Reversible Saliva Is What Makes A Frog's Tongue So Sticky

How do frogs manage to get their prey to stick? Candler Hobbs/Georgia Tech 

Josh Davis 02 Feb 2017, 11:38

They might not look like much, but frogs have some pretty impressive tricks up their sleeve, or in their mouths, as it turns out. They can flick their whip-like tongue faster than you can blink, and smash into their hapless prey with a force five times that of gravity. Researchers have now also discovered the secret of another one of their tricks: What is it that makes their tongues so sticky?

It turns out that frog saliva has an unusual quality, in which it can change properties during prey capture. When the amphibian eyes its quarry and makes its move, the saliva is thin and watery until the extended tongue hits the insect. The thin saliva fills the bug’s crevices, at which point it then becomes thick and sticky as the prey is reeled in, before turning back to being watery so the frog can actually swallow it.

The saliva is transformed from one state to the other due to shearing forces, making it a non-Newtonian fluid. The best example of this is custard, in which you can sink something into the liquid slowly, but hit it with a hammer and it’s like hitting concrete. The frog saliva, however, works the other way round.

When the frog flings its tongue from its mouth, the speed at which it travels thins the saliva until it hits the insect. As the liquid slows down, it becomes more viscous than honey, trapping the prey and allowing the frog to slowly retract its tongue. In the frog’s mouth, the insect is simply sheared off in a similar way to how it was caught.

But that’s not the only reason the tongue is such an effective weapon. The tongue of the frog is also surprisingly soft, around as soft as a brain, in case you were wondering. Combined with the reversible saliva, it means that a frog's tongue is 50 times more adhesive than any polymer humans can produce.

As the tongue stretches, it also stores the energy in the tissue like a spring. “The tongue acts like a bungee cord once it latches onto its prey,” said Alexis Noel, from the Georgia Institute of Technology, who led the study published in the Journal of the Royal Society Interface. “It deforms itself as it pulls back toward the mouth, continually storing the intense applied forces in its stretchy tissue and dissipating them in its internal damping.”

The findings of the study, while fascinating for their own reasons, could also have applications in the development of better artificial adhesives, especially for things traveling at high speed.

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