored adequately except by Miller.

I think he’s onto something. We shouldn’t neglect this because it’s the one evolutionary mechanism we know that produces sudden expansion in organs in quite surprising directions. And there is something very sudden about the human brain size getting bigger, which didn’t happen in other species. Why not? Why isn’t the world full of creatures with big brains? What was it about human society that made a big brain advantageous? Possibly, it was a mate-choice thing.

Big Think: So why do you think the idea has generally been thrown by the wayside?

Ridley: Well I think it’s partly because sexual selection was very out of fashion even in evolution until quite recently. It barely got mentioned in the great books about evolution in the 20th century. They’re all talking about natural selection.

Big Think: So not only has our understanding of birds been hampered by a puritanical hangover, but so has our understanding of ourselves in a way?

Ridley: That’s partly true. And you know you can sound a little prurient, frivolous, and odd if you start talking about the brain as a seduction device or the sexual role of the mind. It’s a slightly embarrassing way of joining the conversation. But why not have a go at it? I’m old enough that I don’t need to worry too much about what people think of me.