E38: Okay, but what’s up with feathers?

Scott Taylor: Don't you hate when one little quill works its way through the fabric of a down jacket and pokes the shit out of you in the arm? You pinch it out, look at it for half a second, then flick it away. Same with the one that escapes your pillow. Same with the one you pick up on a walk. And if you're like me, inspect it and figure out what kind of bird it came from. Here's what you'd be holding if you stopped. Down the middle, a shaft called the rachis. Off the shaft, branches called barbs. Off each of those, smaller branches called barbules. And the barbules end in tiny hooks that grab the barbules next door, depending on the type of feather. That's why you can run a wing feather through your fingers, split it wide open, then smooth it right back together like nothing happened. It's Velcro, basically. Birds got there roughly 150 million years first. That is a wing feather. A down feather skips most of that and just traps air. A whip-poor-will grows feathers around its mouth that look like stiff whiskers. A woodpecker's tail feathers are reinforced into a kickstand. A peacock has whatever a peacock has going on. Feathers are one of the most structurally complicated things any animal grows out of its skin. And coordination while a feather is growing out of the feather bud is critical for both feather structure and coloration. Which brings us to the topic of today's episode. Okay, but what's up with feathers? To help us unpack that, I'm joined by Dr. Cody Limber, who just finished his PhD at Yale. Cody spent his PhD taking growing feathers apart one cell at a time to find out what they're actually made of. After the break, we get into what happens when you smash a growing feather into individual cells and then have to figure out where every one of them came from. We talk about how the keratin in a feather is not the same as the keratin in your fingernails, and Cody calls B.S. on the most repeated claim in all of feather evolution. Stay tuned.

Cody Limber: Hey.

Scott Taylor: Well, welcome back to the podcast, everyone. I'm really excited to have Cody with us today to talk all about feathers. Thanks for joining us, Cody.

Cody Limber: Yeah, it's great to be here.

Scott Taylor: Feathers are often in our day-to-day lives, whether they're stuffed into something we put our heads on or something we wrap ourselves in to keep warm. But I would hazard a guess not a lot of people think too deeply about feathers on a regular basis. Yeah. I know you do, though. So tell us what a feather is. So what is a feather? And what made you so curious about them?

Cody Limber: Well, maybe I'll start with the second part of that, because that's the less hairy portion or less feathery portion. I got really interested in feathers just basically by looking more closely at them. So I grew up in Boulder, Colorado. I grew up banding birds. And banding birds is one of these unique times where you get to hold a bird in your hand and get to look at it really closely. And I feel like the closer you look at a bird, the more you start to become interested in feathers. Because if you look at a bird, you realize, oh, they've got all these different kinds of feathers all across the body. And there's all these feathers that have these really unique roles and properties. Mm-hmm. And so I got interested in feathers just really by starting to look closely at them. Now, what is a feather is a little bit harder of a topic because there are some gray lines. In general, a feather is a thing that grows out of the skin of birds. And it grows from a structure that we call a feather follicle that's really in the skin of birds. All feathers grow from this feather follicle. Most feathers have this stereotyped sort of branched, pennaceous appearance. A lot of feathers have branches that have branches. A lot of feathers are flat, but there's also a lot of feathers that don't look anything like that. And that's when we have to start looking for things like a feather follicle to actually tell if something is or is not a feather.

Scott Taylor: I guess if you see a chicken feather, you're like, oh, all feathers are the same. But then you find like the tail feather of a woodpecker and you're like, wait a second.

Cody Limber: Something's different. Yeah.

Scott Taylor: Yeah. Something's different and interesting. How do they compare to other things that grow out of skin?

Cody Limber: Yeah, there are a lot of these things that grow out of skin that we call epidermal appendages. And basically, that's just a fancy way of saying things that grow from skin. So some good examples of this are teeth and hair and scales. And in general... While there are certain complexities, and even skin itself is a little bit complex, I think that feathers are definitely in their own realm of complexity. A scale is sort of the easiest thing to compare to a feather. Birds have both scales and feathers. They've got scales on their feet. But scales really, they grow out of the skin. They sort of grow as this lump that sort of falls over and flattens out. And it's still formed from layers of the skin and it still has this unique development, but it's not very structurally complex. And then when you compare that to a feather, a feather has branches that have branches. And these branches have to have specific shapes to allow them to be in the right position and to allow them to hook together to form these like flat planar feathers. There are all these different parts of feathers that all have to work together, and they'll have to grow together simultaneously to form something that is a feather. And for a while, people thought, oh, maybe a feather is just sort of a different kind of scale. Maybe it was a scale... You know, it was already flat, and they just had to get divided up into these branched parts. But it turns out it's totally different.

Scott Taylor: Oh, cool.

Cody Limber: And we've actually known about this for at least 100 years. There's some great papers from the late 1890s that describe this process, and it seems like people just forgot that for 90 years or whatever. Because it turns out feathers grow as a tube.

Scott Taylor: Mm-hmm.

Cody Limber: So unlike a scale that grows, you know, I guess initially as a tube and then just sort of is smooshed, a feather grows from a tube and then the tube unfurls to form this flat thing that we think of as a feather. So they're very complex structurally. They have a really, really unique development. And then obviously they have all these different roles and there are all these different kinds of feathers on the bird that make them very complex and interesting. Yeah.

Scott Taylor: Yeah, so I think we often think of feathers as having pretty specific functions. And maybe the ones we're most familiar with are like the thermoregulatory function because we actually collect feathers and use them to keep ourselves warm in coats or comfortable pillows or down comforter. Or we think of feathers in the context of flight. But could you walk us through like there are a lot of other things that feathers are used for, right?

Cody Limber: Yeah, there's very few aspects of avian biology that feathers aren't involved in.

Scott Taylor: Yeah.

Cody Limber: So they definitely help keep birds warm. They obviously help make this flight surface on the wing. They also give the bird this aerodynamic shape. These are called contour feathers that contour the bird. But there are also all these bizarre feathers that are used for different things. So there are some feathers that appear to have a sensory role, like a tactile sensory role. These are filoplumes, or sometimes bristles are often thought of as doing this. I read a paper saying that there are some bristles on some species of birds that might help keep bugs out of their eyes when they're feeding. So that's kind of a bizarre function. They're also used a lot in waterproofing. So we think of water off a duck's back. Well, it turns out that, you know, that's something that feathers are pretty good at. And they're good at it for a couple reasons. One, they give, you know, they're small enough that water tends to sort of bead up on them. But they're also protected with oil. So the bird will actually use this oil gland called the uropygial gland or uropygial gland to actually preen oil all over its feathers to make them really hydrophobic. And so the water actually can slide off of a duck's back. But not a cormorant's back. Not necessarily.

Scott Taylor: Cormorants get wet.

Cody Limber: They do have to stand there and dry.

Scott Taylor: Yeah.

Cody Limber: So it can be a really big problem. But then there are other kinds of feathers. One of my favorite is called powder down. And so it's this downy feather that basically just disintegrates. And, you know, we think it's used for bits of thermoregulation. It's also probably used for a little bit of waterproofing. It's found mostly in doves, and they sort of preen these all around. But it's almost like, you know, powdering your face with makeup is the same thing as what's going on with these feathers, which are really cool.

Scott Taylor: Live-in dust bath in a way. Like, they're kind of preening apart their own feathers in order to... Yeah, like, it's... Yeah, it's very cool because some birds will dust themselves, but it's like a way of doing it yourself.

Cody Limber: Yeah, it's like BYO dust.

Scott Taylor: Yeah. It's very easy if you work on doves or pigeons. I think a lot of people can develop allergies because of how much of that they can produce because they can be quite dusty birds compared to others.

Cody Limber: Yeah, and if you shake a pigeon, it just makes a little dust.

Scott Taylor: Yeah. No, I'm just kidding.

Cody Limber: Don't do that. Don't do that.

Scott Taylor: Liz Carlen.

Cody Limber: Yes.

Scott Taylor: She would be a good one. We've already talked to Liz about pigeons, but I never talked to her about how dusty they are.

Cody Limber: Quite dusty.

Scott Taylor: Yeah. But the feathers that are modified, I mean, I recently saw ostriches in...

Cody Limber: Yeah.

Scott Taylor: And it's so hard to remember that their eyelashes are not the same as mine. They just look so similar, but it's this other structure that has convergently evolved to look like a beautiful eyelash.

Cody Limber: Yep. And my favorite thing about ostriches is they've lost that zipper ability because they don't need it. The most important thing about that is for flight, is for keeping that planar surface. They've all lost all these barbules. Yeah. They have some, but they don't hook together quite as well or at all.

Scott Taylor: Yeah, not really at all. They just look really, really soft. And the feathers of an ostrich's eyelashes are modified feathers, right? So different from a turkey beard. They are just these feathers that look exactly like hairs.

Cody Limber: Yeah, and I mean, we didn't even touch on coloration as one of the uses of feathers. But there are all these different things that colors are used for, you know, display and camouflage and thermoregulation. And that's a whole world of things that feathers do.

Scott Taylor: So you used single-cell sequencing to figure out what a feather actually is made of. Now, single-cell sequencing, you're going to have to describe what it is because it's like pretty new and very cool, but also very weird. And so you figured out what a feather is made of at the cellular level. And so tell us like what single-cell sequencing is and why it's such a cool tool to use in the context of figuring out the complexity of these feathers that you study. Yeah.

Cody Limber: When most people hear the word sequencing, they think DNA. And that is a lot of what sequencing is. But the challenge with thinking about what are the different kinds of cells in the feather is all of the cells within a feather have basically the same DNA. They're from the same bird. And so to actually tell them apart, we have to look at how that DNA is expressed. And so we look at RNA instead of DNA, which RNA is, you know, it is the physical manifestation of the expression of DNA. It's the thing that builds all these proteins. But it's a little challenging because we have to look at the RNA from individual cells and be able to keep track of what cells they're from. And so the way that we do this is we start by taking a feather... And, you know, it's sort of like taking a Lego house and smashing it into individual Lego pieces.

Scott Taylor: Yeah.

Cody Limber: We do the same with a feather. We smash it with enzymes, I guess, into individual cells. And then we take each of these cells and we load them into a... I usually describe this part as magic, but it's like we literally load it into a black box.

Scott Taylor: It's actually a black box, which is hilarious.

Cody Limber: It's actually like a big black box. And what it will do is it'll embed each of these cells in a little tiny droplet with a really, really fancy expensive bead for like 10,000 cells at a time. And what this bead allows you to do is it allows you to capture the RNA from each of these cells and it'll label it. So it'll give each RNA a label saying it's from this cell. And that's great, except for then we have no idea where those cells were in the feather.

Scott Taylor: Right, because you've just taken the feather apart, and now you have to reassemble somehow.

Cody Limber: We have to reassemble it. Yeah, so we do that by asking, what are the different RNAs that are expressed by some subsets of these cells, some clusters of these cells. And then we go into a piece of feather, and we can stain for that RNA. We can say, oh, this RNA, which we know is only expressed in this group of cells, is only expressed here in the tissue. And so we can give the group of cells a label, and that's a thing called a cell type. It's sort of like a species. You know, it's a type of cells that sort of distinct from other types of cells. And feathers have a bunch of different cell types. And they actually, they look different from each other for the most part.

Scott Taylor: Cool.

Cody Limber: Again, sort of like a species, which is pretty cool.

Scott Taylor: Yeah. Yeah. It's a lot of work, a lot of molecular work to figure this all out. And importantly, like Cody said, you're not plucking like a feather from a bird and doing this. You're doing it in a growing feather bud that you can take and then take apart because nothing happens in a fully formed feather, right? It's done.

Cody Limber: Yeah, feathers are dead.

Scott Taylor: Right.

Cody Limber: Just like hair is dead. I guess scales are sort of mixture. But feathers are totally dead.

Scott Taylor: Yeah.

Cody Limber: Yeah. So you have to do it when they're still alive. And when they're still alive is when they're growing. So the way that we get feathers that are growing is we'll pluck feathers from a bird. And what's amazing about birds is that if you pluck a feather, it'll immediately start to regrow. And so you pluck a feather, you wait like two weeks, and then you get a nice growing feather that you can sample for these kinds of experiments. And it's sort of the equivalent of plucking a couple hairs.

Scott Taylor: It's not that bad. Not too bad. And you were doing that with chickens or zebra finch or a combo?

Cody Limber: Or both, yeah. We've been trying it on some other birds recently, too. It seems to work pretty well.

Scott Taylor: That's good. We tried it once. We plucked feathers from golden-winged warblers because we were interested in the... like what genes are being expressed in the redeveloping feathers. And there we weren't as lucky because I think we're going to do a whole episode on molt soon. Some birds, if they're not quite in the right stage of their annual life cycle, won't just start regrowing their feathers. So if they have a really... if their annual cycle is really tied to migration or things like that, I think it's harder to get the feathers to regrow when you want them to versus something like a chicken or a zebra finch, which don't have this constraint of their molt needing to match their migratory timing. But we'll get into that in another episode.

Cody Limber: Feathers grow from their base. So that means that the tip of the feather, the top tip of the feather is the oldest and the base of the feather is the youngest. And if you sample a feather while it's growing, you have this maturation gradient where some cells are older than others. And we can actually, using computational methods, we can figure out where cells are along that differentiation gradient. And so we can watch changes in gene expression over time in the growing feather. And so one of the cool things is feathers are made of beta keratin. So it's similar to, actually, it's not similar at all to keratin in your nails and hair. It's totally different. It's called keratin, but they're actually very different. Yeah. But we can see as cells start to reach this point where they're starting to die, the first thing they start doing is they just produce a ton of keratin because it's the last thing they're going to need to form this structure. So I found that really, really cool and very interesting.

Scott Taylor: That's super fascinating. So can you tell us a little bit more? I feel like I've been misled. It's called keratin, but it is not the same. How is it not the same?

Cody Limber: Yeah. So there are two things that are called keratin. There's alpha and beta keratin. Uh-huh. Yeah. They were the first and second proteins ever described. And so the reason-

Scott Taylor: Of any proteins?

Cody Limber: Of any proteins. The reason that they're alpha and beta is they were named right after the other one. Alpha keratins have alpha helices. Beta keratins have these beta-pleated sheets. Structurally, they're very different. They evolved pretty independently from one another. They just share this name keratin because they were the first two proteins described. Interesting. So people have started calling beta keratin beta-corneous protein to try and make this distinction. I don't know if that's helpful or not.

Scott Taylor: It's more of a mouthful.

Cody Limber: But they are pretty different. Even though they're sort of structurally, they do sort of a similar thing.

Scott Taylor: And birds have which form?

Cody Limber: Birds have both.

Scott Taylor: Oh, okay.

Cody Limber: Early on in feathers, they have alpha keratins. But later on, they almost exclusively switched to beta keratins. And there are so many different kinds of beta keratins. We don't know how many kinds of beta keratins there are. Every time we look, we find more. So it is kind of confusing. And it seems like there are differences in the beta keratins that give the cells that produce them different structural properties.

Scott Taylor: Uh-huh. That makes sense. And then our nails and hair are... Alpha keratins. Alpha keratins. Okay. Interesting. Yeah. Yeah. Well, I did not know that. So that's why we're doing this. All right. So you built an evolutionary tree of feather cell types. The idea about feathers is that, you know, there were feathered dinosaurs. You know, Jurassic Park was inaccurate. There should have been some with feathers. They even had colorful feathers or patterned feathers, which you've been able to figure out. From fossils, from looking at fossils and seeing that, one, there were feathers there, like Archaeopteryx, etc., back in time from Archaeopteryx and forward in time from there. But also that there were color patterns based on melanosomes within the feathers. So what did your evolutionary tree of feather types tell us about how feathers evolved from dinosaur filaments?

Cody Limber: Yeah, it's a really good question. A lot of what we know or what we think we know about feather evolution is actually based on how feathers grow. And so my advisor, Rick Prum, in 1999 came up with this theory of feather evolution based on feather development. And this is sort of classic evo-devo, using development to understand evolution.

Scott Taylor: Yeah.

Cody Limber: The way that feather cells evolved or sort of the groups that they evolved from are actually... we think, related to the organization of the skin.

Scott Taylor: Oh, cool.

Cody Limber: So skin is very stratified. You know, evolution is one of these weird things where you need to build on top of what's already been built. And so we think that's how the feather did this, is that the feather used these pre-existing cell types in the skin and then modified them a bunch to make the cells that we find in the feather.

Scott Taylor: Mm-hmm. Yeah, and just to highlight, I think a lot of people, even if they understand that, okay, dinosaurs had feathers and birds are dinosaurs, that it's really just this simple scale to feather transition. But what you're saying is it's way more complicated than that, right? Yeah.

Cody Limber: Yeah, and there are probably all these things in dinosaurs that looked sort of like a protofeather. Because, you know, if a feather is a tube, the first thing was probably this tube-like structure. And there are a ton of tube-like structures in dinosaurs that we think are not related at all to feathers. So there was probably at the base of birds, this sort of parallel exploration of what you can do with skin. And only one of these ended up being a feather, which is pretty cool.

Scott Taylor: Tell us a little bit about what you've learned about how birds get color into feathers, because it's interesting and you've specifically been looking at melanin, right? Yeah.

Cody Limber: Yeah, so melanin comes in a couple forms, and I'm going to refer to both of them, so I'll define them first. There's pheomelanin and eumelanin. So pheomelanin makes these browns and reds and even some yellows. You know, this is pheomelanin. And then there's eumelanin, which makes darker colors, so blacks and some browns, like sort of, I guess, your hair, or maybe your hair used to be more.

Scott Taylor: Wow, my hair's gray now. Whoa, Cody. Back off. For anyone who's not watching, Cody is a redhead, and I am going gray, so...

Cody Limber: Yeah.

Scott Taylor: I did used to have black hair.

Cody Limber: It turns out birds use both of these melanins in their feathers. And what's really cool about melanin is there's one cell type that produces melanin. It's called a melanocyte. It's actually, it's weirdly related to neurons, which is kind of interesting.

Scott Taylor: Yeah. So that's bizarre.

Cody Limber: That always fascinated me.

Scott Taylor: Yeah.

Cody Limber: Yeah, and they produce these melanin granules called melanosomes that they have to give to whatever they're trying to make colorful. So in our case, it's feathers. It's feather cells. And so what's really cool about these cell types is that, first of all, they're easy to identify. So they were the first cell type I felt confident knowing, like, oh, this one's a melanocyte.

Scott Taylor: Nice.

Cody Limber: So that's really nice. But we can also ask questions if we sample from different colored birds. Yeah. We can ask how their melanocytes are changing based on that color. And this is really sort of a unique way of looking at this. Often to study color, you'd look at one bird that's one color and a different bird that's a different color and compare their genomes. But here we can look at one feather next to another feather, or even one part of a feather next to a different part of a feather and ask, How are these melanocytes changing to make color or not, or to change the color that they're making? And they're doing a lot. So melanosomes are, you know, they have to be transported to the feather cells. So they have to build all this machinery to do this transport, but they also have to make the melanin and package it into these melanosomes. And they also have to get to the place where they're supposed to be in the feather as it's developing.

Scott Taylor: Yeah.

Cody Limber: And so they're doing a ton of things simultaneously. And so there's been a lot to pick through with the data. The first thing that we found that was really interesting to us is that pheomelanin and eumelanin are produced by different cells. And they seem to not be produced simultaneously by the same cell. It seems like you need to either choose one or the other to produce. Yeah. Which is interesting because there are a lot of feathers that have pheomelanin and eumelanin mixed together.

Scott Taylor: Yeah.

Cody Limber: And we even sampled some of these feathers and found that there are two separate populations of cells that are doing this.

Scott Taylor: That's so cool.

Cody Limber: The future of this work that we're hoping to do is to start looking at what are the feather cells actually doing? What's their role in this process?

Scott Taylor: In terms of like picking up the melanosomes. Yeah.

Cody Limber: We think of it as this passive thing, like I'm giving you a thing, but you also have to take the thing.

Scott Taylor: Yeah.

Cody Limber: And maybe you can make some decisions about whether or not you're taking a melanosome. And there are some feathers, gulls are my favorite example, where there are these cells called barbules. These are the smallest branches of feathers. Birds have two on each of their barbs. They're right next to each other in development. Gulls, only one of those barbules, sets of barbules gets pigment. The other one doesn't. And sometimes it switches. So if you think of like a little gull, which is black under the wing and light on the top of the wing, that's... switched from like a herring gull, which is darker on the top of the wing and lighter underneath.

Scott Taylor: Yeah.

Cody Limber: And we have no idea how melanosomes get into these cells. We don't know how melanocytes make this decision to put melanin in only one set of cells in the other one. Yeah. But we're excited to find out.

Scott Taylor: Yeah.

Cody Limber: And so we're doing a bunch of work on the basics of this process so that we can start to ask these more interesting questions.

Scott Taylor: But for the general person, what would you want them to notice about feathers or think about if they're on a walk and they find a feather on the ground? What do you want to come to mind for them first other than, ooh, this is cool.

Cody Limber: I think if you find a feather on the ground, really take a look at it and really look close. Because one of the cool things about feathers is we were talking about this zipping and unzipping of barbules. And you can hardly see them with the naked eye. But if you look really close, you can start to see how this process works. And then you can also get an appreciation, particularly if you find a feather with pattern.

Scott Taylor: Nice. That's a blue jay feather, right?

Cody Limber: This is a blue jay feather. The fact that that pattern goes across multiple structures.

Scott Taylor: Yeah.

Cody Limber: And it's in different places on each of these barbs that branch off depending on which barb you're in. Yeah. Everything has to work together so precisely to actually form even something simple like bars or stripes on a feather. Yeah. So really take a close look.

Scott Taylor: Yeah, the barring and striping is so interesting. Thinking of what you were just talking about, like melanocytes need to make the melanosomes, get them to the feather. The feather has to accept them. It has to do that across multiple cell types. So it actually gets a stripe that goes all the way across every kind of part of the feather. It's just kind of crazy to think about. And yeah, you can think of the most complex feathers, like Argus pheasant's feathers.

Cody Limber: Yeah, or a peafowl.

Scott Taylor: Yeah, exactly. Yeah, they're... That's just incredible. Do you think you'll eventually be able to really understand the development of those like eye spots and all these other really cool structures? I guess that's the goal, right?

Cody Limber: That is the goal. And there is some work in 2002, Prum and Williamson, 2002 or maybe 2004, maybe both, where they've tried to model how you form these different patterns in the feather. And it turns out you can model that using Turing patterns, which is kind of nuts. So a Turing pattern was developed by Alan Turing, and it's sort of this way of creating a sort of uniform but not stereotyped pattern. And so you can model these things, and if you know how a feather grows, you can put it on a feather and you can get all these patterns that feathers make. And importantly, you can't get patterns that feathers don't make, which is also kind of interesting and maybe tells us something about how this process works.

Scott Taylor: Yeah.

Cody Limber: But we don't know any of the genes that are involved in this. We don't know any of the signaling pathways. I mean, we know a lot about how melanin is produced, but we don't know how it's controlled and regulated at such fine scales.

Scott Taylor: Yeah.

Cody Limber: And so these are the things that we're hoping to be able to find by looking at the cells that are in these feathers. And yeah, if we come up with a model that's actually informed by molecular biology, that would be kind of amazing.

Scott Taylor: Yeah. It would be incredible.

Cody Limber: To be able to understand how you make these complicated things.

Scott Taylor: Yeah. We opened with, okay, feathers are, most people think of them as simple, but they're not simple. But there's some pretty insane kinds of feathers out there.

Cody Limber: There are some pretty bizarre feathers. And we actually, we have a whole project looking at like weird things that we found that we weren't sure whether they were feathers or not.

Scott Taylor: Okay, cool.

Cody Limber: So there are some things that are really weird. A bunch of birds of paradise have some pretty bizarre feathers.

Scott Taylor: Yeah.

Cody Limber: You know, wires that it wags on its butt. But we know those are feathers. So, you know, that's pretty easy to tell. The weirdest ones for me are the ones where it's, it's like in a bizarre place with a bizarre form. And then like, maybe it grows in a weird way or something. So my favorite is the horned screamer. Yeah. Which is this big, bulky bird. But they've got this one little dongle on the top of their head. And if you look in paleontological literature, this thing is regularly referenced as not being a feather.

Scott Taylor: Oh, interesting.

Cody Limber: And it totally is. It totally is. If you look closely at it, it's got all these characteristics of feathers. But it is also bizarre because apparently it grows continuously, which most feathers grow once and then they stop and then they're molted, so they're lost. And then they'll regrow from the same feather follicle. But there's very few cases of feathers that grow continuously, which is bizarre. It doesn't have any branches on it. And it's actually... You know, we think of a feather as having this like central backbone and you would think, okay, maybe it's that. Maybe it's just lost these other parts. But it's actually, it turns out it's the base of the feather that this thing is. So it's all base, which is kind of bizarre.

Scott Taylor: So it's totally weird. And it grows continuously. But like, so what wears it down? Do they use it for something?

Cody Limber: They don't really... The end is reported as being pretty fragile.

Scott Taylor: Okay.

Cody Limber: So it can break off and it can wear down.

Scott Taylor: Yeah.

Cody Limber: So that's one of my absolute favorites. My other favorite is not a feather. It's turkey beard.

Scott Taylor: Oh, okay.

Cody Limber: So that's the structure. It grows out of the turkey sort of from its midline in its chest. They can get quite long. They also continuously grow. They wear down. Oh. But they've been reported up to like 18 inches or something. So they can be really long. Yeah. Yeah. They're totally bizarre.

Scott Taylor: And they're not feathers.

Cody Limber: They're not feathers. They don't grow from a follicle. They don't have any of the branching that feathers have. They actually are branched, but in a way that's totally different from how feathers branch, which is also bizarre. They're, you know, they're not molted. They're... apparently composed of beta keratins like feathers, but I'm a little bit skeptical because this was done before we knew about all the diversity of beta keratins. So they could say it's beta keratin, but they couldn't say it's feather beta keratin.

Scott Taylor: All right, we've reached the part of the show we call That's B.S. or That's Bird Stuff, where we give our guests an opportunity to debunk a myth that ruffles their feathers. So Cody, what do you want to call B.S. on?

Cody Limber: I want to call B.S. on the idea that feathers evolved for flight. It's one of these things that you always hear when you're talking about feather evolution is like, oh, feathers are for flight.

Scott Taylor: Right.

Cody Limber: But to actually get a feather that works for flight, you need to do a bunch of things to make a really specific feather. And so we think that the way that feathers evolved was first, they just evolved as these, you know, tubular, almost hair-like structures that were growing out of the skin. We don't know why they became elaborated upon. And as birds started to, you know, glide down from trees, or I guess, jump up from land, depending on who you ask.

Scott Taylor: I like the glide down from trees hypothesis the best, but yes, there are multiple.

Cody Limber: That's my personal favorite. Yeah. But we think that the feathers then sort of became elaborated and helped birds glide longer and longer. And that's how we ended up evolving the flight feather. Evolution doesn't really work for a specific thing. It's sort of evolution just uses whatever a thing is good at best. And it happened that feathers were very simple at first and wouldn't have been useful for flight.

Scott Taylor: Yeah, yeah. That's a great B.S.. And I've heard it a lot. And I think the students in ornithology are always surprised to realize that no feathers were for some other thing we don't fully understand prior to being co-adopted and used for flight. Well, thanks so much for joining us on the podcast today, Cody. It's been awesome to chat all about feathers. Congrats again on recently finishing the PhD. And I look forward to whatever you're working on next. So thanks for joining us.

Cody Limber: Thank you.

Scott Taylor: Birds are dinosaurs, and around here we like our snacks, so we end each episode with a dinosaur nugget. Today's nugget is, a feather is not just a modified scale. It grows as a tube, unfurls into branches that have branches, and every one of those pieces has to be built at the same time as the feather emerges from the feather bud and then dies. Or the whole thing fails. So, feathers came first, flight came later, and we still don't know why feathers evolved in the first place. But every modern bird has them, and they're important for thermal regulation, flight, contouring, waterproofing, sensing the world, and for displays. That's a wrap on this week's episode. Okay, but what's up with feathers? If you like this one, leave us a like or five-star rating, and go find a feather, unzip it, zip it back together, and think about the amazing complexity. We'll catch you next time. Byeee.

This transcript was generated using AI-assisted transcription and may contain errors or omissions. Please refer to the audio or video episode for the most accurate representation.