The amphibians' saliva is what's known as a "shear-thinning fluid," like ketchup—sometimes thick, sometimes thin and flowing. Christopher Intagliata reports.
You might think frogs catch insects ‘cause their tongues are sticky. "But why is the tongue sticky, and how does it actually adhere to these insects at these very high accelerations?"
Those are the questions Alexis Noel, a PhD candidate in mechanical engineering at Georgia Tech, wanted to answer. So she and her team got frog tongues from a dissection lab, and tested their consistency. Ten times softer than human tongues. A texture more like brain tissue. "Their tongue is very much like a sponge. It's infused with this thick, viscous saliva."
That saliva was their next study subject. "And in order to test the saliva we had to get about a fifth of a teaspoon of fluid. Which is a lot of saliva, in a frog's case." They put the saliva in a rheometer, a tool that can measure viscosity. And they found that frog saliva is what's called a 'shear-thinning fluid'—its viscosity changes, depending on conditions.
You might be more familiar with a different shear-thinning fluid. "Ketchup. When you smack the bottom of the ketchup bottle you're actually invoking shear forces within the ketchup itself. And ketchup, because it's shear thinning, its viscosity actually drops and allows it to slide out of the bottle easily."
So back to our frogs: the tongue shoots out, hits the bug and deforms around it. That impact is like a smack on a ketchup bottle—it changes the saliva from thick and sticky to more watery, free to flow all over the bug. Then the tongue bounces back, like a bungee cord, and the saliva thickens up again. What's next is beyond weird.
"Frogs actually take their bulbous eyeballs and bring them down into their mouth cavity and use their eyeballs to shove the insect down the throat." That force turns the saliva watery again, "and the insect slides down the gullet." The study is in the Journal of the Royal Society Interface. [Alexis C. Noel et al., Frogs use a viscoelastic tongue and non-Newtonian saliva to catch prey]
With the case closed on this one, Noel's next inquiries are on grippy fingertips. The dust-collecting properties of earwax. And—wait for it—cat tongues. "Yeah, I get paid to watch cat videos all day."
[The above text is a transcript of this podcast.]