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.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
Transcript
[Music plays]JANNA LEVIN: I’m Janna Levin
STEVE STROGATZ: And I’m Steve Strogatz.
LEVIN: And this is The Joy of Why, a podcast from Quanta Magazine exploring some of the biggest unanswered questions in math and science today.
LEVIN: Steve, hi.
STROGATZ: Hi Janna.
LEVIN: I’ve been thinking you live in a more naturalistic environment than I do. I’m in Manhattan. Do you get to do any birdwatching where you are?
STROGATZ: Oh yeah. Occasionally, I do.
LEVIN: Do you pull out the binoculars? The whole thing?
STROGATZ: 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, yeah. So I don’t know, but I am one step removed from someone who does.
LEVIN: Well, it’s surprising there’s birdwatching 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 Prum. And we had a really fascinating conversation about how birds evolved, how they’re ultimately not just related to dinosaurs, but are dinosaurs.
STROGATZ: They are the living dinosaurs. I remember that in Jurassic Park. They tried to make them kind of look like a bird.
LEVIN: 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.
STROGATZ: That’s really interesting, great.
LEVIN: So here is Yale Professor Rick Prum.
Welcome to The Joy of Why, Rick. It’s a pleasure to speak to you.
RICHARD PRUM: Thanks for having me.
LEVIN: I didn’t realize how many people are absolutely obsessed with birds. This is a real thing.
PRUM: It is, and it’s a growing thing.
LEVIN: Is it?
PRUM: You know, during COVID, when people needed to get out, a lot of people discovered the outdoors and birdwatching — right up there with gardening and getting a dog — were one of those things that really inspired people.
LEVIN: 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.
PRUM: 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.
LEVIN: A lot of things must have come into focus. What was it specifically that drew you to the birds?
PRUM: 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 grosbeak, 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.
LEVIN: And you do travel the world.
PRUM: 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.
LEVIN: Bird watching also strikes me that it’s bird listening. Are you as into the bird song as you are into the visual observation?
PRUM: 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 thirties. 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.
LEVIN: So there’s some bird song that you remember, but you no longer can experience again.
PRUM: A whole world of birds. Actually, I remember, I did some field work on an exotic bird called the velvet asity 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 lekking behavior. This bird is super black with green waddles 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-banded still sitting on the same tree. 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, right?
LEVIN: Wow.
PRUM: So there’s a whole world of birds that, 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.
LEVIN: 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 in, even if it’s just in our human range.
PRUM: Yeah.
LEVIN: 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.
PRUM: Indeed.
LEVIN: Can you help? I thought they were descended from dinosaurs and that was the big revelation.
PRUM: Interestingly, to say that birds are descended from dinosaurs is to imagine the diversity of life as a kind of scala naturae, 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 are always thinking of — the origin of the feather or the origin of the syrinx, 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.
LEVIN: Fascinating. 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?
PRUM: Yeah, it was a phylogeny, 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 a[n] 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.
LEVIN: Fascinating. So is the subgroup that they belong to the therapod?
PRUM: Yeah. Birds are part of that very popular, bipedal, mostly meat-eating, very active.
LEVIN: T-Rex.
PRUM: Yeah. Velociraptor, chasing the kids around in the kitchen.
LEVIN: This is fun.
PRUM: Those are the dinosaurs that are most closely related to the living birds.
LEVIN: And we start to see them appear in the fossil record pretty early, is that right?
PRUM: 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.
LEVIN: 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 it was like that with dinosaurs?
PRUM: Of course, if you were somebody studying beetles, you would laugh at the statement that there are lots of birds. Ah! Ah, we’ve got whole genera that are just one genus that’s got more species in it that 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 there’s like endless summer, they’re riding the surf in different places. 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.
LEVIN: When you’re looking at some of these archeological 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?
PRUM: 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.
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 is 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. 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. 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 zippered 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 evolve to play the piano. That’s only the most advanced thing you can 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 display were all probably very early functions, right? Flight is only the last thing that feathers were put to use for.
LEVIN: 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?
PRUM: Again, that required extrapolating backwards. You know, we’re recreating from these materials what the skin and 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 therapod 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 and particular structural colors, optical colors and bird feathers, 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 fossilize beautifully under the right conditions.
LEVIN: Really?
PRUM: 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 weren’t bacteria — something of the same size scale, really similar-looking — they were melanosomes.
So, we were able to, first, discover that melanosomes fossilize beautifully. They are melanosomes, not bacteria. And then interestingly, it turns out, at least in birds, the melanosomes vary in shape depending on their color. With pheomelanin, the red-brown, red-hair melanins being more jellybean-shaped, and the eumelanin 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.
LEVIN: 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.
PRUM: Well, we have been working on 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 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.
LEVIN: Like outside of the range of human vision.
PRUM: 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. In 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. 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, 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 systems they have, and how they use them in their plumages and social lives.
LEVIN: 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 super-posing colors and creating, in our minds, the impression of something like purple or, in the bird’s case, 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?
PRUM: 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 sort-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, of course, 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, 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.
[Music plays]STROGATZ: Wow. Totally news to me. I had never heard that. Even the idea of the ultraviolet perception was news to me.
LEVIN: 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.
STROGATZ: That’s interesting just to think of in itself because something like a Robin, you know, okay, yes, Robin has that orange-y chest but otherwise kind of drab looking and just makes me wonder, do we know what a bird might see? Is there any way to convey that to us?
LEVIN: 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 Prum 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.
[Music plays]LEVIN: Welcome back. We are here with Yale Professor Rick Prum, discussing the evolution of birds.
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 in some ways. And I believe Darwin just hated that peacock feather.
PRUM: Indeed. He said, you know, the sight of a peacock’s tail whenever I look at it, makes me sick, right?
LEVIN: And, and why did he say this?
PRUM: 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, bird song, brilliant 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, 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? 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.
LEVIN: 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?
PRUM: Yeah, absolutely. And the kind of controlling response, to get sexual selection back on the ranch [laughs] 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 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 evolution in general.
LEVIN: Men knocking antlers, fighting over territory.
PRUM: Right. But his idea that mate choice, in particular female choice, was a force in nature, was a big loser.
LEVIN: It was suggesting females had more power than they ought to have in a Victorian society.
PRUM: 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 tail, 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. 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, in Wallace-ian form, it came back as a kind of natural selection. And that’s still a intellectual debate.
LEVIN: And you don’t buy that from the way you’re, you’re phrasing.
PRUM: 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.
LEVIN: Hmm. 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?
PRUM: 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 for the… 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 the, something like, 12-15,000 species of birds. We got a whole bunch of different situations, right? We’ve got the vast majority of them are 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. We’ve got polyandry species, like Jacanas, 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.
LEVIN: So, if the social organization is part of the adaptation, you know, natural selection can lead to or away from these sorts 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.
PRUM: Yeah, that kind of a 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 explanation, it’s all about survival. That’s not sensitive enough me.
LEVIN: 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.
PRUM: I mean, the reason why birds are so beautiful is because animals are making choices, right? They have sensory perception, some kind of cognitive evaluation, 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 procession, cognitive evaluation, 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 as the beak 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.
LEVIN: 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 objective 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.
PRUM: I guess we’d have to say ugly to whom?
LEVIN: Exactly. So, what do we mean by beautiful?
PRUM: Yeah. Perhaps the most obviously ugly bird is a baby pigeon. And, of course, lots of people have an opportunity 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 valuation, 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 baby’s cute and the babies evolve. So there’s all kinds of wild birds with tuft feathers, and colorful and beautiful mouth patterns inside.
LEVIN: 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.
PRUM: And puffy air sacks too. All of these are interesting aesthetic features in different, what I really consider to be, our world’s natural aesthetic communities.
LEVIN: I guess what I struggle with is, how does this kickoff? 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?
PRUM: How would this kick off, that’s an important question, right? 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?
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 going to like a bigger body size, because that means the individual had a good diet,” something that’s directly related to actionable objective information that might be profitable or 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. And it, 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.
LEVIN: Mm-hmm. 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.
PRUM: Well, if we just stick with the cartoon, initially, of females making choices of male display, which is a very specific setting, not universally broad. 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 interest of theirs in particular. And so the idea of the lying, cheating male, who will do anything to further his fitness, is not remote or fantasy. This is the, kind of, the standard trope of evolutionary biology, right? The contrast, we have eggs, rare, expensive, require more investment, should be more cautious. 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 the 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, 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. 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 reasons to, to imagine that rationally exuberant beauty happens in nature.
LEVIN: Are you suggesting that stilettos are impractical for survival? [laughs]
PRUM: Absolutely. Absolutely.
LEVIN: I don’t understand!
PRUM: And now we get to the idea pioneered by Amotz Zahavi, an Israeli ornithologist, who proposed what he called the handicap principle, back in the seventies. 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, that 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. Zahavi’s handicap principle, it’s probably, in my opinion, irrelevant to nature entirely.
LEVIN: Oh, so you really totally reject this, because I have heard those arguments.
PRUM: Well, the 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 non-linear. You have to get more, and then have enough to like piles up. And, turns out, that the number of studies that have actually tried to test that essential assumption of the Zahavi principle is vanishingly few. People just love the idea, and pursue it, and go with it, without actually thinking whether it, it makes sense.
LEVIN: 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?
PRUM: 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. 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. And, I’m still working on that null model, uh, movement.
LEVIN: Mm-hmm. What is next in your research, that has you really jazzed these days?
PRUM: Wow. Right now I’m 50 years into birdwatching, 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 are doing lots of different things.
Right now, like on my hard drive manuscripts that I need to review, or projects where actually got datasets 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 use their genomes to create cells of different sizes and shapes and colors, right? That’s fun. We are 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 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, and some odd bird pigments. All sorts of different topics like that.
LEVIN: 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?
PRUM: One doesn’t scrutinize where meaning comes from too closely, right? Because you never know. But for me, somehow 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 and dinosaurs, and I was working on pigments and bird feathers, and all of a sudden, unexpected, those two fields came together in fossil dinosaur feather colors, right? 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, then 5% of the time it’s like, uh, my brain’s melting out of my ears. And I’ve gotta go from optics to population genetics to transcriptomics. But it’s a joy to also have amazing students and postdocs that are doing this marvelous work.
LEVIN: Rick, thanks so much for joining us here on The Joy of Why. It’s really been a pleasure.
PRUM: My pleasure. And thanks for having me.
[Music plays]STROGATZ: Huh, that remark of Rick’s really resonates with me. That the more you learn, the more meaningful everything becomes. You know, 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.
LEVIN: So you’re not of the philosophy the more I know, the less I understand.
STROGATZ: There’s also that. There has to be that.
LEVIN: Right. Well, what he was describing also about the multi-varied 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.
STROGATZ: 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 Carl 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 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? Cause it’s more fit, you know.
LEVIN: Yeah, I think what he’s saying is that 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.
STROGATZ: 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.
LEVIN: Right.
STROGATZ: The bird guy loves birds.
LEVIN: Well, thank you for joining us for this conversation.
STROGATZ: Great. This has been fun.
LEVIN: See you next time.
STROGATZ: I’ll see you next time.
LEVIN: Bye.
[Music plays]STROGATZ: If you’re enjoying The Joy of Why and you’re not already subscribed, hit the subscribe or follow button where you’re listening. You can also leave a review for the show. It helps people find this podcast. Find articles, newsletters, videos, and more at quantamagazine.org.
LEVIN: The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine.
The Joy of Why is produced by PRX Productions. The production team is Caitlin Faulds, Livia Brock, Genevieve Sponsler and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzalez. Edwin Ochoa is our project manager.
From Quanta Magazine. Simon Frantz and Samir Patel provide editorial guidance with support from Matt Carlstrom, Samuel Velasco, Simone Barr and Michael Kanyongolo. Samir Patel is Quanta’s editor in chief.
Our theme music is from APM Music. The episode art is by Peter Greenwood, and our logo is by Jaki King and Kristina Armitage. Special thanks to the Columbia Journalism School and the Cornell Broadcast Studios. I’m your host, Janna Levin. If you have any questions or comments for us, please email us at [email protected]. Thanks for listening.
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