The Secret to the Stickiness of Frog Spit

A frog’s tongue can get prey into its mouth in less time than it takes for you to blink – just seven hundredths of a second.

The bug is whipped through the air with a peak acceleration of around 12Gs, way more than astronauts experience when launched into space, twice the acceleration of the craziest theme park ride, and more than the ten g’s permitted in the Red Bull Air Race.

The remarkable thing about this is that the bug doesn’t slide off the tongue. It doesn’t matter how wet, furry, bumpy, or dirty the prey is.
Then once it’s in the frog’s mouth, it easily detaches from the tongue so it can be swallowed. This ability to stick and unstick is better than anything commercially available today.

So how is this possible?

PhD student Alexis Noel from Georgia Institute of Technology became curious about frog-spit when she watched a viral video of a frog attempting to catch digital bugs crawling on a screen.

She and her colleagues gathered up a collection of different frog and toad species the biology department had lying around.

Then they used high-speed cameras to watch northern leopard frogs pull insects into their mouths with accelerations up to 120 m/s-2 12 -g’s

Next, they took a bunch of frog and toad tongues and poked them from above to measure their softness.

Softness is the ratio of stress to strain – how much deformation occurs under a given amount of pressure. Softness determines how the tongue changes shape as it shoots out from the frog’s mouth, connects with the prey, and is pulled back in.

The frog tongues were incredibly soft, 10 times softer than a human tongue. This makes them roughly as soft as rat brain tissue, a comparison the researchers themselves point out in the article.

Finally they tested the frog spit. But getting enough frog saliva to analyse is harder than you’d think. Unlike humans, which have salivary glands beneath the tongue and at the back of the mouth, amphibians have salivary glands actually on the tongue, which meant scraping dead frog tongues to get a big enough sample.

When the researchers looked at it, they discovered that frog saliva is even stranger than they were expecting. It turns out it’s a non-newtonian substance called a two-phase viscoelastic fluid.

Viscoelastic fluids, as the name implies, have a mixture of viscous and elastic properties. Viscous, meaning they have a high resistance to flow, like honey, and elastic meaning they deform when a quick force is applied but return to their previous shape after.

These aspects of the fluid depend on the shear forces applied to the saliva. When there’s not much force acting on it, it’s thick, viscous and springy. But under pressure it loses viscosity and becomes runny.

It’s a bit like tomato sauce, thick and goopy inside the bottle.
But if you smack the bottom of the bottle, the sauce loses viscosity and flows – sometimes too fast.

For the frog, the mix of a super soft tongue and non-newtonian spit is why their prey sticks so well. The tongue pushes against the saliva as it hits its prey, applying a shear force to the liquid as it spreads. This makes the saliva runny, pouring into every bump and crevice of the frog’s prey.

But as the pressure drops, the saliva returns to a thick, gooey and elastic state, helping it grip onto the prey hard enough to withstand the crazy g-forces of being launched into the frog’s mouth.

Once the frog has the bug in its mouth, its eyes sink down and squeeze the saliva and the bug together, once again returning the spit to a liquid state. This releases the bug, allowing the frog to swallow it.

That’s a pretty ingenious mechanism.
Now that we understand how frogs are able to quickly stick and unstick bugs, we may be able to produce technology that works in the same way. This could allow robots to pick up and put down objects in an assembly line, for example
or enable people to climb up walls or on ceilings.