How Do You Know If A Feathered Dinosaur Could Fly?

IRA FLATOW: This is Science Friday. I’m Ira Flatow.

Later in the hour, looking at how artificial intelligence is an energy hog. Yes, perhaps worse than crypto mining. But first, not all birds can fly. I’m looking at you, penguins, ostriches, and kiwis.

Now, it’s pretty easy to figure out if a living bird can fly. But what about extinct birds or, say, bird ancestors, like dinosaurs? Remember, all birds are dinosaurs, but not all dinosaurs evolved into birds. So scientists wanted to figure out if there was a way to tell if a dinosaur could fly or not.

They found out that the number and symmetry of flight feathers are reliable indicators of whether or not a bird or a dinosaur could lift off the ground. Pretty interesting, isn’t it?

Joining me now to talk more about their research and how it might help us better understand how dinosaur flight evolved are my guests from the famed Field Museum in Chicago, Illinois, Dr. Yosef Kiat, postdoctoral researcher, and Dr. Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum. Welcome to Science Friday.

JINGMAI O’CONNOR: So exciting to be here and tell you about this really awesome research that Yosef has led.

IRA FLATOW: OK, Dr. Kiat, let’s get right to the bottom line. What are the characteristics of a bird or a dinosaur that can fly versus one that cannot?

YOSEF KIAT: So we found that asymmetry of the primary feathers, one of the most important feathers to flying in birds, differ between flying and flightless bird species, with only a little overlap between these two groups. And then we applied this result to the extinct species. I mean, dinosaurs and stem birds and non-avian dinosaurs.

And then we can reconstruct the flight ability in this species. And we found that asymmetry of the primary feathers also occurs in some extinct species. And this means that this species already have a flight ability.

And the second important result, we found was that, among modern birds, all flying species have nine to 11 primary feathers. When the birds have fewer or more feathers than 11 primaries, it must be a flightless species. And then we again go back to the extinct species, non-avian dinosaurs or stem birds, and we found that in some species we can find more feathers than 11.

IRA FLATOW: Yosef, when you discovered this nine to 11 feather feature in the birds that can fly, the primary feathers, were you surprised by this?

YOSEF KIAT: I’m not surprised because I see it before I try to test it. But after I test it in a large number of species, yes, it’s become slightly a surprise that this continue to be a strong trait in all tested species.

JINGMAI O’CONNOR: I mean, it’s really astounding on my part, because, see, Yosef has ringed thousands of birds. He’s caught so many birds. So he’s really familiar with modern birds. Whereas I’m a paleontologist, so I don’t really know that much about living birds. If you think about them, they have so many different ways of flying.

You have bounding flight and soaring flight. And if you think about the shapes of birds’ wings, they come in all different shapes and sizes. Some are very long and narrow and some are big and broad. And despite that enormous diversity, only nine to 11 primary feathers.

So yeah, for Yosef, it was something he noticed while doing research, and then tested it and proved it was right. But for me, he’s just sharing this data with me. And I have to say I was quite surprised.

And I think it’s really, really interesting. And I really hope that this opens up new avenues of research and that developmental biologists– people who work on biomechanics, et cetera– I really hope they jump in on this interesting observation and try to help us understand the why that’s underneath it.

IRA FLATOW: When you say hope you understand the why, exactly what why are you looking for?

JINGMAI O’CONNOR: Why do all flying birds have only nine to 11 primaries? I mean, that is a very narrow range, considering this enormous diversity. And so there must be a reason why. And I would love to know why.

And then what we noticed is that when a long period of time has passed since flight has been lost, then the wing develops a new function. And the number of feathers changes with those other functions.

So for example, with penguins, and their forelimb now being a flipper that’s essentially used for underwater flight, the feathers are now very small, but they’re now functioning more similar to body feathers– to just really insulate the surface of the flipper that once, long, long ago, used to be a wing.

IRA FLATOW: Wait, wait. I got to stop you there for a second. You’re saying that a penguin used to fly.

JINGMAI O’CONNOR: Penguins evolved from birds that were able to fly, yes. So one of the prerequisites– one of the very first steps in evolving a penguin– was the loss of flight.

Now, there probably was an early evolutionary stage, where they could basically fly underwater. So if you look at birds that forage in the water, they either forage with their feet or with their wings.

IRA FLATOW: Yeah, I’m thinking of cormorants here.

JINGMAI O’CONNOR: Exactly. Yeah. And so this early stage would have been having a wing that can function for flying underwater and also flying. But eventually, as the animals became more dependent on this underwater foraging ecosystem, the wing and the whole body shapes ever more for life in water.

IRA FLATOW: You were talking about birds that developed feathers earlier in time were different than other birds. Explain that. They had different functionings for their feathers.

JINGMAI O’CONNOR: Feathers are features that all living birds, which all have a common ancestor, they inherited from non-avian dinosaurs. And in fact, you can trace the earliest feathers back to being present in the ancestors of both pterosaurs and dinosaurs. So these primitive early ornithodirans had very simple feather structures that most likely evolved for insulation.

And then, only in dinosaurs closely related to birds do we see modern feather morphologies evolving. And we know that the arrangement of these feathers, forming a winglike structure on their forelimb, also evolved for some other purpose that was not flight. And then, evolution basically hijacked this existing structure for a new purpose– for aerodynamics, for flight.

One thing that we understand from the research led by Yosef is that actually the current fossil record is not capturing the early stages in the evolution of these feathers and the evolution of this proto-wing structure. Right now, in the fossil record, is a taxon called Caudipteryx. It’s an oviraptorosaur. But Yosef’s research suggests that this is a secondarily flightless dinosaur.

We just think anyone who is investigating this area needs to take the soft tissues into account. They need to account for what the feathers are telling us. And this is something that people really haven’t done previously. People try to understand how flight evolved, but didn’t actually look at the feathers that were sustaining flight themselves.

IRA FLATOW: You’ve mentioned secondary flightless a few times. Please explain to me what that means. Why is that a key issue here?

JINGMAI O’CONNOR: So when we think about one of the most important characteristics of living birds is their ability to fly, but not all birds can fly. And the birds that cannot fly– like penguins and ostriches, but also things like flightless cormorants– they all have lost their ability to fly at some point during their evolution. So they evolved from birds that were able to fly. So this is what we mean by secondarily flightless.

And actually something that I think is really interesting about Yosef’s research, basically what it says is that birds that lose their ability to fly are evolutionarily short-lived lineages. Once they lose their ability to fly, they go extinct within several million years.

But if terms of lineages that have lost their flight and have been around for a very long time, there’s very few. It’s essentially penguins and different lineages of paleognathous birds, like kiwis and ostriches. If the feather data is correct and Caudipteryx is a secondarily flightless dinosaur, then it means that secondarily flightless dinosaurs did not have the same problem as secondarily flightless birds.

This would mean that the loss of flight, in the case of non-avian dinosaurs, did not hinder them the way it seems that the loss of flight hinders most modern avian lineages.

IRA FLATOW: Dr. O’Connor, can you paint us a picture of what does a Caudipteryx look like?

JINGMAI O’CONNOR: If you’re going to imagine Caudipteryx, it would be about the size of a large turkey. And it would have very short arms, but with tiny, little wings– wings that are proportionately much shorter than you would see in a chicken or any bird that’s able to fly– just like tiny little wings that were probably used for ornamentation, but maybe they were used for running. We’re really not sure.

The legs would have been really, strong robust legs. So it’s an animal really adapted for running. And it would have had kind of a big belly because most specimens preserve huge masses of gizzard stones, or gastroliths, inside the stomach. The gastral mass in non-avian dinosaurs is bigger than it is in birds. And it’s probably because, once you evolve flight, you’re trying to constrain your body mass. You want to be as light as possible.

So in these dinosaurs that are not flying, they’re able to have really big gastral masses, with these large stomachs. And then it also had teeth.

IRA FLATOW: So are you are you saying that this dinosaur’s ancestors was able to fly and that this dinosaur lost that ability?

JINGMAI O’CONNOR: So that is what Yosef’s feather data indicates. But that doesn’t mean that that is what happened. But the data from the feathers is strongly suggesting that.

IRA FLATOW: Do we know why some dinosaurs went on to be able to fly and why some were not?

JINGMAI O’CONNOR: It probably has to do with ecology. And that’s also why many birds lose their ability to fly. It only happens in certain ecological environments. So basically, if you’re able to get food and to hide from predators without flying, then evolution will select for that. And that is because flight– powered flight– is the most physically demanding form of vertebrate locomotion. It requires enormous energy to fly.

So if you can survive without flying, then natural selection will favor that. But that’s only possible for birds living in certain environments. But it’s most common in birds that are aquatic or semi-aquatic, birds that are able to dive underwater in order to escape predators or hide in reeds on the lake shore.

IRA FLATOW: Well, good luck in your research. I want to thank you for taking time to be with us today.

JINGMAI O’CONNOR: Our pleasure. And thank you so much for your interest.

YOSEF KIAT: Thank you.

IRA FLATOW: Dr. Yosef Kiat, postdoctoral researcher at the Field Museum, in Chicago, and Dr. Jingmai O’Connor, associate curator of fossil reptiles at that same Field Museum.

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