Feb 8, 2019

The Beauty Puzzle

When a female animal is checking out her prospects, natural selection would dictate that she pay attention to how healthy, or strong, or fit he is. But when it comes to finding a mate, some animals seem to be engaged in a very different game. What if a female were looking for something else - something that has nothing to do with fitness? Something...beautiful? Today we explore a different way of looking at evolution and what it may mean for the course of science.

This episode was reported by Robert Krulwich and Bethel Habte and was produced by Bethel Habte.

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Radiolab: The Beauty Puzzle





Jad Abumrad (JA)

Robert Krulwich (RK)

Bethel Habte (BH)

Patty Brennan (PB)


JA: Whose pad is this?


BH: I think that’s mine


JA: Oh my gosh Bethel your handwriting


BH: It’s awful


JA: Your handwriting is not awful. It’s actually very elegant


BH: Is it?


JA: Legible, not so much. But –


JA: Jad here, this is Radiolab. So not too long ago Robert and our producer Bethel Habte


JA: It has a nice lean to it


JA: Pulled me into the studio to walk me through this story that they had been working on together for quite some time.


RK: Everybody hears everybody?


JA: Yes. Here we are.


RK: Alright. So shall we start this?


JA: Yes. So what are we doing? You guys are just gonna tell me the story or –


RK: I think I’d rather tell you a riddle




RK: Once upon a time birds evolved. They evolved from dinosaurs. So they were originally scaley things and the scales turned into feathers and the feathery things began to fly and we call those things birds. But they have – I’m gonna just ask this to you out of the blue because it’s a basic appendage. How many birds


RK: Have (clears throat)


RK: Penises?


JA: Oh. OK


RK: And by the way, before you answer, there are like 10 to 20 thousand different kinds of birds by modern times. So again, how many of those species


RK: Have


RK: Penises?


JA: How – what percentage of currently existing birds –


RK: Modern bird species have –


BH: The male ones, obviously


RK: The males of course


JA: Penises as we would identify –


RK: Yeah like little things that hang out –


JA: Yeah, we don’t have to talk about it –


RK: Yeah yeah yeah. Those things


BH: This is making me so awkward,


JA: Is this making you uncomfortable?


BH: You know, making this draft I was saying the word penis like so many times


RK: At one point she wrote on the page, do you know how many times we’ve said penises on this? 27 in four minutes.


JA: Ok, yes yes so how many birds have penises?


JA: Well, I have absolutely no frame of reference to answer this question. I don’t know. I’m gonna throw out a number, 40 percent.


RK: No


JA: No, 70 percent let’s say 70


RK: No


JA: Uh. 30


RK: Lower


JA: [laughs] 10?


RK: Lower


JA: Wow. Five?


RK: Lower


JA: One


RK: Three


JA: Three?


RK: Three percent


PB: Yeah, 97 percent of birds don’t have penises


JA: Wow


PB: Yeah


BH: So we learned this little fact from a scientist named Patty Brennan


PB: The thing that’s so weird about birds is precisely the fact that they lost the penis


RK: Well you just said – you just used an interesting verb. So you say the birds lost the

penises, so does that mean that earlier editions of birds did have them?


PB: That’s right, yeah. So penises are a widespread trait of all vertebrates right? Except

for you know some amphibians and fish –


RK: And according to Patty, if you go back 200 million years or so, birds were like all those other creatures.


BH: Yeah. Basically a hundred percent of them had penises


PB: Exactly right.


RK: But then over time in the vast, vast majority of birds


PB: The penis was then lost.




RK: Now –


JA: Why would that be?


RK: Well, that’s the question.


PB: How do you lose the penis? Like it seems like a pretty handy thing to have when you

want to put your sperm close to female eggs


JA: So the 97 percent of birds that don’t have a penis now, what do they have?


RK: They have a kind of hole – both birds, the males and the females have these holes and they sort of open up and then they line them up


BH: Yeah. It’s called a cloaca


JA: Oh, they have a cloaca


PB: They have these cloaca and they just briefly touch their cloaca when they’re mating

and that’s it. That’s it.


RK: And it works just fine, but if you’re thinking about the engineering, like what appendage is going to be best for you to make more babies, penis is the one.


BH: Because it gets your sperm closer to female eggs


JA: Oh


BH: So for all of these penises to just vanish, like she says like evolutionarily that just doesn’t make any sense, why would you lose these thing that’s so useful?


PB: So for there to be selection for the penis to go away, there’s gotta be a very

important reason.


JA: What’s the reason?


BH: Well, there’s a few possibilities.




BH: Number 1.


PB: People have speculated for example that it might have been sexually

transmitted diseases. You’re putting this penis way deep into the female. And then you’re pulling your penis back into your cloaca like there could be potentially a very easy exchange of sexually transmitted diseases.


BH: Because you’re like digging the germs deeper I guess


PB: Exactly. Yeah yeah yeah.


BH: So even though the penis has its advantages, obviously, maybe it’s slowly went away because the birds that had the penis kept getting these infections.


PB: That could be.


BH: But so far, nobody’s found a link between penises and STDs.


PB: So then the other possibility that’s really intriguing actually


Music In


PB: is that maybe it’s because penises are heavy,




PB: And birds had started flying and getting rid of the penis would’ve been an easy thing for lowering your body weight.

RK: Maybe they lost them so they could fly farther.


PB: But, I don’t believe that. I mean I think that’s unlikely to have been a strong enough

selection because ducks for example are among the most long distance migrants and

they still have penises.


BH: They are in the three percent.


PB: And sometimes they have penises that are bigger than their own bodies, so...


RK: [laughing] They do?


BH: Oh my god


PB: They do, yes


RK: You mean you watch a penis go by with wings, kinda?


PB: [Laughing] Pretty much!


RK; Oh my God.


PB: Yeah so I think the record is a male that had a 43 cm long penis and this is a male that

was only about 40 cm long himself.


RK: So apparently you can have a pretty large penis and fly just fine. But oddly enough the ducks are also sort of a key to a third theory for the disappearance of the penis.


PB: So yeah I was going to say my favorite actually is this idea that actually it had to do

with female choice against males that could controlled reproduction.


BH: To explain, Patty actually thinks that the original bird penises –


RK: Back when they had penises


BH: Yeah. Might have been a lot like the modern day duck penis which is essentially a weapon.




PB: And so one of the things that we learned when we were studying the ducks is that

the males have evolved this erection mechanism that is crazy. They have an explosive erection mechanism. What that means is that the male actually averts his penis and ejaculates in a third of a second.


RK: Basically a pistol penis like a pew (sound effect)


PB: Exactly, yeah. It just goes pachoing (sound effect) yes.


RK: Wow.


PB:What that does though is it allows this males to forcefully inseminate females even when females don’t want to be inseminated, right, so if this male, is anywhere near a female, he’s just going to go pfff, you know, push right through. And so as you can imagine that’s not, you know, that’s not something necessarily desirable for females. Oftentimes, the females will be struggling, right. They’re trying to get away from these males that are trying to forcibly copulate.


[Music In]


BH: So Patty thinks what might have happened here is that female birds trying to get away from males with large penises and these really violent ways of approaching reproduction began systematically choosing gentler and smaller males with smaller penises.


PB: Right. So, so if females start selecting males that are less violent, less sexually violent that that might lead to the disappearance of the penis.


RK: What I think you just said is over time because the ladies were discomforted the gentlemen changed and lost their penises.


PB: Yeah. You got it.


JA: She’s saying that females, essentially, castrated the males?


RK: Well, this is a slower process than that [laughing]


JA: I mean over time.


RK: Over time. Castration over time.


PB: Yeah


RK: Yes, I suppose


RK: [Laughs] Now what makes –


RK: What evidence is there for such a thing?


PB: So none in a way because we’re talking about penises which are soft

tissue and they don’t fossilize so even if we went back into the fossil record it would be really hard to find evidence of what was happening, so that’s the part is difficult, you know, it’s kind of like a wonderful story that I think makes a lot of sense given what we know about the way these penises work now, but we can only speculate.


JA: I like this idea


RK: Good


JA: But is this just a one-off? I mean, what about gorillas and moose who fight and clash their horns, I mean there’s so many species where it just seems like the females have zero choice.


BH: So I know that there are a lot cases like that, bedbugs are especially horrifying


            RK: Horrible


BH: Google it


JA: Oh yeah, don’t even talk about bedbugs.


BH: But there are actually a lot of cases where when the females do get to choose, they choose in ways that completely change what we see in nature.


RK: And we found a group of scientists who now argue that if you look around nature you will see females driving evolution in ways that I certainly didn’t expect. And when they get into the details of how this actually works I think it’s going to flip your idea about evolution in a way that you are totally unprepared for.


JA: Do they have evidence for this idea? Because I gotta say that you haven’t yet convinced me of the first thing you said, that females have evolved the penises away.


RK: Well because that was a guess. Patty said that was a guess.


            JA: She said she’s guessing, yeah.


RK: Fair enough. I will introduce you to a bird called a bowerbird that I think –well it speaks for itself in these regards.


BH: And it’s also, it’s penis-less [laughing]






Jad Abumrad (JA)

Robert Krulwich (RK)

Bethel Habte (BH)

Gail Patricelli (GP)

Richard Prum (RP)


GP: They are really cool birds. They have beautiful plumage. The satin bowerbird is a

beautiful iridescent blue with violet blue eyes.


BH: This is Gail.


GP: Gail Patricelli and I’m a professor of evolution and ecology at UC Davis. Do I wanna say that again without stuttering on and?


BH: No no, that’s fine.


BH: Anyway, so the bowerbird


GP: What makes them most amazing is that they build bowers


BH: So a bower, these things the birds build, it’s basically a structure made of sticks which can be up to three feet tall


JA: Three feet tall?


BH: Yeah. This tiny little bird making this huge –


RK: You could put a five year old in it, in some of these things


JA: Wow


BH: Yeah and it looks like a nest


RP: But it’s really a kind of seduction theater with one seat. And that, and that’s for her.


RK: This is Richard Prum of Yale University. He’s a biologist there. And he says that on and around this bower


RP: The male will array a whole bunch of found objects


BH: Precious objects


RP: Beautiful things


GP: Parrot feathers and berries and flowers and leaves


BH: Stuff that the bird gathers from the forest so these structures can be very ornamented and elaborate


RK: And every bowerbird species has its own, like, style.


RP: In some species it will all be blue everything. Blue feathers, blue

flowers, and then of course blue trash like drinking straws and bottle caps.


BH: And the male bowerbird basically dedicates his whole life learning how to build these structures. Like take the satin bowerbird.


GP: They take seven years before they reach sexual maturity. And during that period of time they look just like females and they’ll often fly around the valley and get courted by adult males. So they play the role of the female and learn how courtship works from other males in the valley and then when they get a little older, as teenagers they’ll start building practice bowers and they’ll court each other.


RK: And so eventually, once a bowerbird figures out how to make a really good structure


CLIP, David Attenborough: A female has arrived


RK: Females show up


GP: So they’ll fly down to the bower and they’ll often check out the bower itself, is it symmetrical, is it well-built


CLIP, Attenborough: She seems interested.


RK: And if she likes what she sees, she’ll enter the bower.


BH: Now, this is where things get interesting, because the bower has a very particular and purposeful architecture.


RP: So one of the classic bower designs is called an avenue bower. So it’s two parallel walls of sticks that are close to one another and the female sits between them.


BH: There are different variations on this basic design, but…


GP: In all cases, the female is in a protected position.


BH: There’s always some kind of barrier or wall between her and the male.


GP: So if the male wants to try to force himself on her for whatever reason he has to run around the back and she can just fly away out the front


RK: Even as he’s trying to win her attention completely, he’s built a building that keeps her at a distance?


RP: Absolutely. The bower is like insurance against date rape for the female. It allows

the female to observe him at an intimately close distance for as long as she likes while still maintaining her freedom of choice.


BH: So the female, if she likes the bower, she’ll settle into this protected space where she can back out whenever she likes and then…


CLIP, Attenborough: Time to begin the show.


GP: So he’ll start out with his own displays


RP: Many of them will make very loud, [SFX] electronical sounds


GP: Buzzing and whirring sounds


RP: [SFX] And they do imitations of kookaburra, you know [SFX]


BH: Sometimes the imitate [SFX] cockatoos. Then, along with singing, they dance.


GP: They puff up and they run vigorously back and forth right in front of the female and

she’s standing between the walls of the bower…


BH: So the female watches all this from her safe spot for as long as she likes and she can decide she would like to mate with this guy or…


CLIP, Attenborough: No. Not good enough.


BH: Up and fly away all of a sudden.


CLIP, Attenborough: For the female, the moment has gone.


RK: Now what’s particularly interesting here is that Gail has done studies that show when the males pay very close attention to the female –


BH: Like when the male really watches the body position, the way that she’s postured, that can actually signal whether she’s interested or not.


GP: The males that respond the most to those signals are the most successful in mating.


RK: In other words, if the male makes his move too early and too aggressively, she’s gone. And that male doesn’t have any babies.


JA: Huh.


BH: But it also seems like the females are kind of toying with the males, like seeing what they’re willing to do for them, like if you watch video of this, you can see in one bowerbird, she comes in. He like lifts up his wing like a cape then just like sort of rotates his wing like this, like [laughs] it’s so funny.


JA: Oh. Like a matador kind of?


BH: Like a matador. Totally like a matador. And then right after that she like picks up a blueberry from the ground and then she drops the blueberry. And then he does the matador thing with the blueberry in his mouth.


RK: Like he has a rose in his mouth


JA: That’s hilarious. She’s like, you know, it’s alright, but –


BH: This is good, but like –


JA: It’d be so much better with this.


RK: It seems to be all about her.


JA: I see what you mean. In this case, yeah, the females are definitely driving the bus here.


RK: And when you start to really think about the implications of this, about it being all about her, that can lead you to a fairly deep rethink of the of the very basic rules of how evolution works.


JA: Really?


RK: Yeah.


JA: Meaning what?


RK: Well, normally the classical argument, and you’ve heard it, like the reason this male bird is so colorful, so red or whatever, the reason peacock has these gigantic and beautiful feathers on its tale is sort of a signal that the male is sending to the female of fitness. Look at me, I am healthy to have a tail like this, I don’t have parasites, I’m strong, I’d be a great mate.


BH: Like survival of the fittest.


JA: Right.


RK: But let me ask you, you just heard the bowerbird story. You have these females with these crazy tastes. Like this one likes blue, only blue. And this one like iridescent shells, so it’s shells shells shells shells shells. And this one likes green leaves but they have to be right side up and never right, you know, upside down. Are these fitness signals or is this more like art?




RK: Maybe she just likes blue, or she just likes shells.


JA: Wait. You’re saying that these birds have evolved then based on whims, and tastes?


RK: Well Rick and there’s other scientists like him say…


RP: Absolutely.


RK: Yes. It’s about beauty.


RP: I’m really focusing in on aesthetic choices. Choices that are based on what it is the

animal likes.


JA: Wait, I thought the whole Darwinian thought was “well yeah okay there might be beauty there but on some level the logic behind those beautiful things is survival, survival of the fittest.” This does not sound very Darwinian.


RP: Well.


RK: Since you dropped the name -


RP: I propose that my view is the legitimately, authentically Darwinian view.






Jad Abumrad (JA)

Robert Krulwich (RK)

Bethel Habte (BH)

Kim Bostwick (KB)

Richard Prum (RP)


JA: Darwin, he had an idea about beauty?


RK: Oh yes. He – he actually –


BH: Do you want to read the quote?


RK: Did I bring it here –


BH: Yeah, you brought it. Yeah.


JA: Okay. Page 397, Descent of Man. Stripes and marks and ornamental appendages have all been indirectly gained through the influence of love? Huh. Jealousy. Through the appreciation of the beautiful and through the exertion of a choice. Oh, interesting. So he’s saying that it’s love, jealousy, and beauty. And choice.


RK: Yeah. And choice.


JA: Yeah, that is definitely not what I learned when I learned about Darwin.


RP: So it’s over 130 years later and I’m still pissed.


RK: He thinks there was some kind of plot to reduce Darwin's idea into something smaller and eventually eliminated entirely.


RP: I would like to go historical. Let’s go about what Darwin said.


RK: Okay.


RP: Let’s go with what Wallace said. And then let’s go…




RP: ... to the 20th century, and where we got so screwed up.


RK: Okay.


RK: So Darwin spent 20 years working on his theory of natural selection. He was not particularly noisy about it, because he knew it would very much disturb his wife who was quite religious, and other people in his church. So during the 20 years while Darwin was working away at his book…




RK: ...this other guy comes on the scene. His name is Alfred Russel Wallace. Kind of skinny and scrappy and self-taught. 14 years younger than Charles Darwin. But Wallace also ...




RP: Went around the world collecting specimens.


RK: And he also came up with a theory of natural selection.




RK: Independently.


BH: What was their relationship to each other? I mean, did they – were they friends, were they collaborators, were they –


RP: Well you know, they were close collaborators.


RK: At first, they kind of both got credit for the idea, but a little later…


RP: Darwin rocketed out his book into – into press.


RK: When he published his On The Origin of Species, a very popular book…


RP: Darwin got the lion’s share of the credit, as I think rightly so. Because he’d been working on the idea for literally for decades.


RK: Then, 20 years later Darwin dies. Meanwhile, Wallace…


RP: Lived until the dawn of World War I.


RK: And during that time, this is Rick's argument, after Darwin died Wallace kept saying over and over and over again, that when it came to female choice…


RP: Animals didn’t have the sensory and cognitive capacity to make aesthetic judgments in nature.


RK: So these elaborate decorations and be – artistic behaviors…


RP: Could only evolve if it communicates information about vigor, quality and general fitness to survive.


BH: And Rick would say that, because Darwin wasn't around to argue back...


RP: Wallace may have lost the – the battle over credit for the discovery of adaptation by natural selection. But he won the war over what evolutionary biology would become in the 20th century. We have inherited both the science and the culture of flattened, dumbed down, and ideologically purified version of Darwin’s actual richness.


JA: Whoa, those are – those are fighting words.


RK: Well you see Rick is pretty fired up about this. And he's written a book, “Evolution of Beauty," where he argues that ever since Alfred Russel Wallace, scientists have been trying to squeeze everything they see in these male patterns, male dances, male songs, male plumages, into a single explanation. A dogmatic category called "fitness." And that female choice they claim is always, always about fitness, fitness, fitness. But Rick argues, "No. No, that is – that's a stretch." There's no way to boil down all this variety into fitness.


RP: I’m not saying that the emperor wears no clothes. I’m saying that the emperor is

wearing a loincloth. And what I mean by that is that humble garment covers about the same percentage of the human body as the idea of adaptive mate choice covers all the ornaments of – sexual ornaments in the world.


RK: Oh, my god! What you just said is that most, or most of...


RP: Vast majority. So let's – another way ...


RK: Most of what you see in the world, you think is – is desire of – the desire between

creatures expressed in – in beautiful forms.


RP: Absolutely. And...


RK: Not of fitness. Not that oh, she’s just looking at him thinking, "He’s strong. He’s

genetically trustworthy." None of those, just, I love him!


RP: Yeah.


JA: Yeah, so he's saying most – most of the time when you see the ornaments, that's not fitness at all, that's beauty?


BH: Yeah, most.


RK: That's what he says.


JA: Most.


BH: Yeah.


RK: He's big time on most.


JA: Well, how would you even know that? I mean, as a human how would you ever even know the inner workings of another creature's mind? To know enough to say what it is that is driving them or not?


RK: I asked this very – I – no, I asked this very question to ornithologist Kim Bostwick.


KB: Right


RK: Isn’t it impossible for you, as a human being, to have any idea what a lady animal wants to see in a guy, if that lady animal is a peahen, which you aren't. Or a trout, which you aren’t.


KB: Oh, yeah. I'm ready to answer this.


RK: Okay.


KB: Here it is. I don't know what she wants now, but I know historically she’s wanted exactly what you see in those males. The peahen has wanted that big ol’ tail. She’s wanted the show. She’s wanted the blue chest. She has, at some time or another in the last I don’t know how many thousands of years, she has wanted everything you see on him, from the fancy feathers to the white skin around his eye to the iridescent blue, the shaking, that’s what she has wanted in the past.


RK: How do you know that?


KB: Because it’s there. Seriously.


RK: Isn't there some lawyer in the room that can say, "Wait a second, wait a second!” Is that -- what do you mean, because it's there?


KB: How else did it get there? Well, how else did it get there?


RK: Well it could’ve gotten there by accident.


KB: By chance?


RK: Yes.


KB: Yeah, so you just happen to have this, like, fine, nano-structured, iridescent colors in your feather because, like, 42 interacting genes across five different chromosomes came together to give you iridescent blue? No.


BH: The way you get to iridescent blue, says Kim, is you start small.


KB: You give your female a bunch of choices, which are the males in her block.


BH: So imagine a female surrounded by a bunch of males who are all just boring or grey or black or whatever. Then, totally randomly, a male shows up who just happens to have a little bit of blue on him. And she notices.


KB: When the sun glints off of his black plumage there’s just a little hint of blue in that

black. It’s kind of a glossy, shiny blue black. And she’s like, “Mmm. I like that. I like that. That is a little different than everybody else and I like it.”


BH: And who knows why, it could be totally random too, but for whatever reason she decides to mate with the blue guy.


KB: Mmm. I like that


RK: Because she's passing on his genes, her sons are like gonna be a little bit blue too.


KB: Her offspring are gonna have those beautiful genes and we already know that the

females chose him because he was attractive, and so she’s gonna have attractive sons. And her daughters are gonna also find those traits attractive.


BH: And so what you get generations after generation of females who say…


KB: Mmm, I like that.


SFX ROBOT: I like that. I like that. I like that. I like that.


KB: And then off it goes. You’re -- you're creating -- you’re evolving the males to please the females.


BH: And it all just starts with ...


KB: I like that.


JA: But "I like that" doesn't feel to me opposite of fitness. Like, "I like that" could be a desire that is driven by instinct. I mean, couldn't, like, a Wallace person walk in and be like, "Yeah, she likes that because she was designed to like that." She's not thinking about the – the "why" there, she's just having a desire. But behind that desire is maybe a drive to get her to the male that is the fittest. Like, and I – and I – frankly, it could be the same with us, too. With people. Like, the – what we find sexy might have a deep logic to it.


RK: A hidden – a hidden logic.


BHL: Yeah, I see that.


JA: A deep, hidden logic to it that maybe in the end is about fitness.


RK: Okay, that's a perfectly reasonable objection.


JA: Thank you.


RK: But, what if I told you about an animal who has a fierce desire for beauty, and this particular desire for beauty is so strong that fitness is out of the picture?


JA: It's another bird?


RK: It's another bird.


BH: Big surprise, I know.




BH: Don’t hate them because they’re beautiful, Jad. We’ll just take a break. We’ll take a break, and after that we’ll come back with another bird.





Jad Abumrad (JA)

Robert Krulwich (RK)

Bethel Habte (BH)

Kim Bostwick (KB)

Richard Prum (RP)


JA: Jad.


RK: Robert.


BH: Bethel.


JA: Radiolab.


  1. We’re back mid-battle between beauty and fitness.


JA: Bird battle?


RK: Bird battle. And we’re about to introduce our third bird.


BH: Yeah, you want me to go?


RK: Yeah. Go ahead. Go.


BH: All right. OK, so there’s a bird out there called the club-winged manakin…




BH: And this manakin lives in the jungles of, I forgot where she was. Somewhere in the jungles [laughs].


RK: South America.


BH: In South America. I think Colombia.


KB: Well, okay. So, the – the club-winged manakin has a very small range in Colombia

and Ecuador.


JA: Is it a big bird, small bird?


RK: No. Very little.


BH: It’s a tiny bird. The males…


KB: The males are mostly red


RP: He’s got a bright red head


BH: Kind of an auburn body and black and white wings with little flecks of yellow underneath them. So these are small birds, lot of color. And…


RP: The female can raise the babies all on her own.


RK: So she doesn’t need guys except for the sexual act.


RP: Right.


BH: He doesn’t have to provide for her, he just has to attract her.






BH: So, to that end, this is what our little red Romeo will do. He'll sit on a branch in the Andean jungle and he'll wait awhile until a female shows up.




BH: And then when she does...


KB: He has a courtship display. And he bounces back and forth on the perch. And while he’s bouncing around…


BH: Every so often he stops, lowers his head, sticks his butt up in the air.


KB: Gets his wings all positioned, and he throws them up.


BH: Behind his back.


KB: And then, he's – he vibrates.




KB: Or just shivers. He shivers his wings together. So that the only things that touch are those feathers across the back. The tips of those funny feathers are going knock-knock- knock-knock-knock-knock-knock-knock. They knock 39 times in a row. And it takes about a third of a second for those 39 knocks.


BH: Their wings vibrate so fast that they make this sound.


RK: Can you make a – can you make the sound that…


BH: Tick tick tick




JA: Wow!


BH: It's impossible for a human to make with their voice.


JA: Is that seriously the sound it makes?


BH: Yeah!


JA: Yeah. It's like a truck backing up.


RK: Exactly.


BH: Eeeee!


JA: So he's not using his voice for that? It's just his wings?


KB: He’s got this instrument


[Bird SFX]


RP: Yeah. It’s called strigulation. This is a cricket-winged manakin would be a perfectly

good name for this bird.


BH: In any case, something about that sound, that specific sound, excites the female. We don't know why or when it started, but somewhere in the bird's past a female decided she liked that sound, and so the males just started to make it. But here's the thing, in order to make that sound?


KB: Yeah, they really have paid. They have paid.


BH: Because to vibrate your wings that fast, 107 times per second, you need a wing bone that can really control. And so the club winged-manakin has done something unheard of. Their wing bones went solid.


KB: It’s just like a rock inside there.


RP: This is a big deal because all flying birds have hollow wing bones.


RK: Right. So they can fly, so they’re light.


RP: Yeah. Well actually, even velociraptor and t-rex have hollow forelimb bones, right? So this

is a feature that predates the origin of birds and the origin of flight. But this guy has given this up in order to make these extraordinary wing songs.


RK: Well, doesn't that hurt your ability to fly?


KB: Yes. Yes. They are slow, heavy, unable to leap buildings in a single bound.


RP: Well it’s the cost of doing business for a – a displaying male, right?


BH: Think about that. You're in a crowded forest. Lots of competitors, lots of predators trying to eat you. And you have made yourself slower, more vulnerable. And it gets worse. Because the manakins have to start this process of building these hard bones really early, like when they're very, very tiny in the embryo.


RP: Before the embryo has become either male or female.


BH: So you’ve got an embryo that can go either way, and they’re already making the big bones. And some of them are gonna be male, but some of them are gonna be female.


RP: So by choosing males with weird wing bones because they make great songs, the female also has daughters with distorted and inferior wing bones that they will never use.


RK: Both the females and the males get these thick bones. So she is choosing to hear that sound and has designed him to produce that sound. But in the bargain, he comes out with heavier bones and can fly less well, and weirdly enough she comes out with heavier bones and can fly less well. So both of them are hurting their chances to survive for the chance to hear the beautiful tone that she wants to hear that he wants to give her.


JA: Wait, what?


JA: Wait. So she has heavy bones too?


RK: Yeah


BH: Yep


JA: But she doesn’t use them?


BH: Nope


RK: Nope.


KB: Rick saw it immediately. Like, I was like, “Oh this is weird. The female’s got the bumps on her bones too, and he's like, "[gasp] decadence!" And I was like, "What? What are you talking about, decadence?" He’s like, "The females have it but they don’t use it! She’s bearing the cost of her own choices! Decadence!” And this was like 20 years ago [laughs]


RK: You science people have such strange moments of ecstasy.


KB: Oh yes.


RK: So when he says “decadence,” what did he mean?


RP: So everybody in the population becomes worse off because females are so choosy

and choose beauty.




BH: So from Rick's point of view, you've got a contest going on here. Two primal drives. And on one hand, there's the desire to survive. Survival of the fittest, right? And according to that logic, the manakin should go for things that make him swift and powerful and agile. But on the other hand, there's a second drive: to see, or in this case to hear, something they like. Something beautiful. And in this case, that second drive is so strong that it's winning. It's pushing the birds, like, away from fitness.


RP: So - so we’ve gone through the math and others have as well and there is nothing, in theory, nothing to prevent this kind of process from leading to extinction.


RK: Oh my god.


            RP: Right?


RK: You just – wait, you’ve just turned what I formerly think of Darwinian evolution on its head. Like, we’ve always been taught that what these animals are doing is they’re adapting as best as they can to new and changing conditions, and they’re getting better. But here you say that they are so hung up on desire and beauty that they even are willing to get worse.


RP: Right. Well, you know, that’s why this example is checkmate.







JA: Jad Abumrad

RK: Robert Krulwich

BH: Bethel Habte

JC: Jerry Coin

RK: Richard Prum

KB: Kim Bostwick


BH: But there are biologists




RK: Eminent biologists.


JC: Hello?


RK: You don’t want to be sucking a lollipop or whatever you’re doing there, Jerry.


BH: Like Jerry Coin of the University of Chicago.


JC: I’m not - I’ll be through with it when I go on the air.


BH: Who disagree with Rick.


BH: What is it, is it a lollipop or –


JC: No, it’s a cough drop


RK: Oh, it's a cough drop. Okay.


RK: So Jerry read Rick's book, and was not - not convinced by the argument.


JC: Well I think it’s a good book, but it’s not a great book. It’s good because it has really

great stories about mating behavior which are accurate stories as far as I know. And they’re quite absorbing, Prum is a good writer. The reason it’s not a great book is that it’s tendentious, that is it’s written to promote a cause.


BH: Which is that, of course, that males with beautiful ornaments are shaped by female sense of beauty, not fitness.


RP: I think the word fitness is a huge problem. And I don’t use it anymore.


JC: That – you can’t just say that and have it accepted by scientists unless you contest it, okay? And that is the problem…


RK: For Jerry, when it comes to the stories that we’ve just heard, like the bowerbird or the manakin, he thinks that -


JC: More is going on here than what Richard says in his book, you know?


RK: Take the female bowerbird’s preference for bowers that keep her apart from males.


JC: I mean Richard’s explanation that it allows the female more time to choose. It doesn’t freak her out, because she feels protected. That might be the right explanation. But we don’t know. I mean, it could be that females have an innate preference for being sheltered, because it gives them a sense of security. She - like cats. Cats like to be in boxes, right? Because they feel sheltered and they feel safe and it could be that female bowerbirds are the same way. And that will lead to exactly the same situation, but with a completely different explanation. And it’s not just a random aesthetic phenomenon.


BH: Or, he says, maybe some female bowerbirds like blue so much because blueberries are really good for them, and they're innately drawn to blue because of that.


RK: Or go to the manakin tale with that, you know, 1,500 Hz tone. Rick says simply delights the females, but -


JC: Suppose that the females prefer males that have a certain frequency of wing-beating because they were attuned to that frequency, perhaps because it indicates something in their environment that’s useful to them, like the presence of a predator…


RK: It could be anything, really. The sound of something manakin-friendly animal or a protective animal, or maybe the call of their own baby, the baby chicks.


JC: And so they just have their nervous system innately tuned to that frequency. That’s called

sensory bias and that’s another theory of sexual selection.


RK: Jerry doesn't dispute the fact that female birds like bowers, they do. Or that they like the beautiful tone, they do. He just thinks that there might be a reason behind their liking. And that reason could include fitness.


JC: Correct, yeah. I mean ...


RK: But are you – are you hoping, I guess? Is that the right word? Hoping that there might be a parasite signal involved here? Like, you don't know. You just want to ...


JC: No, I don’t know. I mean that’s my whole point and that’s why I don’t think, I don’t propose my own theory of sexual selection. I’m not sure which one is – does account for these manakins with the heavy bones. We just don’t know.


RK: Oh my God! This conversation we’re having keep ending up in the same place, which is we just don’t know.


JC: Well it – the difference between me and Dr. Prum is I admit I don’t know.




RK: The suggestion is here is that Rick is clinging to a faith in one explanation. For Wallace it was fitness, for Rick it’s beauty. But it’s still that dogmatic insistence.


RP: I just think that adaptation by natural selection, fitness is – is pretty boring.


[RK laughs]


RP: You know? You know, 150 years in, we've – we've – we've whipped that pony quite a bit, and we made a whole bunch of progress. But you know what? There's this huge world of opportunity in aesthetic evolution that – that has been missed.


RK: In fact, Rick would say, the whole point of a sense of beauty is that it can be many, many different things. It is funda – beauty is fundamentally subjective.


RP: So the aesthetic model requires that we put the subjective experience, desire itself, at the center of our – of our scientific explanation




JA: Can you even have a science that’s based on subjective experience?


RP: I think that -- that a science of subjective experience is not only a good idea, it's necessary to understand the natural world.


KB: Yeah. There is idiosyncrasy.


BH: Again, ornithologist Kim Bostwick.


KB: You can’t take the individuals out of it.


RK: So you don’t – is that science or is that just, it’s just saying “okay it really comes down to she the carp likes what that male carp has, she the worm likes what the worm has, she the bird likes what the bird has. And then you have a trillion explanations, each one different and depending upon the lady a different one, so you don’t – doesn’t it worry you that to call beauty and desire a category, an explanation as to not tell you very much?


KB: That doesn’t worry me at all. I – that doesn’t worry me at all. I, you, all of us, we are individuals. We have unique histories. And life on this planet has a unique history. Science has a hard time dealing with unique instances. But biology is just in your face with it. There are individual females making choices. If that individual female had not made that choice, history might have been different, and the species might look different. And that is true. That’s just true.


RK: In – in physics and in chemistry there's this sort of conceit that what a scientist is supposed to do is take all this variety that you see in front of you and say, "Look, I can boil this down to a rule which is always true."


JA: Yeah, which transcend ...


RK: And usually it's one rule. And transcends...


JA: Yeah. Transcends everything.


RK: So – so what the job of science usually is, is to find some kind of oneness inside the manyness. But now we've got nature and living things. And they have this crazy, spectacular variety. And now you've got a group of people who say, "You know, maybe we shouldn't even try to explain all this with one rule or one paradigm." But you know, I wonder if, um – if that’s still science. Or is it more like history?


JA: Hm.


RK: What if every species or every animal comes with its own story?


            JA: Oh that’s interesting.


RK: So it’s once upon a time and then a once upon a time and a once upon a time and a once upon a time and a once upon a time and a once upon a time…


JA: This was reported by you


            RK: Yeah


JA: Robert Krulwich, and Bethel Habte. And Bethel produced this story. Thank you, Bethel.


BH: Sure thing.


RK: And special thanks also to Jessica Eurinsky for her works on peacocks and Michaela Gunther for her work on hyenas. We didn’t mention the peacocks and hyenas as much as we thought but we are very grateful to both of you.


JA: All right, well we should say goodbye.


RK: Goodbye.


JA: Well, to –


RK: To our audience?


JA: To our audience.




JA: I’m not done with you.


RK: No, okay.


JA: I’m Jad Abumrad.


RK: I’m Robert Krulwich.


JA: Thanks for listening.