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Special Spit Helps Frogs Get a Grip on Insects

Secretly sticky spit snatches snacks, study shows

smithsonian.com

Let’s just get this out of the way: Frogs are cool. They jump. They thrive in water and on land. And their tongues are capable of sticking to bugs like glue—even ones heavier than they are. And now, at last, the mysteries behind those incredible tongues are being revealed. As Ben Guarino reports for The Washington Post, the secret sauce that makes frog tongues so deadly to insects turns out to be a simple one: spit.

A new study published in the Journal of the Royal Society Interface reveals that frog saliva is more fascinating than previously thought. Tests showed that it’s both uniquely sticky and physically astonishing—it can actually change physical properties.

For the study, researchers examined frog saliva in combination with the frog’s soft and elastic tongue. Fluid tests of the spit showed that it’s a non-Newtonian fluid—that is, that it has properties different from normal liquids.

Your average Newtonian fluid (as described by Sir Isaac Newton) has the same properties as other such fluids. They freeze at the same temperatures, move into containers in the same ways, and flow with the same characteristics. But then there are the non-Newtonian fluids—liquids that seem to have a mind of their own. Among their ranks are ketchup, melted chocolate, lava…and apparently frog saliva. These fluids take on different properties at different times, and they don’t behave the same way. (Just think of how the face of a cliff can turn to water during the sudden movement of an earthquake.)

When they studied the non-Newtonian frog spit, researchers learned that it's reversible. That's right: It can change from a glue-like substance into a very thin fluid and back again. But the tongue's important, too. So the researchers then studied frozen frog tongues and found that they are ten times softer than the human tongue. They learned that, when combined with the non-Newtonian spit, the uniquely soft tongues have two functions.

The thin spit (which is still 50,000 times more viscous than human saliva), helps the tongue hit and release from bugs. When it hits a bug, the tongue deforms and its contact area becomes bigger. The force of this impact against the bug turns the spit into a thin liquid, allowing it to ooze around its prey. But as the tongue retracts, the saliva thickens, sticking to the bug and making it easier to get the critter into its mouth.

What’s the point of studying frog spit, pressing on frog tongues and videotaping eating frogs? Researchers tell Guarino that the research could one day inspire new, resealable adhesives. The future could very well contain Post-its or envelopes with frog spit-inspired glues—or devices that capitalize on the frogs’ unique ability to grab bugs. Until then, though, the amazement of the little amphibians make it well worth trying to figure out what makes their tongues tick.

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