'Pretty Disgusting Snot-Like Spit' Lets Frogs Catch Their Prey Frog tongues are super soft and wrap around their prey while secreting a sticky spit that changes consistency. Alexis Noel of Georgia Tech tells NPR's Scott Simon how she studied the amphibians.
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'Pretty Disgusting Snot-Like Spit' Lets Frogs Catch Their Prey

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'Pretty Disgusting Snot-Like Spit' Lets Frogs Catch Their Prey

'Pretty Disgusting Snot-Like Spit' Lets Frogs Catch Their Prey

'Pretty Disgusting Snot-Like Spit' Lets Frogs Catch Their Prey

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  • <iframe src="https://www.npr.org/player/embed/513388242/513388243" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript

Frog tongues are super soft and wrap around their prey while secreting a sticky spit that changes consistency. Alexis Noel of Georgia Tech tells NPR's Scott Simon how she studied the amphibians.

SCOTT SIMON, HOST:

The frog shoots its tongue out in the blink of an eye to trap its prey - thwack (ph) - how does that tongue actually work? Alexis C. Noel, who's a biomechanics Ph.D. student at Georgia Tech, has figured that out. Here's a hint - think of ketchup. Alexis Noel joins us from member station WABE in Atlanta. Thanks very much for being with us.

ALEXIS C. NOEL: Thanks for having me.

SIMON: So it's all in the yuck - spit?

NOEL: That is right. It's the very super soft tongue and the pretty disgusting snot-like spit that exists on the tongue.

SIMON: Thank you. Thank you for saying snot-like. Not a - I don't believe we've had that phrase on our show before. So what is there...

NOEL: It's a first.

SIMON: What is there about this delightful mucus of the tongue?

NOEL: Well, we spent a few years studying the frog tongue.

SIMON: Yeah.

NOEL: We were able to test the tongue and found that it is uber soft. It is actually 10 times softer than the human tongue.

SIMON: Tongue-wise, what does this accomplish?

NOEL: So the soft tongue and the sticky saliva which exists on the tongue work together to actually increase adhesion. And the frog is able to catch very hairy things, like tarantulas or even other frogs, which are slippery. And it's able to use this amazing tongue with this amazing adhesive to capture things in the blink of an eye.

SIMON: And so the ketchup bottle.

NOEL: Yes.

SIMON: Why should we think ketchup?

NOEL: Frog saliva is actually what's termed a shear thinning fluid. The viscosity or the thickness - the gooeyness (ph) - of the saliva can actually change with how much you shear it. So if you've ever tried to get ketchup out of a bottle, and you turn it upside down, the ketchup doesn't really want to come out. But if you tap the bottom, you're actually invoking all of these these shear rates within the fluid, and the fluid in the ketchup comes right out of the bottle.

SIMON: Here I was crediting the palm of my hand, but I guess that's - I guess it's something different. It's the shear rate of the fluid, eh (ph)?

NOEL: Yes. It's this category called non-Newtonian fluids where viscosity can change based on how you move it.

SIMON: Wow. I mean, how do you research this? Do you like line up a dozen, you know, frogs and then dangle a dozen flies and say, OK, guys, go at it?

NOEL: Pretty much, yeah (laughter). We've done a few things. The first thing we did was, of course, do some high-speed film of frogs eating insects. And we filmed - oh, I don't know - anywhere from six to eight different frog species, some very exotic. Then we also had to look at the tissue properties. So we actually got some frog tongues from a local dissection class at Georgia Tech, and we also tested the frog saliva. Now, what's really interesting about frogs...

SIMON: Oh, just thinking of that.

NOEL: (Laughter).

SIMON: Yes, go ahead. Yeah?

NOEL: Frogs actually secrete saliva from their tongue tissue, and this is actually different from how humans or mammals do it. We actually have glands located all over our mouths which drip saliva onto our tongue, but a frog tongue is kind of like a sponge just filled with saliva. So I ended up having to scrape about 15 frog tongues to get a saliva sample large enough to test the fluid properties, and that was a lovely couple of hours.

SIMON: What might this discovery about frogs spit lead to for our world?

NOEL: We're hoping that this research kind of paves the way to looking at novel high-speed adhesives. So how do you grab things - difficult things - very, very quickly? And so, like, if you were to grab something that's dusty, like a brick, or furry, duct tape's not going to be able to stick to either of those things. You know, you're going to have to use something like claws or grabbing mechanisms. But we think that with this new type of thick, viscous fluid coating a very soft substrate, that it will be able to grab oddly textured things very quickly. So in my mind, I'm dreaming of a - like, a frog tongue attached to a drone, and the drone can fly around and pick up packages really quickly and fly away.

SIMON: Do you mind me asking - after a day in the lab - I'm just guessing- do you just want to go home and fast? I mean, it must be difficult to eat anything.

NOEL: (Laughter) There are times that - especially in the beginning - where I had the smell of frog in my nose for at least an hour, and I couldn't eat anything. But by the end of the study, I'd been - you know, I was eating frog legs every week - part of the study.

SIMON: Oh, no. Seriously?

NOEL: Yeah, seriously. Might as well use the whole frog.

SIMON: Oh.

NOEL: Waste not.

SIMON: Well, Alexis Noel is a biomechanics Ph.D. student at Georgia Tech. Thanks very much for being with us.

NOEL: Thank you for having me.

SIMON: And if you'd like to say frog tongues in action, (laughter) you can find them on our website, npr.org. But if you're the kind of person that likes to look at frog tongues in action - never mind.

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To Catch Prey, Frogs Turn To Sticky Spit

leopard grab

Frog spit might be some of the catchiest spit on the planet.

That's according to new research on frog saliva, which shows that the sticky stuff is tailor-made to grab bugs.

It helps to explain how frogs can snatch flies out of the air at incredible speeds, and hang on to them using only their tongues.

Researchers had suspected a frog's saliva might be an important tool for hunting.

A leopard frog catches a cricket using its sticky tongue. Courtesy of A. Noel and D.L. Hu/Georgia Institute of Technology hide caption

toggle caption
Courtesy of A. Noel and D.L. Hu/Georgia Institute of Technology

A leopard frog catches a cricket using its sticky tongue.

Courtesy of A. Noel and D.L. Hu/Georgia Institute of Technology

To find out, they needed quite a bit of frog saliva. "I actually got 15 frogs, and scraped their tongues for a couple of hours one night," says Alexis Noel, a PhD student at the Georgia Institute of Technology who led the study, "It was pretty disgusting."

The researchers then used high-speed photography and an instrument called a Rheometer to analyze frog saliva under prey-capturing conditions.

They found that when the frog's tongue hits the fly, the saliva actually changes properties in order to trap the bug on its tongue. Frog saliva is normally thick like honey. But when the frog hits an insect with its tongue, the force causes the thick saliva to liquefy. The watery saliva spreads all over the nooks and crannies of the insect's body, trapping the helpless bug. The saliva then becomes thick again, and the frog can pull it the insect into its mouth.

That puts frog spit in a unique class of fluids that can change properties. They're known as non-Newtonian fluids.

Frogs Use Eyeballs To Swallow

A Tiger-leg Monkey Frog frog uses its eyeballs to swallow, pushing the trapped insect off the tongue.

Monkey eyeball

A common example is wall paint. Paint in the can is fairly thick, but when apply it to a wall with a paint brush, it becomes thinner. The force of the brush makes the paint liquefy and spread evenly across the wall. Remove the brush, and the paint becomes sticky again, staying in place on the wall.

So how does the frog get the insect off of its super-sticky tongue once it's in the mouth? With their eyeballs, of course.

"They actually bring their eyeballs into their mouth cavity and push down on the insect against the tongue," Noel says. The force from the eyeballs liquefies the saliva, releasing the bug from the tongue.

It's not all about the saliva, the frog's tongue also plays a big role. "The frog tongue is like a bungee cord, in that the tongue as it is pulling back actually stretches and deforms," Noel says. This stretching helps to keep the tongue in contact with the bug. The tongue is also extremely soft – 10 times softer than a human tongue. That softness helps keep the saliva thick as the bug returns to the frog's mouth.

The study, which was published Tuesday in the Journal of the Royal Society Interface is the first to study the viscosity of frog saliva using these techniques.

The authors suggest that frog tongues could one day help engineers design reversible soft adhesives that could work at high speeds. "Maybe even on a conveyor belt in a manufacturing plant, if you had to pick up very delicate components very quickly," says Noel.

The authors received help from the The Amphibian Foundation, a group that brings together top researchers in the field of amphibian biology, conservation, and applied science to address the causes of global amphibian declines.