(SOUNDBITE OF MUSIC)

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. Evolution has given the creatures that inhabit the earth a number of skin coverings: scales, fur and skin, for example; but one of the most remarkable, our guest Thor Hanson says, is the feather. In his new book, Hanson traces the evolutionary origin of feathers, noting that they probably adorned some dinosaurs. He explains how feathers keeps birds warm in freezing temperatures and cool in blazing heat, how they function in attraction and mating, and how they enable flight.

Thor Hanson is a conservation biologist who has studied birds around the world. His first book, "The Impenetrable Forest," was about gorillas in Uganda. His book, "Feathers: The Evolution of a Natural Miracle," just came out in paperback. He spoke with FRESH AIR contributor Dave Davies.

DAVE DAVIES, HOST:

Thor Hanson, welcome to FRESH AIR. In the book, you describe an experience you had observing vultures, which kind of inspired you to look more into feathers. You want to tell us that story?

THOR HANSON: Yes. Well, that story occurred some years ago now. But I was working on a vulture project in Kenya and the project involved studying the feeding habits of vultures. Which, if you've never done it, tends to involve moving around a lot of vulture food in wheelbarrows and such. So here I was with a wheelbarrow full of zebra remains in the hot African sun, and after transporting that stuff around and slopping it hither and yon, I ended up covered with the blood and guts and the gore, so to speak. And I realized, of course, that in that moment I was very much like a vulture myself. But, of course, with my long ponytail and my T-shirt and my jeans I wasn't very well adapted to the lifestyle. I was covered with the stuff and it got me thinking.....

DAVIES: And I just - I hate to interrupt you, but I don't want you to leave out a couple of details. The stuff was like two days old, right, the zebra remains?

(LAUGHTER)

HANSON: Yes.

DAVIES: And you are...

HANSON: Yeah we got, we...

DAVIES: You're handling this bare-handed, right?

HANSON: Yeah. Well, we typically would go to this slaughterhouse where they would save things for us. But we showed up one day and they were closed but they had just left everything in the side yard. And so here was this heap of animal intestines and hoofs and bits of zebras and we hadn't brought anything to put that into the trailer. They usually did all of that for us. So here we were and we had to get it in there somehow. So we were heaving stuff into the trailer and getting it into the wheelbarrow and it was a sticky, sticky, stinky mess.

So at any rate, this got me thinking about vultures and their feeding habits and the advantage of having a bare head. I had originally been wondering what would make a bird lose something so inherent to its nature as its feathers. And of course, in this case for the vultures, it's this awful stuff they're eating every day, and having lost their feathers, that allows them to remain much cleaner and much free from - more free from bacteria and parasites and disease.

DAVIES: Now there are a lot of different skin coverings we see in the natural world. I mean some creatures have scales, we have hair, some have fur. What's so remarkable about feathers?

HANSON: Ah, so many remarkable things. So many. But I think if you want to boil it down to one comment, it is this: there are many, many things in nature that are beautifully adapted to a single purpose. But almost nothing has adapted to so many purposes as feathers. Those other skin coverings that you mentioned are marvelous in their way but you don't see them with the diversity of structure and function that you find in feathers. You have a feather that can be the finest insulation that we've ever discovered. You have feathers that are wonderfully waterproof, structurally waterproof. You have feathers that are beautifully adapted to flight - perfect airfoils. You have feathers, of course that's our wonderfully adapted for display with incredible iridescent colors, long flowing plumes. So the diversity of form and function in feathers is truly unique.

DAVIES: All right. Let's talk about understanding the evolution of feathers. And it's interesting, you know, fossils preserve bones well. Feathers, not so well. What do we now think we know about how feathers appeared in the evolutionary chain?

HANSON: Well, we are probably as close to some kind of scientific consensus as we ever have been about the evolution of feathers and the evolution of birds, because the quality of the fossils coming out of China, and the feather specimens preserved there, tells quite a lot about how they evolve and where they evolved. And that it does appear that all the stages of feather evolution, those steps necessary to build the complex modern plume occurred in a lineage of dinosaurs called the theropods, the meat-eating dinosaurs. It's a group that includes such famous dinosaurs as Tyrannosaurus or the Velociraptors from "Jurassic Park." That story has gained a lot of new evidence based on the fossils now coming out of China.

DAVIES: And it's fascinating that these were not flying creatures, right, but we think they may have been what, covered or partially covered in feathers?

HANSON: Yes. There's great evidence now for feathers evolving in the theropods. And the creatures were primarily runners - they had long, strong, long, strong overdeveloped, marathon-runner legs, and walked on two legs and ran on two legs, which freed up their arms, perhaps, by one of the theories, to be feathered and used eventually for flight. But one thing that we really believe now about the evolution of feathers is that the simplest feathers - the first feathers in the series of steps towards building a complex vein feather - were not aerodynamic. So the first uses of feathers must have been for other purposes. And what's really amazing about the quality of these fossils is they are actually able to determine in some cases the colors of those early, early feathers, which gives real credence to the idea that an early use for feathers was display and color.

DAVIES: You know, you describe de-feathering a wren, a little bird that, what, that you're in the habit of picking up birds and putting them in your freezer? Is that right?

(LAUGHTER)

HANSON: Yeah. The freezer is a, it's a complicated place in our household. You dive in there for a bit of ice cream, you might come out with a vole or a woodpecker.

DAVIES: And the family has gotten used to this, huh?

(LAUGHTER)

HANSON: My wife is very patient.

DAVIES: Well anyway, so you defrost this wren and pluck the feathers. Just describe a little bit about that experiment and kind of what it told you about feathers and birds.

HANSON: Well, in all this research about feathers I realized, you know, I really need to disassemble a bird, essentially, to get my head around this diversity of form and function. I need to look at these things in detail and see how they're arranged on a specimen. So I went to the freezer and sure enough I had a specimen there that fit the bill. And I headed down to my office with the only reference on plucking I could think of, "The Joy of Cooking," and looked up how to pluck a bird, and by gosh, I set to work on the thing. And it took me hours. There is a fellow in Ireland who holds the world record for plucking a bird. He can do a turkey in about a minute and a half. So it was hours for me to get through this wren but I learned a great deal from doing so in terms of how the feathers are arranged and how many different types, and just how many there are on a bird for this tiny wren. I had well over 1,500 feathers by the time I was done, an incredible number of feathers and it's a real testament to their importance for birds that in general the weight of their feather coat outweighs the dry weight of their skeleton by more than two to one in most species.

DAVIES: Yeah, you were struck by when you got - when you'd plucked the feathers, how little bird there was left.

HANSON: That's absolutely right. The poor bird was, you know, the remainder was just a bedraggled little carcass whereas I had piles of feathers all over my desk.

DAVIES: We're speaking with Thor Hanson. He is a conservation biologist and has a new book called "Feathers."

We'll talk more after a break. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

DAVIES: if you're just joining us, we're speaking with conservation biologist Thor Hanson. His new book, "Feathers: The Evolution of a Natural Miracle," is now out in paperback.

I want to talk about some of the ways that feathers function in animals. But first, tell us what are feathers made of? What's their structure?

HANSON: Well, feathers are a protein. They're made of keratin. It's a specific keratin to feathers, but it's similar in general to the keratin that makes up our fingernails or our hair. Keratin is a very common protein in nature because it's tough stuff. And so you find it in the skin of animals or in these hard and horny surfaces or hair, things that have a protective function. So feather protein is one that is very well suited to feathers because it is lightweight and very durable and very tough and it takes color well. So it is a protein-based structure.

DAVIES: So the shaft grows and then the little - what do you call them?

HANSON: The barbs.

DAVIES: The barbs coming off - come off the feather in a variety of ways and then they interlock as they grow, right?

HANSON: That's right. The growth of feathers is truly remarkable. Now we know how hair grows, more or less; it grows from a follicle. But it's a simple process compared to feathers in that hair is just a bunch of dead cells stacked on top of one another and they come out of the follicle. And we know they're dead cells because we don't bleed when we go to the barber, unless we have a very clumsy barber. But on the other hand, feather growth is very complicated and when feathers are growing, in fact, they are infused with blood because it's a complex process. If you injure the pinfeather, the growing feather of a bird, that's quite a significant injury, it will bleed a lot. It's only after that growth is completed that the blood vessels are retracted from the base of the feather. So while it's growing, it's alive because you have this complex sort of helical process going on with the feather barbs growing from around the sides of the follicle and being attached to the rachis, that central shaft. So because it's so complex you can do a lot of complex structural things with it. You can make these incredibly branched and delicate structures that have so many functions.

DAVIES: All right. When you really want to stay warm in the winter, you get a down quilt, I guess goose down. Why are feathers particularly good at keeping birds and us warm?

HANSON: Well, feathers are, to this day, they are the most efficient insulation known. We haven't been able to match them with synthetics. And I think it boils down to that growth process and the fact that you can make these fine, fine branch instructors. The key to insulation is what they call loft. How much air can you hold in a small space? And because feathers are so beautifully and finely branched, they can hold a great deal of tiny, tiny air pockets in that branched structure. And that's what people try to mimic with synthetics but we haven't been able to match feathers for that yet because it's difficult to manufacture finally branched structures.

DAVIES: Now there are seabirds that you say always stay dry because the feathers amount to waterproofing. And you described a story, once, of finding a seabird on a highway, a murre . Is that what it is, a murre?

HANSON: Yes, a common murre.

DAVIES: Right. And you say that when you picked it up you wanted to make sure that before you tried to return it to those sea that none of its feathers were disturbed because well, why would that be the case? Why couldn't you take it back to the water if feathers had been rearranged or disheveled?

HANSON: Well, murres and other seabirds occasionally, when they're flying over land, they will mistake a road or a parking lot for water and they will land. And again, the real danger to them if this happens is that they damage their feather coat, because you can't put one of those little birds out in the icy water with a damaged feather coating over its belly, because if the water touches the skin of that bird, the thing is going to suffer immediately from hypothermia. The birds must maintain a waterproof feather coat at all times. And the way they do it is really quite remarkable. For many years, ornithologists believed that birds maintained waterproofing by applying preen oils to their feathers and that the oils must be repelling the water. And the oils play a role, but they play a small role. Because what's been learned is that you can wash all the oil off of a contour feather and then drop water on it and it will bead up and wash away. You know, you can scrub that feather with acid and remove everything and - leaving only the structure of the feather itself - and the structure is waterproof.

DAVIES: If you're just joining us, we're speaking with conservation biologist Thor Hanson. His book, "Feathers: The Evolution of a Natural Miracle" is out in paperback. Well, let's talk about flying. How do feathers permit birds to fly?

HANSON: Well, feathers are beautifully adapted for flight, but we do know now that flight came after the feather, in that the early stages of feathers were not aerodynamic. It's only the most advanced stages in feather evolution that have that aerodynamic structure.

But ultimately, the result of all of this evolution is an incredible adaptation for flight. If you look at a bird's wing it has a particular shape that is similar to the shape you would see if you looked out the window of an airplane and that is an airfoil-shaped wing with a curved upper surface that gives that wing a bit of extra lift in the air.

And a bird wing has that shape, just as an airplane wing does. But what's amazing about a bird wing is that the individual flight feathers are also shaped like airfoils. You can see these feathers. They're often the ones that you might find on the ground that have been shed by a bird. The long tapering, beautiful flight feathers of a bird are airfoils.

So what you get for a bird wing is an airfoil made up of airfoils and the bird has muscle control over all of those feathers. So it can constantly adapt and change the position of feathers and the shape of the wing to react to any change in air temperature or wind direction or air pressure, making it a truly incredible way to fly.

DAVIES: Now, all right. So in addition to, you know, insulation and waterproofing and flight, feathers make for very colorful plumage. And of course a lot of birds will use them to attract mates. And I wonder why is it so often in the natural world that it's the females that have these bland, camouflaged, you know, paler colors but the males who are so bright and ornate?

HANSON: Great question. So this boils down to a very important theme throughout nature and that can be summarized as this: Females are choosy. Females are choosy and so they have the ability to choose a mate and because they can choose a mate, it produces competition among males.

And once you have it set up where females can choose a mate, then you see competition among the males. And in birds, that competition has resulted, in many instances, in incredible feathered displays. So that choosiness on the part of the females is what they call sexual selection, another one of Darwin's great ideas. Really, his second great contribution to evolutionary theory.

This idea that the choice of mates can drive evolution and it's very clear in birds. He spent four chapters on this in one of his books, in the colorful displays of the males and their wonderful feathers. You see it in the birds of paradise, you see it in a wide range of birds where the males are incredibly plumed, in some cases to the point where they're so ornate they seem they can hardly function.

I mean, look at the display of a peacock, for example. And so that has driven this wild variety of colors and displays. And on the other hand, you often see in those instances the females retaining a much more drab coloration. Well, that has an evolutionary purpose too, in that for many bird species it is the females who are sitting on a nest to brood the eggs.

And if you're going to be sitting there on a nest all day it's a much better idea to blend in than to look gaudy.

DAVIES: Well, Thor Hanson, it's been really interesting. Thanks so much for spending some time with us.

HANSON: Well, thanks for having me on and thanks for all the great questions.

GROSS: Thor Hanson spoke with FRESH AIR contributor Dave Davies. Hanson's book "Feathers" just came out in paperback. You can read an excerpt on our website, freshair.npr.org.

Copyright © 2012 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information.

NPR transcripts are created on a rush deadline by a contractor for NPR, and accuracy and availability may vary. This text may not be in its final form and may be updated or revised in the future. Please be aware that the authoritative record of NPR’s programming is the audio.

Comments

 

Please keep your community civil. All comments must follow the NPR.org Community rules and Terms of Use. NPR reserves the right to use the comments we receive, in whole or in part, and to use the commenter's name and location, in any medium. See also the Terms of Use, Privacy Policy and Community FAQ.