Frog saliva switches between being thin and watery as the whip-like tongue hits its target, to thick and sticky as the insect is reeled in, research shows

This article is more than 3 years old

This article is more than 3 years old

Scientists have discovered what makes the frog’s tongue uniquely sticky, allowing the amphibian to snag passing flies while squatting motionless by the water’s edge.

Frog saliva, the study revealed, has a bizarre property of being able to switch between thin and watery as the whip-like tongue hits its target, to thick and sticky as the insect is reeled in, creating an almost inescapable trap.

Alexis Noel, a mechanical engineering PhD candidate at the Georgia Institute of Technology in Altlanta and the paper’s first author, said: “For frogs, saliva seeps easily when it hits the insect, then thickens up during retraction. It’s really an incredible process.”

The transformation is brought about by shearing forces – forces working in unaligned directions – on the saliva, making it a so-called non-Newtonian fluid. Custard is a well-known example of the phenomenon – you can slowly sink your hand into a bowl of custard, but if you punch it, it becomes solid.

With frog saliva – a “shear-thinning” fluid – the reverse was found to be true. A globule of frog spit, stationary on a surface, has a thick snotty consistency. But when the frog lashes its tongue against its prey at high-speed, the shearing force dramatically thins the saliva, allowing it to fill all the insect’s crevices.

“Then, when the tongue snaps back, the saliva changes and becomes more viscous –thicker than honey, actually – gripping the insect for the ride back,” said Noel.

Finally, as the frog rubs its tongue on the inside of its mouth, the insect is sheared off again, and swallowed.

The scientists identified the precise shear rate when viscosity drops by analysing saliva samples from 18 frogs using a rheometer, a highly sensitive device for measuring properties of fluids.

The property arises due to high concentrations of long-chain proteins in frog saliva, which give it a more mucus-like consistency than human spit. Lower concentrations of the same proteins in human spit is what makes drool form long strings before dripping. “It’s what makes saliva and snot in your nose disgusting,” said Noel.

The paper, published in the Journal of the Royal Society Interface, also describes how the extreme softness of the frog tongue – it is as soft as brain tissue, or 10 times softer than a human tongue – further enhances the stickiness. As the tongue hits the prey it coils around it, and stretches out like a bungee cord, allowing the tongue to be retracted smoothly enough to avoid simply batting the fly away.

The researchers believe there may be practical applications for the discovery in future.

“We’re hoping to look into reversible adhesives,” said Noel, citing the future possibility of drones fitted with artificial frog tongues that could whip out and grab objects in flight.