An Aggressively Glossy Bird
The cassowary, a large flightless bird native to Australia, New Guinea, and nearby islands, has a reputation for aggression and wickedly clawed feet that can cause serious injury. Indeed, they’ve been known to attack humans dozens of times, and even occasionally kill people.
But they also have a beauty trick: New research published in Science Advances reveals their glossy black body feathers have a structure for producing shine that’s never before been seen in birds. Where other black birds like crows are shiny because of structures in their feather barbules, the cassowary instead derives its shine from a smooth, wide rachis—the main “stem” of the feather.
University of Texas paleontologist Julia Clarke explains how the cassowary’s color could help shed light on the feathers of extinct birds and dinosaurs—and how paleontologists are investigating the evolution of birds as we see them today.
Julia Clarke is a professor of Vertebrate Paleontology at the Jackson School of Geosciences at the University of Texas at Austin in Austin, Texas.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Just a quick note before we get started, we miss you. We miss talking to you and we want you to say hello. So please talk to us on the Science Friday VoxPop app on Twitter. Or even email us, firstname.lastname@example.org.
Later in the hour, we explore new insights into the genetic story of the early humans who populated East Asia.
But first, in the bird world, you can be one of two branches of the evolutionary tree. Either you’re one of the 10,000 flying songbird shorebirds, and raptors. Or you’re one of just 70 species in the group of mostly flightless birds, like ostriches, emus, and the fierce giant cassowary, a native of Australia, New Guinea, and nearby islands.
Cassowaries, with their strong legs and vicious toes, have a well-earned reputation as aggressive, dangerous birds. They’re not just a scary face. Producer Christie Taylor takes a closer look at new research on the evolutionary mysteries of cassowaries and their close relations. And she begins, of course, with their feathers.
CHRISTIE TAYLOR: If you’ve ever looked at a cassowary, I hope you’ll agree that they’re not just weird-looking, flightless murderers. They’re also kind of gorgeous. They’ve got these bald, bright blue necks and really glossy, fluffy-looking black feathers that drape their very powerful bodies.
It’s that black glossiness that caught the eye of Dr. Julia Clarke, a paleontologist at the University of Texas in Austin. She joins me to explain how a close look at feather color can help scientists understand not just the evolution of cassowaries, but also their extinct cousins and the dinosaurs that gave rise to them. Welcome back to Science Friday, Dr. Clarke.
JULIA CLARKE: It’s my pleasure to join you, Christie.
CHRISTIE TAYLOR: Julia, when I look at a cassowary, my first question is really not, why is it so shiny?
JULIA CLARKE: Well, yeah, I guess that isn’t most people’s first question about cassowaries. Maybe their first question is, is it going to kill me? Shininess is not unusual in many, many groups of birds. But in the group of birds to which cassowaries belong, it is. So that’s kind of what got us started on this.
And I happened to be in a collection space in the Netherlands, and they had some cassowary specimens. And it was so striking. And these were even kind of a little bit dusty, you know? And I was struck by how shiny these feathers were. And I was like, how?
CHRISTIE TAYLOR: OK, so this feels like a question I would ask in a fable. But how did the cassowary get so shiny? You set out to investigate. What did you find out?
JULIA CLARKE: Well, we at first thought there might be modifications of the structure on the outside-most part of the feather, so that the feather keratin could be particularly smooth and maybe more reflective. So we investigated that, and it didn’t seem to be explanatory. It didn’t seem to explain the phenomenon.
And then my co-author, who did the biophysics of this structure, found that it was actually some aspects of the dimensions of the rachis, or the center part of the feather itself, that was contributing to the shine. And to be clear, what we found was that cassowary’s shiny blackness is among the brightest known shiny blackness in living birds. So this wasn’t just a moderate thing, but this is up in the zone of shiny crow feathers, for example. But that shine in cassowaries is produced by having really fat, wide central parts of the feathers. It’s actually aspects of the feather geometry that are most important to determining that shine.
CHRISTIE TAYLOR: Is being shiny a useful adaptation for either a bird or a dinosaur? Like, if you look at a cassowary, I feel like it has everything it needs just in those really powerful feet.
JULIA CLARKE: Man, cassowaries are so fascinating. And if we think about what colors do more broadly, is that they can be used in crypsis, or camouflage, or they can be used in communication, and typically communication with other members of their own species. But it also could be, since cassowaries live in these typically heavily forested environments, and we can’t rule out the possibility that these black feathers could function in crypsis.
It could function in somehow making the cassowary blend better into a shadow-dappled forest environment. But I will say that most shiny feathers in birds that have been studied are related to communication. So they function in what we call the sexual selection system, which might be in things related to finding a mate.
CHRISTIE TAYLOR: Because I feel like I don’t have this picture in my head quite yet, if we’re looking at a feather, the part that the cassowary has that’s shiny is like the trunk of a tree, right?
JULIA CLARKE: Exactly.
CHRISTIE TAYLOR: So how is a crow’s feather, a black shiny crow feather, where is that shine coming from, if it’s not like this tree trunk part of the feather?
JULIA CLARKE: So if a feather is tree-like and it has a central trunk– that’s the rachis– and off of that come barbs. And then off those barbs come the twiglets, which are the barbules. And in the crows and in many other birds, the shiny black color is conferred by changes in the structure of those twiglets.
So in birds like a shiny pheasant or a shiny iridescent peacock, for example, you’re going to have shine conferred by the arrangement of these melanin pigment-filled structures within the feathers themselves. And there could be one layer of these or multiple layers. And the spacing of the layers could even determine whether they refract more of a reddish tone shiny black, or more of a blue-green tone shiny black. So that’s basically the mechanism that we’ve now found evidence for in non-avian dinosaurs that have branched feathers.
CHRISTIE TAYLOR: I’m really glad you mentioned dinosaurs because I know you’re a paleontologist. And I really want to know why you’re looking at living modern birds and their color right now.
JULIA CLARKE: So I think sometimes kind of moving between being motivated by questions about things we see in deep time can bring us to ask new questions of animals in the present day. And conversely, we go the other direction. Maybe we understand a mechanism.
Now that we understand this mechanism of shine in the cassowary, we could bring that understanding to look back at the fossil record and say, could we find evidence of this form of shininess in the fossil record? What would those fossils look like? How would we maybe ask that question.
So right now, we have a dinosaur that’s about 120 million years, that shows that those long, skinny, melanin-containing structures that make shiny blacks in things like crows or things like chickens and ducks. But we also have evidence for about 140 million years of another form of structural color. So that would have been potentially some shiny component.
But what was really cool is that, in dinosaurs that are even more distantly related to birds, we call these, among friends, fuzzy dinosaurs.
CHRISTIE TAYLOR: [LAUGHS]
JULIA CLARKE: These are things– yeah.
CHRISTIE TAYLOR: Adorable.
JULIA CLARKE: Well, it’s clear. They’re not hair-covered, but what they have are thin filaments, maybe several filaments connected together at the base. Then maybe they have thicker filaments that would be with something we’d call a bristle. And those vary in different species of fuzzy dinosaurs, these dinosaurs that are more distantly related to birds.
But we haven’t found any evidence of these melanin pigment-based mechanisms for making shine in those filament structures. And so what’s interesting about the cassowary thing is, this is a mechanism of coloration that does not depend upon the shape of those pigment-containing organelles, that’s really conferred by the shape of the bristle itself. And this might be a mechanism by which fuzzy dinosaurs got shiny.
But now we can turn to the fossil record and maybe think, how might this be preserved? Maybe there are feathers in amber, for example. Maybe there are other just exceptional cases. Because it really depends on what fossil records you look at, what kind of information you can get from them.
CHRISTIE TAYLOR: I got to say, when I look at a cassowary, when I look at an ostrich, when I look at an emu, I feel like I’m looking at something that is a lot more closely related to non-avian dinosaurs than like a pigeon. Do we see that they’re more primitive?
JULIA CLARKE: So I think what we see when we look at them and are like, oh, my gosh, big dinosaurs, is that we can recognize their affinities with these flightless– like all of the bipedal raptors and other dinosaurs that we more commonly encounter in the media, in museums. We can make that relationship more easily, because they’re big. They’re bigger than the other things.
So what we know happens in the evolution of our living birds today is that, they generally got smaller and they evolved flight. And then in these living guys, like cassowaries and ostrich, they reverse that trend. So they lose flight and they get big. So it’s not an ancestral bigness, but it does hearken back. It helps us make that connection with non-avian dinosaurs.
CHRISTIE TAYLOR: So is there any living bird that you would say probably has the most to tell us about how these birds arose from non-avian dinosaurs?
JULIA CLARKE: Oh, man, I love them all, you know? I mean, you can’t pick just one species as– people used to do this. And actually, the most notable case was Scott’s polar expedition, right? The thinking at the time was that penguins were one of the groups that were going to shed the most light on the origin of birds, because their feathers are highly– well, now we know, modified and, quote, unquote, “scale-like.” And people thought that was a primitive trait. And they thought that, if you could observe the ontogeny from the egg of a penguin, like an emperor penguin, you might know about the origin of birds.
Well, all of those things are completely false. You would learn a lot about the development of penguins, which independently lost flight. But you wouldn’t learn about bird origins. So there’s no one species that’s going to show you differentially more about that origin. But I will say that, when you look at songbirds outside our windows, those are among the most heavily modified of living birds. They have a lot of new traits that are absent in most other birds.
And when we look at things like a chicken or a duck, those species have been parts of groups that have been evolving independently for, I want to say, more than 65 million years. So they have had their own separate history. Each one of those is, let’s say, if it’s alive today, it’s equidistant from the bird ancestor. So we got to look at them together to get that picture of, what are traits that ancestral bird might have that we could bring down and look at in non-avian dinosaurs. We can’t just single one out.
CHRISTIE TAYLOR: Good luck to you in finding that. Thank you so much for joining me today. Julia Clarke is a professor of paleontology at the University of Texas at Austin. Thank you again so much.
JULIA CLARKE: Thank you, Christie. That was a lot of fun.