The Secrets of Sticky Frog Saliva

8:37 minutes

Attempted prey capture in Rana pipiens. Red dots indicate tracking of tongue tip. Credit: C. Hobbs
Attempted prey capture in Rana pipiens. Red dots indicate tracking of tongue tip. Credit: C. Hobbs

Frogs use their tongues to snatch insects out of the air in less than a second. Reporting in a study out this week, mechanical engineer Alexis Noel found that the softness of the tongue, along with the viscosity of the saliva, enable these mid-flight catches. When the tongue hits an insect, the force turns the normally thick, viscous saliva into a thin liquid that can coat the prey, effectively ensnaring it. Noel calls the saliva a “reversible” non-Newtonian fluid and describes how its properties could be useful in adhesives and soft robots.

Stretching of tongue epithelium during prey capture. The gaps indicate areas where prey has been released. Credit: C. Hobbs
Stretching of tongue epithelium during prey capture. The gaps indicate areas where prey has been released. Credit: C. Hobbs
Finger retracted from tongue surface showing its strong adhesion. Credit: C. Hobbs
Finger retracted from tongue surface showing its strong adhesion. Credit: C. Hobbs

Segment Guests

Alexis Noel

Alexis Noel is a PhD candidate in mechanical engineering at the Georgia Institute of Technology in Atlanta, Georgia.

Segment Transcript

IRA FLATOW: As a kid, you probably had a picture book showing a frog, right? A frog slurping up a fly– frogs eat flies and insects. And it’s one of those facts that you don’t remember learning, but you always knew that, right? But if you sit down and think about it, it starts to make a little less sense.

Like this– how can the flat surface of a frog’s tongue grip and hold on to a flying insect? It’s flat. My next guest found out that the secret is in the saliva.

Alexis Noel is a PhD candidate in mechanical engineering at the Georgia Institute of Technology in Atlanta. Welcome to “Science Friday”. Hi there, Alexis, are you there?

ALEXIS NOEL: Hi, thank you for having me. Hello?

IRA FLATOW: You’re welcome. Can you hear me? Hello, Alexis.

ALEXIS NOEL: Yes, I can hear you. Hear me?

IRA FLATOW: Yes. We’re doing a telephone commercial. Let’s talk about your study. Your study looks at frog tongues, and your last study looked at cat tongues. You’re interested in animal tongues as an engineer? What’s the connection here?

ALEXIS NOEL: Yes, yes. In my lab, we study a lot of interesting things. So I studied frog tongues for about three years. I’m currently studying cat tongues. Really, looking at how do you grab things with very soft, squishy surfaces. That’s really the crux of my research.

IRA FLATOW: Mm-hmm. And you found that the saliva is sort of the glue that holds it all together.

ALEXIS NOEL: That is true. Yeah. The frog has a super soft tongue, almost 10 times softer than a human tongue, and it uses this very sticky fluid that’s kind of infused within the tissue. And you can kind of imagine it’s like the snot within your nose, even thicker than honey. It’s all very disgusting.

IRA FLATOW: Yeah. [LAUGHTER] Well, it’s interesting. I didn’t realize the tongue is 10 times softer than a human tongue. And it’s about, I understand, the same consistency as brain tissue?

ALEXIS NOEL: That’s correct. We also tested brain tissue. It kind of reminds me of when you have a piece of chewing gum that you chew just a little too long and you pull it out of your mouth. That’s kind of what a frog tongue feels like.

IRA FLATOW: Have you– I don’t want to get too personal here, but did you actually go about feeling tongues of frogs?

ALEXIS NOEL: I had to do a lot of that during the past three years, yes.

IRA FLATOW: Now I understand that you say the key to this stickiness is that the saliva is reversible. What do you mean by that?

ALEXIS NOEL: Yeah, that’s correct. That was probably the most fascinating part of this whole study. We actually scraped about 15 frog tongues to get a sample large enough to test. Because you need about a fifth of a teaspoon of fluid to test in a rheometer. And a rheometer tells you basic fluid properties, things like viscosity.

So we scraped all these frog tongues and put the saliva in there. And what we found was that the frog saliva is actually a shear thinning fluid, meaning that viscosity can change based on the shear rate. And this is actually very similar to ketchup. And I know everybody has dealt with ketchup at some point, maybe not frog saliva.

You know, if you turn a ketchup bottle upside down, it’s very difficult for the ketchup to flow out. But if you smack the bottom, the ketchup flows. Well, that’s because you’re imparting shear stresses in a shear rate within the fluid. So the viscosity actually drops.

So similar things are happening with frog saliva when it hits on the insect. When the tongue hits, the saliva actually becomes very liquidy and water-like. And it will penetrate all the cracks in the insect. And when the tongue comes back into the mouth, the saliva hardens up and becomes more viscous than honey. And it maintains very high grip on the insect.

IRA FLATOW: Wow. How fast does the tongue need to be traveling to create just the right amount of force on the saliva?

ALEXIS NOEL: Well, so when the tongue goes out of the frog mouth, it travels at about 9 miles per hour, or about 4 meters per second. So it’s traveling pretty fast relative to the frog. And so that’s how fast it impacts the insect. And when it pulls back, the acceleration at which it pulls the insect back into its mouth is about four times the acceleration astronauts feel when they go into space. So it’s about 12 G’s.

IRA FLATOW: Wow. This is “Science Friday” from PRI, Public Radio International, talking with Alexis Noel, a PhD candidate. She’s a ramblin’ wreck from Georgia Tech and a hell of an engineer. Sorry, I can never stop myself when I have somebody from Georgia Tech.

OK. So let’s take this one step further. The frog has caught the insect. It’s whipped it back into its mouth at four times the speed of an astronaut in G-forces. It’s got it in its mouth. Now how does it get the insect off the tongue if it’s so stuck to it?

ALEXIS NOEL: Well, I think that’s one of my favorite parts of the study. So that was the question that bugged us– ha, pun– that bugged us–

IRA FLATOW: I like it.

ALEXIS NOEL: For a very long time. How does the insect get off of the sticky tongue once it’s inside the mouth? And so we observed these frogs using high speed videography equipment. We observed them actually swallowing. And what we noticed was that the eyeballs actually go and go into their skull and into their mouth cavity and push on the insect inside of their mouth. And so it’s that pushing motion from the eyeballs that actually causes the insect to slide off the tongue and into the throat.

IRA FLATOW: Now we’ve been doing this show over 25 years. I think that is the most unusual explanation I’ve ever heard for a natural phenomenon. The eyeballs push the insect off the tongue.

ALEXIS NOEL: That’s correct. Yes. If you see– frogs are known to have these big bulbous eyeballs on the surface of their skull, and they just completely drop into their skull. And you don’t see them. It’s amazing.

IRA FLATOW: As a scientist, how do you discover that?

ALEXIS NOEL: Well, people have known, a lot of biologists have known that frogs use their eyeballs when they swallow. Many thought that it was just a reaction mechanism. But when we looked at the videos and we actually saw some x-ray videography of insects inside of the frog’s mouth, and we saw the insects slide right off of the tongue.

IRA FLATOW: Wow. OK. So now you have studied cats’ tongues, now frog tongues.


IRA FLATOW: What tongue is next, if I might ask?

ALEXIS NOEL: What tongue is next? Well, that’s a great question. We actually, in addition to the cat research, we actually got a tiger tongue at the local zoo. There was a tiger that recently died.

And so we’ve been able to compare a tiger tongue next to a house cat tongue and we’re making some amazing discoveries that the spines on the tiger tongue are actually the exact same size and shape as the housecat, just with more of them. So that’s been fun. The next research topic is going to be earwax.

IRA FLATOW: You got me. You got me on this one. Not with the tongue though, earwax? Tell me.

ALEXIS NOEL: Not with the tongue, yes.

IRA FLATOW: What do you want to know about earwax?

ALEXIS NOEL: So the– well, we’re looking into how earwax is actually a very efficient dust collector within the inner ear. And it actually creates these web-like effects within the ear canal, captures dust, and then falls out of the ear by crumbling when there’s enough dust in the earwax. So we’re looking at novel applications for dust collection systems with air filtration.

IRA FLATOW: Wow. Alexis, will you promise to come back when you do this study on earwax? Because–

ALEXIS NOEL: Absolutely.

IRA FLATOW: Well. It’s going to be great. Thank you, thank you very much for taking time to be with us today.

ALEXIS NOEL: Thank you for having me.

IRA FLATOW: We didn’t get to talk about gecko tongues and how they use their tongue to clean their eyeballs, but maybe we’ll save that for the next one. Alexis Noel is a PhD candidate in mechanical engineering at the Georgia Institute of Technology in Atlanta.

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About Alexa Lim

Alexa Lim was a senior producer for Science Friday. Her favorite stories involve space, sound, and strange animal discoveries.