Do Beautiful Birds Have an Evolutionary Advantage?

Birds are not merely descendants of dinosaurs — they are dinosaurs. For Yale evolutionary biologist and ornithologist Richard Prum, birds have been a lifelong passion and a window into some of evolution’s most intriguing mysteries. In a wide-ranging conversation with co-host Janna Levin, Prum traces the deep evolutionary origins of feathers, which he argues first emerged not for flight but for insulation, camouflage and display. Their colors — often invisible to the human eye — come into sharp focus under birds’ ultraviolet vision, suggesting a sensory world far richer than our own. Prum also explains why he champions Darwin’s once-marginalized theory of sexual selection, which proposes that traits such as the peacock’s tail evolved not for survival, but simply because they were attractive. Beauty, in other words, may shape life as powerfully as utility.

• 0:00Speaker 00

  Hi, I'm Terry Tao. My new book, Six Math Essentials, explores six concepts that have guided mathematicians like me throughout history. With a simple introduction to numbers, algebra, geometry, probability, analysis, and dynamics, I share how these ideas helped me think mathematically and can help everyone better understand our complex world. Mathematics isn't magic. It's a way of thinking anyone can learn. Please visit quantobooks.org to learn more.

• 0:31Speaker 01

  I'm Janna Levin.

• 0:32Speaker 02

  And I'm Steve Strogatz.

• 0:33Speaker 01

  And this is The Joy of Why, a podcast from Quantum Magazine, exploring some of the biggest unanswered questions in math and science today.

• 0:44Speaker 01

  Steve, hi.

• 0:45Speaker 02

  Hi, Jana.

• 0:47Speaker 01

  I've been thinking, you live in a more naturalistic environment than I do. I'm in Manhattan. Do you get to do any bird watching where you are?

• 0:55Speaker 02

  Oh, yeah. Occasionally, I do.

• 0:57Speaker 01

  Do you pull out the binoculars, the whole thing?

• 0:59Speaker 02

  I'm not a binocular person, but I have a very good friend. And so she will point out all kinds of bird behavior and calls. And yeah, so I don't know, but I'm one step removed from someone who does.

• 1:11Speaker 01

  Well, it's surprising there's bird watching in Manhattan. People love it in Central Park. There's like a real place to go. And obviously, birds are very connected with their environments. They adapt very much to their environments. They fly from one part of the world to another part of the world. And so it's all kind of connected. I had the chance to talk about this with a birder who loves to watch and listen to birds, but is also an evolutionary ornithologist over at Yale. His name's Rick Prom. And we had a really fascinating conversation about how birds evolved, how they are ultimately not just related to dinosaurs, but are dinosaurs.

• 1:49Speaker 02

  They are the living dinosaurs. I remember that in Jurassic Park, they tried to make them kind of look like a bird.

• 1:54Speaker 01

  Right. All of this is a really interesting aspect of understanding not just bird species and dinosaurs, but also fundamental ideas in evolutionary biology itself.

• 2:07Speaker 02

  That's really interesting. Great.

• 2:09Speaker 01

  So here is Yale professor Rick Prom.

• 2:16Speaker 01

  Welcome to the Joy of Why, Rick. It's a pleasure to speak to you.

• 2:20Speaker 03

  Thanks for having me.

• 2:21Speaker 01

  I didn't realize how many people are absolutely obsessed with birds. This is a real thing.

• 2:28Speaker 03

  It is, and it's a growing thing.

• 2:30Speaker 01

  Is it?

• 2:30Speaker 03

  You know, during COVID, when people needed to get out, a lot of people discovered the outdoors and bird watching, right up there with gardening and getting a dog, were one of those things that really inspired people.

• 2:43Speaker 01

  As I understand, you were birding very young. You were like a kid and you would hang out with adults who would come fetch you to go birding.

• 2:50Speaker 03

  Yeah, I got my first pair of glasses in fourth grade and the world came into focus. And within a few short months, I was a bird watcher and already realizing that my life would be bird-filled and trying to figure out what that meant. And at first it was bird watching and later academic study of bird evolution and everything else.

• 3:11Speaker 01

  A lot of things must have come into focus. What was it specifically that drew you to the birds?

• 3:16Speaker 03

  Hard to say. I remember distinctly seeing a copy of the Peterson Field Guide in a bookstore and looking at the cover, which had a puffin and an evening gross speak, and then looking at the maps and imagining all the places you would have to go to see all the birds. And the romance of that, the travel, this hunt, was immediately clear to me.

• 3:38Speaker 01

  And you do travel the world.

• 3:39Speaker 03

  As much as possible. This kind of job thing interferes with that because there literally is an open-ended need to go to basically every country, every little isolated mountain range in the world, every island to see the birds they have there. That's a lower priority, but still a life priority.

• 3:55Speaker 01

  Bird watching also strikes me that it's bird listening. Are you as into the bird song as you are into the visual observation?

• 4:03Speaker 03

  Absolutely. You know, because I was drawn to birds at the same time that I was getting glasses, it turns out that my advantage of birding was always my ears, right? Acoustically identifying birds, tracking them down, that was really my edge. But unfortunately, I started losing my hearing a lot, you know, bad luck, disease in grad school and then later on in my 30s. And so now I'm really kind of hearing impaired, which is a bad thing for an ornithologist. But hearing aids are fantastic technology. They help out a lot, but they don't help out completely.

• 4:35Speaker 01

  So there's some bird song that you remember, but you no longer can experience again.

• 4:41Speaker 03

  A whole world of birds. Actually, I remember I did some fieldwork on an exotic bird called the Velveta Citi in Madagascar in 94. And that was at the time when I had some hearing loss, but had not really advanced very far. And I got a grant from the National Geographic Society to go back and study its courtship display, its leching behavior. This bird is super black with green wattles over its eyes. And we had some insight into its breeding season, but not enough. So I got this grant. I went back. I recruited three people. We went first thing, straight to the point on the trail where we knew we would find these birds. And there was orange, orange bandit still, sitting on the same tree.

• 5:26Speaker 03

  And he kicked back his head and opened up his mouth. And he sang, and I couldn't hear it. And this was a song that I had described for science.

• 5:36Speaker 01

  Wow.

• 5:37Speaker 03

  So there's a whole world of birds that I'm missing. I now have transposing hearing aids, which help me hear them. But you know, when a piccolo and a saxophone and a flute are all transposed down to the level of a bassoon, they all sound the same. So I'm missing some aspects of that. I had to develop a new way to relate to my life's work. And that kind of personal challenge, you hope that it works, but it can be a challenge.

• 6:03Speaker 01

  You adapted. I mean, it's sort of a metaphor for life. We're all limited in our senses, right? We can't see the way the birds see. We can't hear the way we're all kind of confined. And even if it's just in our human range.

• 6:14Speaker 03

  Yeah.

• 6:15Speaker 01

  I want to get into some of the evolutionary forces, the real hardcore science of what shapes birds and the proliferation of birds. And I want to begin with this fascinating idea that birds aren't descended from dinosaurs, that they are dinosaurs. I find that remarkable.

• 6:33Speaker 03

  Indeed.

• 6:34Speaker 01

  Can you help? Because I thought they were descended from dinosaurs, and that was the big revelation.

• 6:39Speaker 03

  Interestingly, to say that birds are descended from dinosaurs is to imagine the diversity of life as a kind of Scala Natura, where things come from lower levels. But one of the most fundamental and I think important commitments in evolutionary biology is the idea that the history of life is a tree, that it's a hierarchy. And tree thinking, which is actually about how these historical lineages are related to one another in time and to the events in evolution that we're always thinking of, the origin of the feather, or the origin of the seringues, the gizmo that birds sing with. So, yes, the birds are dinosaurs, implying that they are a branch within the dinosaurs, and that branch doesn't go away just because some of the branches went extinct.

• 7:24Speaker 01

  Fascinating. And as I recall, correct me if this is wrong, that in Darwin's Origin of the Species, there's only one drawing, and that's the Tree of Life. Is that right?

• 7:34Speaker 03

  Yeah, it was a phylogi-- it was a sort of micro, a little imagining of speciation over temporal strata. But he, of course, concluded the book with a statement that if all this makes sense, then all life is related in one great tree of life. And really kicked off the idea of the Tree of Life as both an intellectual construct and an empirical area, a thing to discover. And of course, that's a big job, and we're still working on it. But it is a major and important focus of evolutionary biology right now.

• 8:05Speaker 01

  Fascinating. So is the subgroup that they belong to, the theropod?

• 8:09Speaker 03

  Yeah, birds are part of that very popular bipedal, mostly meat-eating, very active--

• 8:16Speaker 01

  T-Rex.

• 8:17Speaker 03

  Yeah, velociraptor, chasing the kids around in the kitchen.

• 8:20Speaker 01

  This is fun.

• 8:21Speaker 03

  Those are the dinosaurs that are most closely related to the living birds.

• 8:26Speaker 01

  And we start to see them appear in the fossil record pretty early. Is that right?

• 8:30Speaker 03

  Sure. The classic, of course, is Archaeopteryx, a fossil bird that was discovered in Germany in lithographic limestone in the mid-19th century. And it goes back to the late Jurassic, so 170, 160 million years ago. And that was, of course, almost the whole literature was about that one bird. Now, in the last 30 years, coming out of Liaoning in northeastern China, are a whole series of discoveries that are just quite amazing. And those have contributed a huge amount to our knowledge. And now dozens and dozens of species up and down the tree from that area are present.

• 9:10Speaker 01

  I was also wondering about the tremendous variety of bird species. Is this unique to birds, this incredible diversity of species? And is it modern? Or do we think that was like that with dinosaurs?

• 9:25Speaker 03

  Of course, if you were somebody studying beetles, you would laugh at the statement that there are lots of birds.

• 9:33Speaker 03

  We got a whole genera that are just one genus. It's got more species in it than you got, all your birds, right? So, yes, all those kind of statements are relative. But one of the things that's really interesting, I think, and has contributed, I believe, to the diversity of birds, is their cognitive complexity. Their social and sexual choices lead to differentiation, which means that they can speciate rapidly, that they can become different and irreversibly different. And I think that has really contributed. Another one is migration, that they can live in one place and then fly a continent away for another time of year. And that means that they're like endless summer. They're riding the surf in different places.

• 10:16Speaker 03

  And that allows something that sessile or even slowly moving organisms can't pull off. And these sorts of things have really, I think, contributed greatly to the number of species of birds.

• 10:28Speaker 01

  When you're looking at some of these archaeological records, can you tell when feathers started to originate? And if they were immediately attached to the functionality of flight? Or was there a different reason why they might have evolved feathers?

• 10:43Speaker 03

  Yeah, the origin of feathers has always been right up there with the origin of birds as a central question. Birds themselves, feathers and flight. It's like the holy trinity of ornithology, right? So where and how did the feathers come from? Again, since the mid 19th century, we have all focused on archaeopteryx. And that was this amazing fossil with lots of intermediate states between modern birds and reptiles. But one thing about it, you look at its feathers, they were almost entirely modern. So you couldn't learn anything about the origin of feathers from archaeopteryx that you wouldn't learn from, you know, a roadkill pigeon out on the street, right? It really is, they're feathers.

• 11:24Speaker 03

  For more than a century, the literature consisted of imagining backwards from modern complexity to some ancestral feather type thing. And most people, because the idea of adaptation as a strong force, natural selection as a strong force dominating everything in evolutionary was so strong, people thought, oh, well, feathers are obviously good for flight. So as we imagine backwards from modern type feathers to some antecedent, we should probably think about something that would evolve for flight. And so the main idea was elongate scales, that feathers are skin structures, they share some developmental features with scales.

• 12:06Speaker 03

  What we imagined was scales getting longer like shingles on a house and then catching air and then really, birds really evolving to invest in gliding and flying and that ultimately you would get feathers. So of course, that guided what people were looking for and they never found it. And one of the reasons is that they were looking for something that didn't make any sense. It wasn't actually related to how birds evolved. What we really need to do is try to understand the pattern, tree thinking, right? What were the stages of feathers and how did they distribute on the tree? And in this case, development. So we came up with a theory of the origin of feathers based on how feathers grow.

• 12:47Speaker 03

  And that implied that feathers were first tubular, then a tuft, then a vein, a flat surface, but not integrated, and then a vein that zipper together. So only this most complicated kind of feather could have been functioning in flight. So in other words, saying that feathers evolve for flight is like saying that digits evolved to play the piano, right? That's some of the most advanced thing you could do with your fingers, right? We have, I think, a really good idea of feathers were in those early stages, but still a very broad idea of what they could have been for. Could have been insulation, if they were fuzzy like hair, could be like a porcupine quill, right? It could be defense. But it's clear that thermoregulation and water repellency and camouflage and social

• 13:32Speaker 03

  display were all probably very early functions, right? Flight is only the last thing that feathers were put to use for.

• 13:42Speaker 01

  Fascinating. And do we think that dinosaurs were colorful? I was always shown in the natural history museums, these sort of gray brown dioramas. Right?

• 13:52Speaker 03

  Again, that required extrapolating backwards. You know, we're recreating from these materials, what the skin muscles might have looked like, and then what the surface might have looked like. Right? So a lot of speculation there. But certainly for most of the history, people looked to alligators or crocodiles to imagine what dinosaurs were like. And that's drab and greenish and not ornamented. But now that we know that feathers evolved in deripod dinosaurs, prior to the origin of flight and prior to the origin of birds, the real question becomes, what were they good for? In my work in ornithology, I had done work on the origin of feathers and then lots of work on bird coloration, in particular, structural colors, optical colors in bird feathers,

• 14:38Speaker 03

  and more recently, pigments. But I never really expected these two directions of my research to come together. But that's what happened in the late 2000s. An opportunity arose to look at extraordinary preservation of pigments. It turns out that melanins, like what make red hair and brown hair and black hair in humans, actually fossilized beautifully under the right conditions.

• 15:01Speaker 01

  Really?

• 15:02Speaker 03

  Yeah, they're packaged in the cell, in the living cell, in what's called a melanosome, where the pigments are polymerized into a durable molecule. And it looks like a little package. And it's membrane bound and it's passed over to the hair or to the feather cells. And people had started looking at feather fossils with an electron microscope. And they saw these little granules and they thought they were bacteria that had eaten the feather at the time of its fossilization. But it turns out they were bacteria, something of the same size scale, really similar looking. They were melanosomes. So we were able to first discover that melanosomes fossilized beautifully, that they are melanosomes, not bacteria.

• 15:46Speaker 03

  And then interestingly, it turns out, at least in birds, the melanosomes vary in shape depending On their color with theomelanin, the red brown, red hair melanins being more jelly bean shaped and the the umelanin being more hot dog shaped. So that meant we could actually diagnose the color of some dinosaurs. So that was really exciting and it's now a huge area of paleontology to look at fossilized coloration.

• 16:12Speaker 01

  How do we understand the bird's perception? For instance, or the dinosaur's perception of these colors? Even living birds, modern birds, they look one way to us, but they might look very different to each other.

• 16:28Speaker 03

  Well, we have been working that, of course, that's a big area. Many labs in the world focus on avian sensory ecology, visual sensory ecology. It turns out that looking around us, almost any environment you go into, you're going to see a wild diversity of colors. I'm looking at, I've got all kinds of books. I've got a baseball hat. I look outside, I see amazing flowers in the garden, right? All those colors. Well, it turns out that we have kind of a crappy retrofit color vision compared to other vertebrates. We ain't got nothing compared to birds. It turns out birds see in four colors, red, green, blue, but also violet or ultraviolet. There's a fourth cone.

• 17:07Speaker 01

  Like outside of the range of human vision?

• 17:10Speaker 03

  Yeah, it sees well down into the near UV. And to make a long story short, birds see these colors. They make them in their feathers, often with structures, sometimes with pigments. And they have evolved in their daily lives. What's really interesting is how it's more than just an expansion of the breadth of color sensitivity. When astronomy, they're always saying, oh, wow, we put up this new telescope. It's sensing a whole new set of wavelengths. We're getting new images we never imagined, right? That's a product of the breadth. But it turns out that sensation of color is always a result of a comparison of the relative stimulation of different channels. Yellow is a stimulus of both the green and the red channel.

• 17:54Speaker 03

  Turquoise is a stimulus of both the blue and the green channel. So the play of color is a result of this relative stimulus. Well, it turns out that birds, since they see UV. They have a whole new dimension of color perception. They see colors like ultraviolet yellow and ultraviolet green, which are as different from green and yellow as purple is from a red or blue. We now know that they perceive these colors as distinct. They can learn them and make choices based on them. And that speaks to the richness of the sensory system they have and how they use them in their plumages and social lives.

• 18:33Speaker 01

  Fascinating. I guess it never occurred to me that some of what we perceive as color is just part of this hallucination in our minds, right? That we're not seeing colors just linearly on the spectrum. We're superposing colors and creating, in our minds, the impression of something like purple, or in the bird's case, the ultraviolet yellow. So what do you think the role is of this rich color perception and display for birds? Is it a defense mechanism or is it just a mating advantage? What's going on there that makes birds so incredibly colorful?

• 19:11Speaker 03

  Yeah, well, a lot of levels to that question. One is that at least part of this system evolved in fishes, right? The four colors sensitivity, that's ancestral. Great idea, probably necessary for murky perception in water and trying to find food and all sorts of things like that. But once vertebrates came onto land, suddenly they had all this sensory capability. They weren't going to chuck it out, they were going to use it. And they have used it in all ecological, food finding, awareness of predators, looking for prey, all the things that vertebrates do. But of course, when they become socially complicated and they're interested in mate choice or other kinds of social interactions,

• 19:56Speaker 03

  these sensory systems are going to be brought to bear on those decisions. And you end up with a dynamic interaction between sensory biology and the biology of the organisms.

• 20:07Speaker 02

  Wow, totally news to me. I had never heard that. Even the idea of the ultraviolet perception was news to me.

• 20:14Speaker 01

  Yeah, I had heard this before, but he really expanded on this point that birds really visualize differently. Sometimes we'll look at a bird, we'll see a plain bird, but they will see something very vibrant.

• 20:27Speaker 02

  That's interesting just to think of in itself because something like a robin, you know, okay, yes, robin has that orangey chest, but otherwise kind of drab looking. And it just makes me wonder, do we know what a bird might see? Is there any way to convey that to us?

• 20:39Speaker 01

  Right. I mean, I think it's like anything, if we look astronomically in the ultraviolet, we have to map it to something we can see anyway. But yes, I have seen that sort of as best we can, right? And it is incredibly different, but it really led to something that I thought was quite deep, which is beauty and the role of beauty. Well, we're going to take a quick break, but when we get back, Rick Prom is going to describe to us why some birds are so beautiful, why some birds are so ugly, and why this might actually be controversial and lead to some deeper insights into Darwin and natural selection.

• 21:28Speaker 01

  Welcome back. We're here with Yale professor Rick Prom discussing the evolution of birds.

• 21:35Speaker 01

  Now, back in the 19th century, I know you've talked about this a lot in your work, Charles Darwin really struggled with the role of the aesthetics of birds. Famously, the peacock display seeming so egregious and counter to the need for fitness and survival. And I believe Darwin just hated that peacock feather.

• 21:57Speaker 03

  Indeed, he said, you know, the sight of a peacock's tail, whenever I look at it, makes me sick, right?

• 22:03Speaker 01

  And why did he say this?

• 22:04Speaker 03

  Well, because he had an intellectual challenge. He was trying to come up with a naturalistic explanation of biodiversity. So, having already in 1859 published The Origin of Species and proposed natural selection as a force in the evolution of diversity, he realized that he had a problem. And one of the problems was a beauty problem, right? And he realized ornament and antlers, et cetera, birdsong, plumage, will not help with survival or fecundity, you know, raising lots of babies. They needed to have a different function. And so, instead of just resting on his laurels as the most famous scientist in the world,

• 22:46Speaker 03

  he spun around and proposed another mechanism of evolution, which he called sexual selection, which was a consequence of differential mating and fertilization success. And he proposed there would be two kinds of ways it could happen. Physical competition for control over mating opportunities, usually among males, and mate choice, where individuals of both sexes could, based on their perceptions, choose a mate. And in that area, he explicitly pulled upon aesthetic language. He described the ability of birds to charm. He described mating preferences as aesthetic standards. And he was also explicit that this idea of sexual selection was distinct from adaptation, right?

• 23:29Speaker 03

  He said that these advanced kinds of beauty could function for attraction and for no other purpose, by which he meant no other adaptive purpose, right? He proposed a whole new idea.

• 23:40Speaker 01

  Are you really saying that sexual selection was proposed as being fundamentally different from natural selection, as opposed to a complex subset of natural selection?

• 23:48Speaker 03

  Yeah, absolutely. And the kind of controlling response to get sexual selection back on the ranch was a part of the immediate response to Darwin's proposal in 1871 in The Descent of Man. He wanted to be able to describe these ornaments, and he knew that in some cases they could Could be congruent with adaptation by natural selection. That is, that one male could actually be signaling that he was better. But he explicitly kept it as separate. But Darwin's theory that male competition structured the social and sexual world of animals was so congruent with Victorian culture that it was a big winner. It was like instantaneously. I think it went a long way to contributing even further to the acceptance of the idea of

• 24:33Speaker 03

  evolution in general.

• 24:35Speaker 01

  Men knocking antlers, fighting over territory.

• 24:39Speaker 03

  Right. But his idea that mate choice, in particular female choice, was a force in nature was a big loser.

• 24:47Speaker 01

  It was suggesting females had more power than they ought to have in a Victorian society?

• 24:51Speaker 03

  And indeed, even in some of the initial reviews of the book, many of the critiques were explicitly misogynistic. Female preference is going to be one way one day and one way the other. How could it ever arrive at something as amazing as The Peacock's Tale? Right? But the real critique that shut it down was the idea that if it were to happen, it would be under the control of natural selection. That is, that mate choice would always be for betterment, for objective improvement, the mate that was absolutely the best. Right? And that kind of response really led to people saying, well, if that's the case, then sexual selection is just like natural selection and we don't really need it anymore.

• 25:34Speaker 03

  That was Wallace's response, Alfred Russell Wallace. We don't really need sexual selection anymore. And that's really what happened. For almost a century, sexual selection was kind of a, oh, an odd idea by Darwin and put away in the archive of ideas. And when it was revived in a century later, in the 1970s, it came back really in Wallacean form. It came back as a kind of natural selection. And that's still an intellectual debate.

• 26:00Speaker 01

  And you don't buy that from the way you're phrasing.

• 26:03Speaker 03

  Obviously, we can structure these ideas about process however we want. Animals don't care. It's still going to be happening. The question is, what are the kind of definitions that actually further the advance of knowledge? Best. And I think, and have argued for a long time, that we ought to entertain or create a biology that is authentically Darwinian, reestablish this separation of sexual selection and natural selection, and think of the adaptive forms of sexual selection as a special kind of interaction between those two forces.

• 26:35Speaker 01

  I want to ask about the social aspect of the birds that show these displays. Do they tend to be more social birds? The way not all birds sing, and those that sing also dance, and those that sing and dance tend to have more social cohesion and more social networks.

• 26:51Speaker 03

  All kinds of ecologies support all different sorts of social arrangements, and those really come first. The food is distributed like this, and you need so much help to raise the babies. All those kinds of ecological features are going to structure how breeding systems or family life occurs in birds. And what's great is that something like 12,000 to 15,000 species of birds, we got a whole bunch of different situations, right? We've got the vast majority, we're monogamous, with at least two social parents at the nest raising the offspring. We have these wild displaying species, the most extreme, with male display and all-female care.

• 27:34Speaker 03

  We've got polygynous species like Chicanas with the long toes on the lily pads, where the female is 40% larger than the male. That's like almost as big as a mountain gorilla, male to female. But this is female to male. So the females are huge. They occupy a large portion of marsh. And if they have enough resources, then multiple males will nest with them. The female lays the eggs, and the males do all the parental care. So there's a lot of varieties of social system and breeding system. And in all of them, communication, color, song are a vital part of how they get there.

• 28:10Speaker 01

  So if the social organization is part of the adaptation, you know, natural selection can lead to or away from these sort of social arrangements. Could I also imagine that sexual selection is just a variant on that spectrum? That animals that sexually select are doing so because that's the survival of the social organization? That it's not just the survival of the individual, but you know, the social way that species was successful?

• 28:38Speaker 03

  Yeah, that kind of unifying idea is attractive. However, it's clear that when we study biodiversity, we see that the song is a really specific part of the phenotype, the behavior of the individual, and is displayed in certain contexts, certain times. And so to explain that portion of the phenotype, we need to look more specifically at how does song work? Or in what way do these plumage patches function? And when we do that, we find that those generalized explanations, it's all about survival, that's not sensitive enough to me.

• 29:11Speaker 01

  I have also heard that there may be the suggestion that females are conducting the song. They like this song, they don't like that song, and guiding the song to change over time.

• 29:24Speaker 03

  I mean, the reason why birds are so beautiful is because animals are making choices, right? They have sensory perception, some kind of cognitive valuation, do I like it or not? And they choose which one they like, and they go with it. Or in the case of a coral snake, it's like the opposite. A genre of horror in the natural world, ah, run away, right? Nature doesn't just do beauty. It can do beauty, or revulsion, or lots of other kinds of aesthetic. So when you have sensory perception, cognitive valuation, and choice, you end up with an evolution of some aspect of the body or behavior that functions in the brains of other birds. And that is not subject to the same kind of constraint

• 30:07Speaker 03

  as the beaker of the finch cracking a nut or a woodpecker pounding into a tree to get food. And I think those kinds of features are appropriately called aesthetic. And I mean that as a scientific statement in the way that Darwin did. So aesthetic evolution is an important feature of the social, sexual lives of animals, and it stands out at requiring a distinct explanation compared to adaptation.

• 30:34Speaker 01

  It's very easy to get caught up in, oh, the peacock is beautiful and all these birds are beautiful and we, maybe beauty is somehow this subjective character that is sought out in the animal kingdom, but you've raised other variants and I guess you're, those are falling under the category of aesthetic, but there are some really ugly birds.

• 30:53Speaker 03

  I guess we'd have to say ugly to whom?

• 30:55Speaker 01

  Exactly. So what do we mean by beautiful?

• 30:57Speaker 03

  Yeah, perhaps the most obviously ugly bird is a baby pigeon. And of course, lots of people haven't had to see that on the windowsill in their apartment or whatever, right? Truly ugly to us. And yet, when parents care for baby birds, they have sensory perception, cognitive evaluation, and choice. Which baby bird am I going to feed? And based on those things, you have the evolution of cuteness, the co-evolution of cuteness, where parents find the babies cute and the babies evolve. So there's all kinds of wild birds with tufts of feathers and colorful and beautiful mouth patterns inside.

• 31:30Speaker 01

  One of my favorites is the incredibly long waddle of some of these birds that just seem utterly impractical, especially if you need to fly.

• 31:39Speaker 03

  And puffy air sacs, too. All of these are interesting aesthetic features in different, what I really consider to be art world's natural aesthetic communities.

• 31:48Speaker 01

  I guess what I struggle with is, how does this kick off? Are you suggesting that there's some objective reason why a long waddle is beautiful? Or these hideous little pigeons are actually cute without some natural selection or evolutionary advantage? How does this emerge? Why would a bird select for this outside of natural selection?

• 32:09Speaker 03

  How would this kick off? That's an important question, right? And typically, the way it's deployed, at least in ornithology, would be, how do you explain the origin of song in the house wren, right? But the fact is that mate choice in birds started in the Jurassic. They have been making choices for a very long time. So it's unscientific to imagine that we should think about the origin of song in a species. Their greatest grandparents were already choosing. It's like, how do you explain the origin of sex in primates? It didn't happen in primates. So- If we wanted to know about the origin of choice, we should look at particular species where that is on the bubble or happening, and none of them are birds, right?

• 32:54Speaker 03

  But yeah, there's a lot of theory in that area. And obviously, one of the things about it is to say, even if you start out by saying, oh, I'm gonna like a bigger body size because that means the individual had a good diet, something's directly related to actionable, objective information that might be profitable, that might be evolutionarily advantageous. The challenge, of course, is that, once you have choices, you have the opportunity for lying, for fakery. Lying and cheating corrupt the information content of the ornament, and that process is just like part of life. And the idea that birds are somehow special and maintaining the purity and information content of their signals is a fantasy.

• 33:39Speaker 03

  And indeed, there's an intellectual problem trying to get people to imagine that there are things that can evade adaptation by natural selection, that can be less than optimal, or not even admit that it's everywhere. So that's kind of my intellectual job, try to keep pushing that view.

• 33:57Speaker 01

  I guess that's an interesting distinction. In some sense, you're suggesting it was selected for, perhaps, but the female was lied to. It was an exaggeration of fitness, or it was a sort of overindulgent display, and it spiraled away from what might have initially been selected for just on the basis of physical fitness.

• 34:19Speaker 03

  Well, if we just stick with the cartoon initially of females making choices of male display, which is a very specific setting, not universal abroad. But usually, the idea here from behavioral ecology generally is that sperm are cheap, there are many of them, and all sperm producers are competing to try to advance the interests of theirs in particular. And so the idea of the lying, cheating male, who will do anything to further his fitness, is not a remote or fantasy. This is the kind of the standard trope of evolutionary biology, right? Contrast, we have eggs, rare, expensive. Require more investment, should be more cautious.

• 34:59Speaker 03

  And so saying that lying and cheating by signalers is expected is not a remote or difficult hypothesis, right? The idea, of course, is that way evolutionary biology maintains the interest in what we call honest signaling, the idea that beauty is encoding information about quality, is with this knife edge, right? Where the signal is somehow maintaining its honesty and that cheaters cannot undermine it. I think that if we look at other aesthetic communities, like, for example, high fashion, right? You see that there's a whole lot of stuff that's extraordinarily successful and extraordinarily impractical.

• 35:39Speaker 03

  And indeed, I love now reading the fashion pages, which I find completely unfathomable. And clearly, a lot is invested. We've got millions and billions of dollars on the line of this color, that color, this hemline, this style. And then there's some real just artistic expressions. And it's almost like a world I will never understand. But it shows the fundamental nature of market bubbles within a system based on choices. I think there's lots of reason to imagine that rationally exuberant beauty happens in nature.

• 36:16Speaker 01

  Are you suggesting that stilettos are impractical for survival?

• 36:20Speaker 03

  Absolutely.

• 36:21Speaker 01

  I don't understand.

• 36:23Speaker 03

  And now we get to the idea pioneered by Amos Zahavi, an Israeli ornithologist, who proposed what he called the handicap principle back in the 70s. And this was a kind of intellectual idea about how you would maintain honesty, like if everybody's lying and cheating. And he proposed that the way to encode honesty is through the cost of the signal, right? And that if it was more costly than individuals didn't have those resources to pony up to make this incredibly expensive tail or to maintain their poise in stiletto heels, that they were showing that they were actually better because they could waste this energy, waste this investment.

• 37:03Speaker 03

  Zahavi's handicap principle is probably, in my opinion, irrelevant to nature entirely.

• 37:09Speaker 01

  Oh, so you really totally reject this because I have heard those arguments?

• 37:12Speaker 03

  Well, the only way that it could work, too, is if resources are distributed like money. That is, that you actually have money to waste, right? There has to be nonlinear. You have to get more and then have enough to like piles up. And it turns out that the number of studies that have actually tried to test that, essential assumption of the Haeve principle is vanishingly few. People just love the idea and pursue it and go with it without actually thinking whether it makes sense.

• 37:41Speaker 01

  Do you think there will be a way to perform experiments, observations that will resolve this? It sounds to me quite hotly debated contentious difference in perspectives in the community.

• 37:54Speaker 03

  Yeah, one of the sorry features, and it actually applies to the handicap principle, but to adaptive mate choice in general, is what counts as falsification. Currently and extensively in the history of the literature, people, because they put sexual selection by definition within natural selection, and because natural selection is by definition adaptation, then they don't actually think that they need a null model, right? They say, well, this is obviously relevant. And so they search and search for relevant costs or association with good genes or high quality territory, the kind of benefits that you might get from choosing a beautiful male or a beautiful mate. And if they find them, then they publish, they get tenure.

• 38:35Speaker 03

  If they don't find them, they may not finish the degree, they may not publish those papers, they don't get tenure. And what I've argued is that what this means is that adaptation by natural selection becomes a faith-based assumption of the field, right? If you can't falsify it, if everybody says, oh, you just haven't looked long enough to find how this bird song encodes quality, which is almost the standard in the field, then that becomes non-science. So what I propose is that arbitrary or authentically Darwinian aesthetic sexual selection is the null model. It is up to those who believe that beauty has meaning encoded in quality to demonstrate that. And when they haven't demonstrated it, that means that it's arbitrary.

• 39:18Speaker 03

  And I'm still working on that null model movement.

• 39:21Speaker 01

  What is next in your research that has you really jazzed these days?

• 39:26Speaker 03

  Wow. Right now, I'm 50 years into bird watching and 40 years into the academic study of ornithology and evolutionary biology. And I keep getting new ideas and I don't lose old interests. So that means we are spread so thin that we're doing lots of different things. Right now, like on my hard drive, manuscripts that I need to review or projects where actually got data sets coming in this week include transcriptomics on feathers. So we're looking at the RNAs that are expressed in individual cells in a developing feather. So we want to know how different cells that make up a complex feather

• 40:07Speaker 03

  use their genomes to create cells of different sizes and shapes and colors. Right? That's fun. We're doing population genetic theory, trying to show that when we remove sexual selection from natural selection in this authentically Darwinian fashion, that we can still do all the bells and whistles that people like to do with thinking about how evolution works. We are interested in the evolution of inside birds, a delayed plumage maturation, why some birds, instead of growing directly into an adult plumage, have intermediate or younger adolescent stage. It's really about the evolution of adolescent signals in birds. And then got some optics and physics of color

• 40:52Speaker 03

  and some odd bird pigments, all sorts of different topics like that.

• 40:56Speaker 01

  Clearly, you have very broad range of interests. And what is it about your work that brings you joy across all of these varied ranges of ideas?

• 41:07Speaker 03

  One doesn't scrutinize where meaning comes from too closely. Right? Because you never know. But for me, somehow or other, I just love birds. And so when I have an opportunity to learn something new about them, it's delightful. And also, like we said before, I was working on fossil feathers in dinosaurs and I was working on pigments in bird feathers. And all of a sudden, unexpected, those two fields came together in fossil dinosaur feather colors. So there's this kind of feedback that happens when you've been in the business enough decades that is really, really marvelous. I would say that 90% of the time, it's a joy. 5% of the time, it's like smiley mouth.

• 41:50Speaker 03

  Then 5% of the time, it's like, ah, my brain's melting out of my ears. And I've got to go from optics to population genetics to transcriptomics. But it's a joy to also have amazing students of postdocs that are doing this marvelous work.

• 42:05Speaker 01

  Rick, thanks so much for joining us here on The Joy of Why. It's really been a pleasure.

• 42:09Speaker 03

  My pleasure, and thanks for having me.

• 42:14Speaker 02

  That remark of Rick's really resonates with me, that the more you learn, the more meaningful everything becomes. The cross-connections are so much a part of the pleasure. And it's not just of being a scientist, it's the pleasure of being old and remembering things while you still can, right? The world just makes more and more sense. Not all of it, but it becomes richer and more integrated.

• 42:38Speaker 01

  So you're not of the philosophy, "The more I know, the less I understand."

• 42:42Speaker 02

  There's also that. There has to be that.

• 42:43Speaker 01

  Right. Well, what he was describing also about the multivariate approach of something like feathers was just so fascinating. The chemistry of the melanin and the coloration of feathers. How did feathers evolve? Was it for the purpose of flight? Or did flight come later? There's lots of reasons why feathers emerged and were so prevalent and widespread. Flying is like, just that's the height of what you could do with them. So it's all of these different things coming together in something that we really take for granted about birds, which is their feathers.

• 43:18Speaker 02

  It's great. It's a lot of different perspectives on things that we don't normally think about. Or when we do, we tend to think in certain confined channels. There's an ancient tradition, going back at least to Karl Popper, of saying that there's circularity in traditional Darwin arguments about natural selection. You know, that they're not falsifiable. This is this Popper criterion. And so the evolutionary theory has this big problem that... How do you know if something's fit? Well, it gets to reproduce more. And why does it reproduce more? Because it's more fit, you know?

• 43:51Speaker 01

  Yeah, I think what he's saying is that if you are so swayed by natural selection on the basis of fitness, that you're not even going to question that there might be something outside of that, then you're never going to discover it. You're just going to, you know, presume that it's all based on the original argument, and you're just going to keep going till you find it.

• 44:13Speaker 02

  I was also really charmed by Rick's answer that he loves birds. You know, so often, our guests say things like, I love the puzzle solving, or I love the collaboration with my wonderful grad students, or the international aspect. Those are all fine answers, of course, and I'm sure a lot of people feel that. But what a direct answer. The bird guy loves birds.

• 44:37Speaker 01

  Wow. Thank you for joining us for this conversation.

• 44:40Speaker 02

  Great. This has been fun.

• 44:41Speaker 01

  See you next time.

• 44:42Speaker 02

  And I'll see you next time.

• 44:43Speaker 01

  Bye.

• 44:47Speaker 02

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• 45:04Speaker 01

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• 45:45Speaker 01

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• 46:09Speaker 01

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• 46:34Speaker 04

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