Examining The Hummingbird Tongue

Hummingbirds can extend their tongues great distances — in some cases the length of their heads — to retrieve nectar. Biologist Margaret Rubega, of the University of Connecticut, explains how the structure of the hummingbird tongue traps liquid, and the evolution tales tongues tell.

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IRA FLATOW, host:

Here with us now is Flora Lichtman with our Video Pick of the Week. Hi, Flora.

FLORA LICHTMAN: Hi, Ira.

FLATOW: You've got something wonderful, I'm sure, as every week.

LICHTMAN: Yes. It's kind of a weird one, I think.

FLATOW: I love it. I love it already.

(Soundbite of laughter)

LICHTMAN: OK. So there's been a rush or a slew of news about tongues in the science journals recently, and so we took a look at two of them. It is - the video pick this week is a tale of two tongues.

FLATOW: Oh, it was a dark and stormy night.

(Soundbite of laughter)

LICHTMAN: Yeah. I know. I don't even think we should go there.

(Soundbite of laughter)

LICHTMAN: It gets weirder.

FLATOW: A tale of two tongues. Tell us about...

LICHTMAN: So the first tongue...

CONAN: Yes.

LICHTMAN: ...is a dog tongue. And I think most people are familiar with the dog lapping technique. Most people have seen this happen, but you've never seen it like this. You've never seen it in X-ray, high-speed video.

FLATOW: Wow.

LICHTMAN: It's pretty cool, actually. So they're looking at what happens, how the dog gets the water into their mouth. And there was this kind of notion that dogs scooped in or ladled in the water because they sort of turn their tongue backwards. And when they stick that tongue in the water bowl, a bunch of water comes up. But it turns out that's not what's going on. It's really an adhesion technique. Anyway, you can see it better. It's easier to see.

FLATOW: Wow. Yeah. Yeah. And it's up on our site, a Video Pick of the Week, up at sciencefriday.com. That's one tongue.

LICHTMAN: That's one tongue. But the second tongue is a tongue that I have never - I've never seen a tongue like this. And I think we should bring on the researcher, Margaret Rubega, who's a professor in the Department of Evolutionary Biology at the University of Connecticut in Storrs, to tell us more about it. Dr. Rubega, can you - will you describe this tongue for us, the hummingbird tongue?

Dr. MARGARET RUBEGA (Ecology and Evolutionary Biology, University of Connecticut, Storrs): Sure. Well, the first thing I'm going to do is try and shift everybody's mind off of the dog tongue...

(Soundbite of laughter)

LICHTMAN: Yeah. Let's get away...

Dr. RUBEGA: ...because the dog tongue is a whole other deal.

(Soundbite of laughter)

Dr. RUBEGA: Mammals have very fleshy, very muscular tongues because they've given up their tongue skeletons in exchange for this system that uses sort of wet masses inside the tongue that they can put under pressure to make it change shape, right, so...

LICHTMAN: It's muscle-y(ph), right.

Dr. RUBEGA: It's muscle-y. You can roll your tongue up. But birds and a lot of other vertebrates don't have tongues that are that fleshy, birds especially. People don't think about bird tongues very much because they don't see them very often. They're relatively speaking really small, and they don't have very many muscles out in the - out near the tip.

So. a hummingbird tongue is especially weird even for a bird because, in addition to being really flat and not fleshy at all at the tip, it split into two branches like a snake tongue is. It's bifurcated at the tip.

And furthermore, each of those tips out at the very end is edged with this fringe, kind of like you would see on the edge of a buckskin moccasin, this sort of division of the edge of each of those two tongues tips is cut up into what we call lamellae, these little flaps.

LICHTMAN: They're these like feathery things that are at the end of the tongue, right?

Dr. RUBEGA: They sort of make the tongue tip itself look like a feather, exactly, because it's subdivided into these flaps. And when the tongue is at rest, those flaps are rolled up in a way that makes each of those tongue tips into a tubular shape. And when the bird is not doing anything with the tongue and it's just in its mouth, those two tips are stuck together, so their tongue just looks like a kind of a flat-pointy thing.

FLATOW: Because their inside a long tube, right? The bird has a long tube, sort of snout to it.

Dr. RUBEGA: It does. It has a long tube-shaped beak, and the way we've always understood that is that that long beak and the equally long tongue - in fact, their tongue is quite a bit longer than their beak. They can extrude that tongue, stick it outside their mouth as far outside their mouth as their beak is long. So it's way, way, way out of their beak when they want it to. It has to lay inside that narrow tubular thing that we've always understood to be related to their habit of feeding inside of long, skinny flowers.

LICHTMAN: Yeah. So what are they using these tongues for?

Dr. RUBEGA: Well, they're using them to pick fluid up in the form of -you know, the flower is making this sugary water nectar to attract the bird so that the bird will stick its face down into the flower and the bird gets fed. But what the plant gets in return for that is these pollination services because the bird has jammed its face down into the flower.

The flowers have various ways of brushing their pollen-bearing parts against the bird's face. And then when the bird goes to the next flower and jams its face into that flower, it transfers the pollen over. So it's kind of a trade. Everybody is getting something out...

LICHTMAN: It's a win-win.

Dr. RUBEGA: A win-win. So they're sticking their tongues out of their mouths to get into these little pools of nectar that are sitting in the very bottom, at the very back of the flower.

LICHTMAN: And so the tongue is really important to basically feeding them. I mean, they rely on that tongue to get the nectar out. And you showed sort of exactly how that tongue does the grabbing of the nectar without any muscles in it, right?

Dr. RUBEGA: Yeah. It's critical to them, in fact, to get the nectar out because the beak itself can't pick the fluid up and...

LICHTMAN: They can't like slurp through their beak.

Dr. RUBEGA: They can't slurp through their beak because, in order to do what we think of as slurping - slurping is suction. You have to suck on something to make that slurping noise. And in order to suck on something, you have to keep the sides of your mouth shut.

And that's among the many advantages of having a pair of lips. But birds don't have any lips. Their lips - flaps for them, and so they use their tongue instead of just to move their food around inside their mouths, which is mostly why we use our tongues for, they actually are using it to catch their food to get outside the mouth, get a hold of the food and bring it back into the mouth where they can swallow it.

So the tip of the tongue in these particular birds was always sort of conceived off from the first time biologists started looking at it closely at the structure as kind of structured like a pair of little capillary tubes that the bird was sticking into the fluid and almost using like a drinking straw, except there was no suction involved. The idea was just that the physics of the shape of that tube was allowing the fluid to rise up into the tongue.

And it turns out, from our studies that that's not how it happens at all. The tongue actually - once those tips get into the fluid, they spread wide apart from one another, and those flaps we were talking about, open out flat so that they're covered with as much fluid as they can be. And then when the bird takes its tongue back out, pulls it back out of the nectar pool, the little flaps roll over and trap the fluid on the tongue that they're actively grabbing, in a way, the fluid that's in the pool. So that when the bird pulls its tongue out, some of the fluid comes with it.

FLATOW: You're listening to SCIENCE FRIDAY from NPR. I'm Ira Flatow with Flora Lichtman, talking about tale of two tongues on the hummingbird tongue.

LICHTMAN: Yeah. Who even knew hummingbirds have tongues.

FLATOW: And like a fork tongue, right? It...

LICHTMAN: It looks a lot like a snake tongue.

FLATOW: ...looks like a snake tongue.

LICHTMAN: You should see it on the website because it's really surprising how it changes its shape in water, and that's what Dr. Rubega was just talking about.

FLATOW: And, Dr. Rubega, you're saying that the tongues goes out as long as the beak is and then collapses and with a little - we'll just say there were little hairs on it or something that sucks - that draws in by what? Just the fluid sticks to the hairy part of the tongue?

Dr. RUBEGA: Well, hair sort of implies something that you would think of like your own hair, which is a rounded structure, right?

FLATOW: Right.

Dr. RUBEGA: You hair is kind of a - it's very fine, but it's a round thing in cross section. These are flat, but they're curled up around the flat side of the tongue when they're closed. So when the bird put its tongue in, it's almost like it's got fingers at the tips of its tongue.

Imagine this: If you stuck your tongue into your soup, and instead of the soup just kind of, you know, wetting the surface of your tongue, your tongue actually had fingers, and they would open up as soon as you put them into the soup. And then as you pulled your tongue back out, they would close again and grasp that soup and pull it along with...

LICHTMAN: Wow. That is some fun experience.

FLATOW: Wouldn't it be great if we had pictures of that - oh, we do have pictures of that.

(Soundbite of laughter)

Dr. RUBEGA: Yeah, you have pictures of that.

FLATOW: up on our website. Flora has that.

LICHTMAN: Yeah. Up on our website. You should see. So, Dr. Rubega, how did you get into hummingbird tongues?

Dr. RUBEGA: Well, we got into hummingbird tongues because in my lab, generally, we're interested in how birds feed and the details of the mechanics of that because it helps us make some sense of how much birds can adjust to environmental change, if we know exactly what the limits are to the way they can go about feeding.

And hummingbirds have been of interest for a long time just because they're so extreme. They are little tiny birds. They're eating this fluid diet that's, you know, that's very high in calories, but their energy costs are huge, so they're just an extreme case.

And the more we looked at the literature about how this nectar-drinking thing was supposed to work, the more it seems to us that something really important had been missed. Because, as I said, the idea was previous to this that it was this very passive thing where the tongue went in and the fluid was supposed to just rise up into these little tubular-shaped things at the tips of the tongue passively through capillary action.

Just like when somebody is taking a blood sample and you put a capillary tube in a pool of liquid, the fluid just rises on its own. But the problem with that idea is that the fluid rises only so high and then it stops by the force of its own weight sort of pulling it back down and also the air pressure...

LICHTMAN: Yeah. And this - so that wasn't it. I mean, it was something different.

Dr. RUBEGA: No. It didn't make any sense to us, but that was kind of the widely accepted idea, that the whole process just happened by capillary action.

LICHTMAN: Can I ask you a quick question, because we have, like, 30 seconds left. But it's something I'm dying to know. Is there a tongue a sort of original tongue, a tongue Eve? Did all tongues come from one tongue?

(Soundbite of laughter)

Dr. RUBEGA: That's a great question. And the answer to that is that there - the tongue is a land animal invention, right? Because once you're out of the water, you can't use the water that surrounds you to move your food around anymore. It's a land animal invention. So there was presumably in the ancestor of all land animals, a structure that evolved specifically because, you know, a fish will just use the water that surrounds it to move its food around, but a land animal can't.

LICHTMAN: That's really cool. Well, thanks for joining us today, Dr. Rubega.

Dr. RUBEGA: Oh, thank you.

LICHTMAN: Dr. Rubega is an evolutionary biologist at the University of Connecticut at Storrs.

FLATOW: Thank you, Flora. You can see Flora's Video Pick of the Week. You can actually see this hummingbird tongue in slow motion. It's really fascinating...

LICHTMAN: It's cool.

FLATOW: ...up on our website at sciencefriday.com. And they also have a dog - see their tongue. It's - you're not gonna believe how your dog drinks water when you actually see it in slow motion. It's not what you think it is.

LICHTMAN: X-ray slow motions.

FLATOW: Wow. It was the best of tongues, it was - never mind. Tale of two tongues up there on our website at sciencefriday.com. Thanks, Flora.

LICHTMAN: Thanks, Ira.

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