Instant Genius - How birds’ bizarre mating rituals have played a key role in their evolution

Episode Date: March 21, 2025

Be it the extravagant displays of peacocks fanning their tails, the beautiful, complex song of nightingales or the meticulous art installation-like structures built by bouwerbirds, the animal kingdom filled with a huge variety of fascinating mating rituals. But more than being mere quirks of evolution, these behaviours play a significant role in driving the process itself. In this episode, we catch up with science writer and best-selling author Matt Ridley to talk about his latest book, Birds, Sex and Beauty: The Extraordinary Implications of Darwin’s Strangest Idea. He tells us about the runaway processes that have led to the development of these flamboyant displays and behaviours, why choosing a mate may well be a popularity contest, and why it’s so important to give birth to sexy sons. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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now. Book on Hilton.com or the Hilton app and save up to 20% to get the stay you expected. When you want savings, not surprises. It matters where you stay. Hilton, for the stay. This podcast is sponsored by name, audio and focal. Streaming has made music more accessible than ever, but true listening is about more than ease. It's about quality. British audio experts name Audio, alongside French acoustic specialist focal, combine handcrafted tradition with cutting-edge innovation and high-end materials, delivering digital precision with analogue warmth. So you can experience exceptional sound at home. Music just as the artist intended. Visit name audio.com to learn more. Hello and welcome to Instant Genius, a bite-sized master class in podcast form. Every Monday and Friday,

you'll hear world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Jason Goodyear, commissioning editor, a BBC science focus. Be it the extravagant displays of peacocks funding their tales, the beautiful complex song of nightingales, or the meticulous art installation-like structures built by bowerbirds. The animal kingdom is filled with a huge variety of fascinating mating rituals. But more than being mere quirks of evolution, these behaviours play a significant role in driving the process. itself. In this episode, we catch up with science writer and best-selling author Matt Riddley to talk about his latest book, Birds, Sex and Beauty, the extraordinary implications of Darwin's

strangest idea. He tells us about the runaway processes that have led to the development of these flamboyant displays and behaviours, why choosing a mate may well be a popularity contest, and why is so important to give birth to sexy sons. So welcome to the pocketbooker. So welcome to the Just, thanks very much for joining us. Thank you, Jason. Good to be with you. So today we're talking about your latest book, Birds, Sex and Beauty, the Extraordinary Implications of Darwin's Strangest Idea. So as you lay out in the preface of the book,

it's a bit different from the previous ones that you've written. So could you explain the sort of credo of it? Yes, I've written non-fiction books about technology, genetics, evolution in the past. But what I've not done is written a sort of. natural history book. That is to say the kind of book I loved reading as a child by Gerald Dural or someone like that, where you just glory in how wonderful nature is. And this book is partly that, and it's partly a popular science book trying to understand the evolutionary science behind the story I'm telling. But there's quite a lot in this book for good old birdwatchers

like myself. So the subtitle in the book refers to Darwin's strangest idea. So let's start with that. What is this idea? The idea is sexual selection or evolution by mate choice. And this was an idea that Darwin had pretty early on before he published the origin of species. And he said there is natural selection in which survival of the fittest is what happens. And there is also mate choice. You know, mates can be very selective in who they mate with. And that can influence. influence what happens to the genes of the population in surprising and rather different ways. The simplest way that I put it these days is that it's seduction of the hottest rather than survival of the fittest. And most of his contemporaries thought that this was either not

a significant force in evolution or where it did happen, it was just a sort of survival of the fittest at one removed because what the females were after, were genes that enabled their offspring to survive. Darwin didn't think that. He thought it was a very different species of evolutionary mechanism and one that would produce very different outcomes. So you kick off the book talking about your observations of something known as a LEC. So I personally never heard of this before. So, you know, could you explain that for us? Yes, no, it's a Swedish word, LEC, L-E-K, and it is used for animals, usually birds, which gather in a communal way, to display to members of the opposite sex.

So what it usually means is a group of a dozen or 20 birds that come to the same spot every day and for months on end, and indeed the same spot every year, in order to display. play their plumage and sing and dance in such a way that when the females turn up, the males have got a sort of competitive market going in saying, here's your chance to choose the best male. It's a curious way of doing mating, but it's one that a number of species, particularly of birds, but also of other species do. So you talk about the black grouse leck quite a lot.

So this is quite interesting in that it's a kind of, it's a case of the winner takes all in a way, isn't it, with the victorious cock getting to mate with the majority of hens. How do they reach that point? Yes, it's an extraordinary system in that sense, because you can have a dozen or 20, as I say, black grouse males on the leg. One of them will do possibly all, but certainly nearly all, of the matings. And when I watched a particular leck for several years, I described one year in particular when a male called Black Spot, who had a tiny black spot on one of his white feathers, basically did nearly all of the mating. And so what's the point of being one of the other males?

And how do the females decide which one is the one they want to mate with? And why are they so extremely choosy? Because to the naked eye of a human being, there is not much difference to tell between these males. So you talk about the females choosing their mates there. And this was sort of anathema to a lot of early naturalists, particularly Wallace. So was this coming from more of a sort of cultural viewpoint, sort of largely based on the idea of women and society, human society at the time? I think the cultural background is quite important to the debate between Charles Darwin and Alfred Russell Wallace and others. because Darwin said, I think females have been choosing the male they want in certain species for thousands of generations,

and this has resulted in features like peacocks tails and other flamboyant plumage. And they have been behaving like bird breeders, you know, like people who breed bantoms, he said. He had in mind his friend, Sir John Seabright, who bred a particular kind of bantam. And he said, if a man can do that, why can't females do that to males? And his colleagues said, that's not, that doesn't make sense. And partly because they couldn't imagine women being in charge of whomates. They couldn't imagine male beauty mattering more than female beauty. And they were uncomfortable with the whole idea of female lust.

The second half of the 19th century was a particularly puritanical time in Victorian England. And so I genuinely think that it just didn't compute as a suggestion for most Victorian men, including Alfred Russell Wallace. And he said, I mean, I find it hard to believe that a female would be able to distinguish one male from another and mind about it. And anyway, why should beautiful feathers matter to the females? Now, that very last question was a good one and one that Darwin did. didn't have an answer to. But I do think you're right that the cultural background of these Victorian naturalists stopped them being open-minded about this question. And Darwin's genius was that he was always open-minded. He was very good at considering strange ideas.

So you mentioned peacock tails then. So let's have a look at this. So lots of birds have these extravagant features like long tails, colourful plumage, complex songs and so on. And a lot of these at first glance seem to be just totally impractical. So are these the product of something that you talk about called runaway selection? Well, that's the interesting question. The conventional answer, which Wallace insisted on, which Darwin didn't agree with, and which dominated sexual selection theory for the next century, was that the information that the female got from the beautiful tale of the peacock was enough to tell her that the male had good. genes, disease resistance, general vigor, healthy in some way. And that remains the sort of

default idea at the moment. But another idea, which occurred to particularly a man called Ronald Fisher, a statistician and geneticist in the early 20th century, writing in 1930, and was then proved mathematically to be spot on by a guy called Russell Landy and someone called Mark Kirkpatrick in 1980, this is the idea of runaway sexy sons. Yeah. What this means is that suppose the peacock's tail is genuinely useless, does not have information about the male being high quality, but the female has nonetheless decided that it matters, because other females in the population share the same preference for

decorated tales, then she is bound to follow the fashion, because if she doesn't, her sons won't get to mate. So the genes she's after are not for survival in the next generation, but for the ability to seduce females. And once you start thinking this idea through, you realize that it's a sort of circular argument, and it would result in a rapid acceleration of both the size of the tail and the ornamentation of it and of the female preference for the tail, that the two would co-evolve in an accelerating runaway fashion. That's the central argument. And I think the best evidence that it's right, at least partly right, is that the features that get exaggerated in male birds are so random, so arbitrary. And by the way, just to go back to Darwin for a second,

He said, the feathers in a peacock's tail when I gaze at them make me feel sick. It's an extraordinary remark, and this was in a letter to his friend, Aza Gray. And what he was saying was, I can explain anything through evolution by natural selection, except useless beauty. What is the point of it? The conventional answer is that God made the peacock's tail beautiful, that human beings could appreciate it. Darwin didn't find that a satisfactory explanation. He needed another one. He didn't in the end come up with the fish a runaway idea. That had to wait

another 60 or 70 years, but it does seem to me to be quite a good one. So one interesting point about these that I think a lot of people will raise is surely if the birds are putting so much sort of evolutionary energy into producing these flamboyant ornaments, these huge cumbersome tails, the bright colours that make them stand out, then surely that makes them more prone to predation, which is going to go against their survival. So what's the balance going on there? Well, this is exactly the point, and this is why Wallace might well have been wrong

to say that this is really just natural selection at one remove, because natural selection is, as we say, survival of the fittest. Now, if growing an enormous flamboyant tail or a lot of colourful plumage makes you stand out and costs a lot of energy, then the chances are that you're more likely to get picked off by a fox or an eagle at some point and you won't even get to mate at all. If that's the case, then why doesn't a dull-colored bird emerge instead and do better? And the answer is because the female has a preference for the plumage of the bright, colored birds, and the brightly colored bird therefore gets to mate with more females, or gets to mate sooner with females, and leaves more offspring behind.

So, yes, it can be true at the same time that this thing is deleterious to the survival of males, but advantageous to the reproduction of males. So I really like this idea about the possibility that females, they don't make choices sort of based on personal preference. they kind of go for the popular option. So what do we know about that? Right. Well, it follows from the Fisher runaway hypothesis

that females are going to really want to be following the fashion of other females because what they're concerned about is producing a male who doesn't manage to attract mates in the next generation. And therefore, it's possible that females might genuinely, be copying each other. They might be looking round at a leck and saying, right, who's she mating with? Ah, that chap. Perhaps I better mate with him too, because that's obviously the attractive male. So it's a completely circular argument, as you can see, you know, but it's one that nonetheless the females might follow. And the evidence for copying by females in various species in terms of their choice of mate is quite good, but not overwhelmingly good. There's some experiments which

have failed to find evidence of copying and some which have found evidence of copying. No one goes to Hank's for his spreadsheets. They go for a darn good pizza. Lately though, the shop's been quiet. So Hank decides to bring back the $1 slice. He asks co-pilot in Microsoft Excel to look at his sales and costs to help him see if he can afford it. Co-pilot shows Hank where the money's going and which little extras make the dollar slice work. Now, Hank has a line out the door. Hank makes the pizza. Co-Pilot handles the spreadsheets. Learn more at M365 copilot.com slash work. When you need to build up your team to handle the growing chaos at work, use indeed sponsor jobs. It gives your job post the boost it needs to be seen and helps reach people with the

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digital precision whilst sustaining Pratt, pace, rhythm and timing, the elusive quality that makes music feel alive and gives it emotional texture. Today, in partnership with French acoustic specialist's focal, name audio creates systems that deliver. a exceptional sound and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit focal powered by name.com for more information. So how about the difference in appearance between males and females of some species? Like typically the females, they're much duller.

So is that just a consequence of them doing all of the choosing, in essence? If you take birds, in many species the female is brown and well camouflaged, the male is brightly coloured. In some species it's the other way round, phallaropes, dotaroles, jacanhas, these are a small number of birds in which the females are brightly coloured and the males are dull. And then in other species, both sexes are brightly coloured. So puffins, parrots, pigeons and a number of birds like that. Sea birds are often in that category. What's going on here? Well, the general answer, is that where the bird that is going to sit on the eggs wants to be inconspicuous, that's often the female.

But if they're nesting in a hole in the ground or a hole in a tree, she doesn't mind about that. So those species tend to have brightly coloured females, actually. And where the male is going to do the egg sitting, like happens in Jocanas and phallaropes and dodderals, then it's going to be the female that's brightly coloured and the male that's dull. So there was a man named Robert Trivers in the 1970s at half, who suggested the parental investment theory of sexual selection.

He said, whichever sex devotes more energy and time to bringing up the offspring, either by sitting on the eggs or feeding the chicks, will be the dull-colored one that the other sex competes for. The other sex will become the brightly colored competitive one desperately trying to attract the attention. because the one that sits on the eggs is a scarce resource, which it's worth fighting over, if you see what I mean. Yeah, absolutely.

So another sort of well-known feature of birds is their song. So in the book you write about curlews, skylarks, great snipes, etc. And typically these are much less colourful birds because they use their songs, I presume, to attract mates. Some of the bird songs are incredibly complicated. So can we draw a parallel between the sort of, evolution of these elaborate ribbon-like tails and the complexity of the songs? Yes, I think this is a very interesting point. The song of the Nightingale, or the Skylark,

is an auditory peacock's tail, as it were. This bird, a nightingale is a dull brown bird, so is a skylark. There's not much difference between the sexes, but the male is able to be incredibly flamboyant in an auditory way rather than a visual. way. So some birds have chosen to go down that route and some birds have chosen to go down the other route. And as a rule, actually, brightly colored birds don't tend to sing very well and dull-colored birds do tend to sing better. It's not a hard and fast rule, but it is a sort of general pattern. So birds have a choice. Now, the advantage of a nightingale song as opposed to a peacock's tail is that you can shut up when the predator's approaching and you're no longer conspicuous. On the other hand,

you're very conspicuous in all directions when you're singing, etc., etc., even if you're trying to hide. So, yes, the birds have two ways of doing it. One is to go for extremely complex and extravagant songs, and the other is to go for extremely complex and extravagant plumage. So one question that a lot of people will ask then is, if this is for the purposes of mating, why do birds sing when they're not mating?

Well, mostly they don't. Robbins sing through the winter in Britain, but pretty well no other bird does. Robbins use those songs to tell other males that the territory is occupied. Most birds start singing in February, March, as the breeding season approaches, and they go on singing through April May. I'm talking about Britain, obviously. It's different in other parts of the world. Now, interestingly, people were puzzled by the fact that bird,

went on singing after they acquired a mate. Because a lot of birds, one male mates with one female, and they both helped to bring the up the offspring. So hang on, he's already, you know, there's a song thrush that's been singing outside my window this morning, for example. It may well already have a mate. We're well into March now. They might already be, well, let's take a missile thrush, because they will already be building a nest. A missile thrush will. And so if the two are already building a nest, why does he need to sing? Well, we found out the answer to that in the 1990s when we started genotyping birds and we discovered that a nest full of eggs isn't necessarily fathered by one male, that actually there's quite a lot of what's called extra pair copulation

going on in these supposedly monogamous birds. So basically the female is sitting on the nest listening to her husband belting out a song in which he is saying, anyone fancy a bit on the side. And some females do. So the sperm competition theory, i.e. females mating with more than one male, has become an interesting aspect of evolutionary biology that was unexpected. So going back to the different parallels between the songbirds and the flamboyant colorful birds. presumably the sexy son hypothesis stands for the songbirds as well the better singers produce the more desirable offspring

is that right? Well again it's exactly the same argument which is still unsettled either the nightingale's song is telling the female I'm strong I'm healthy I'm disease resistant you therefore would like to mate with me because my genes will be good for your for the survival of your offspring. Or the song is saying, I'm good at singing,

your sons will need to be good at singing in order to attract a mate, and therefore in order to have sexy sons, you better mate with a sexy male. Both are probably true to some degree, but it's exactly the same phenomenon. And the extraordinary elaboration of a song, like the Nightingale song, or some of the warblers, the sedge warbler, you know,

imitates hundreds of different birds that it has heard in Africa during the winter and sings those songs in spring in Britain, that's not necessary for the simple business of telling the female you're fit, but it is necessary perhaps for the business of telling the female that you are the best singer in the neighbourhood. So let's move on to another absolutely fascinating behaviour then. So rather than having these flamboyant features or songs, Bowerbirds set up sort of mini artworks in which to impress females. So what can we say about that? This is, I think, one of the most extraordinary of all evolutionary phenomena, one that I was thrilled to have a chance while I was researching this book to go and experience. In Australia and New Guinea, there is a group of birds called Bauerbirds, some of which are very colourful, but some of which are not particularly.

particularly colourful, but what they do is the male builds a bower out of usually sticks or grass stems and it's shaped like a passage or a tunnel, sometimes like a sort of like a little house, sometimes just a huge pile of sticks. In each species the bower is a different shape. But then it decorates the bower with colourful objects and separates those by colour into different piles or different patches. So for example, the great bower bird, which I watched in Queensland, has a bower made of sort of grass stems and sticks, then lays a sort of platform of grass stems and sticks on the front of it, and then puts green objects to one side, red objects next to them, and then white objects in front of the bower. White objects in the case where I looked at the bird

consisted mostly of man-made objects. There were snail shells and bones, too, but on the whole, bits of plastic, bits of broken glass collected from the nearby town. And that included, in this case, a toy plastic hand grenade and a toy tiara that some poor girl had lost while playing in the garden. The bird had pinched it, presumably. And these art installations that these birds build prove that we didn't invent art. The birds did. Now, experiments in these Bower birds by either enhancing or detracting from the number of coloured objects on the bower prove that that's what the females are looking for. The more coloured objects you have, the more likely they are to agree to mate with you. So it's literally a case of, come and see my etchings, I've got a better collection than the other chap.

Now will you mate with me? There's nothing new under the sun. So, and the females sort of approach the males as if you would, if you were going into a gallery to buy some art, don't they? They sort of have a good look around, see if something there's there that takes their fancy. And if not, then they move on to another one. I think that's absolutely hilarious. Yes, and I watched in the Great Bowerbird case, I watched the female standing in the Bower, looking around and saying this, and the male is terribly excited. And he's buzzing backwards and forwards, making a funny buzzing noise in front of her.

And he picks up a red chili pepper. and he holds it up and says, see, I've got a red chili pepper. And she apparently is, well, she didn't mate on that occasion. So maybe she wasn't as impressed as she could have been. And I haven't seen them, but some of the New Guinea-Bowerbirds in the rainforest have the most extraordinary displays that are really so elaborate and so complex and involve building these enormous structures, decorating them like a sort of Christmas tree, and then hiding behind them and popping out suddenly

and flashing your red crest at the same time, that you almost feel sorry for what the females have done to the males of these species. They've put them through these ordeals. You know, I'm not going to mate with you unless you do all of these fancy displays, but also build this extraordinary art exhibit. Oh, and by the way, the males steal ornaments from each other's bowers. Amazing. So it's quite interesting in that they spend like so much effort, months or whatever.

And then the actual act of copulations over in seconds. Well, in a lot of these species, and this is true of the black grouse that I particularly focus on in the book, the pair bond is extraordinarily short, as you say, a couple of seconds to mate, and that's it. The female will then go off and do all the rearing. In other words, the bear bond goes on for months, years, or even decades, and those species tend to be less ornamented, obviously. For so little effort in terms of actual mating, the male ends up having to put in an enormous effort

in terms of display. So the Blackgrouse are occupying the leck every dawn from about October through to about June. Now, in really bad weather in January and February, they won't bother to go to the leck. But otherwise, they're there every day. day and when they're there, they have to strut, display, flare their tails, croon these sort of bubbling noises and then make a noise like opening a can of lager, a sort of ch-o-h-h-h-noise, and it's a lot of effort.

And what the work of Carl Salsbury and his colleagues in Finland has found is that the male that gets to do all the matings at the centre of the leg, has put in so much effort to impress the females that year that he's knackered. And his chances of being alive the next year are 25%. So three quarters of the males who managed to mate are dead by the next spring. And some of the others survive, but are beyond their best. And I watched one male in people. And I watched one male in particularly who was most of them of course you can't recognize them from one year to another unless you've marked them because they've shed their bloomage and grow fresh feathers but this one's called wonky tail because his tail is at an angle some kind of spinal deformity i presume and he's

the same every year and we watched him for four years consecutively and in the third year i think it was he did get one mating but in the fourth year he was just a shadow of his former self he didn't look at all well and he disappeared before the end of the season. Thank you for listening to this episode of Instant Genius, brought you from the team behind BBC Science Focus. That was Matt Ridley. To discover more about the topics we've just discussed, check out his book, Birds, Sex and Beauty, The Extraordinary Implications of Darwin's Strangest Idea. If you liked what you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to see our

presenters and guests in person, then please do also check out our YouTube channel at ScienceFocus. The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines or download us on your app store of choice. You can also find us on Apple News or online at sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analogue warmth. Alongside French acoustic specialist vocal, Name creates high-end audio systems combining innovation with craftsmanship,

so you can listen to music, just as the artist intended. Discover more at nameadio.com.