What makes a frog’s tongue so sticky? The secret is in the spit

Frogs and amphibians can nab a fly with remarkable speed — but the real secret of their bug-catching prowess is in the saliva.

Sticky frog saliva is a non-Newtonian fluid. That means it can behave as both a liquid and a solid.

This unusual combination of tongue and saliva allows a frog to catch insects, mice or even small birds faster than you can blink — five times faster, in fact. Once captured, the prey is yanked back toward the frog’s mouth at a force up to 12 times greater than gravity. 

So sophisticated is the frog tongue that it’s capable of grabbing prey up to 1.4 times the predator’s body weight — a feat unmatched by any man-made device.

Inspired by a viral video of a frog attacking animated insects on a smartphone, researchers from Georgia Tech set out to unravel exactly what happens in those 70 milliseconds. Their findings were published this week in the Journal of the Royal Society Interface.

Instead of using virtual insects on a smartphone game, the researchers from David Hu’s biolocomotion lab attached actual crickets to strings and hooks. They also set up a high-speed video camera to record the tongue action in unprecedented detail.

Here’s what they discovered:

First, the frog’s super-soft tongue stretches out like a bungee cord and splats against an insect. The tongue wraps around the insect and covers it with sticky saliva before the victim knows what hit it.

This is where the weird, non-Newtonian properties of the frog’s spit come in.

A good example of a non-Newtonian fluid is the corn starch and water mixture known as oobleck or moon mud, said study leader Alexis Noel, a doctoral student in mechanical engineering at Georgia Tech. If you jiggle it around in a cup, it flows like a liquid. But if you press down on it, it turns solid.

Frog saliva behaves in the opposite way. When the slobbery tongue smacks its prey, the saliva becomes more liquid and spreads into all the cracks and crevices of its prey.

As the frog retracts its tongue, the saliva thickens, making it harder for the prey to separate from the tongue. (Imagine trying to pull apart fingers stuck together with peanut butter, Noel said.)

To figure this out, Noel had to scrape the tongues of 15 donated frozen leopard frog specimens. The fruits of her labor resulted in about one-fifth of a teaspoon of the stuff, which she ran through a device that gauges viscosity, a measure of how well a liquid flows.

The frog tongue’s softness also plays an important role in holding on to prey. The researchers found that frog tongues are among the softest biological materials known to science — 10 times softer than human tongues, or about as soft as brain tissue.

This gives the tongue its stretchy quality, much like a bungee cord. As it flies back toward the mouth, it absorbs and stores much of the force that would otherwise cause the insect to separate from the tongue.

Once tongue and prey are inside the frog’s mouth, the saliva regains its watery properties. Then the frog’s eyeballs dip back into the mouth to scrape the meal off the tongue and down the throat.

“It’s all very strange, if you’ve never watched a frog eat an insect before,” Noel said.

The findings could aid in the development of adhesives capable of rapidly grabbing delicate items, such as microchips, off conveyor belts without damaging them. A sticky frog tongue mechanism could also be attached to a drone to capture objects in mid-air, Noel said.

Members of David Hu’s lab have not shied away from unconventional research angles. Hu’s previous work has demonstrated how fire ants form ant-rafts to sail across flood waters, what eyelashes do and the average time it takes for animals to urinate (it’s 21 seconds). That study earned an Ig Nobel Prize in 2015.

sean.greene@latimes.com

@seangreene89

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An earlier version of this article suggested that human saliva is not a non-Newtonian fluid. Human spit does have that property.
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