"Reversible" Spit Helps Frogs Nab Their Prey

A. Noel and D.L. Hu, Georgia Tech.
A. Noel and D.L. Hu, Georgia Tech. / A. Noel and D.L. Hu, Georgia Tech.

Watching frogs eat is far from boring, but knowing how they do it makes it even more interesting. Engineers studying the frogs’ feeding habits say their saliva’s variable viscosity is the key to securing prey to their whiplike tongues. The researchers described their findings in the Journal of the Royal Society Interface.

Mechanical engineer Alexis Noel with a frog friend. Image Credit: A. Noel and D.L. Hu, Georgia Tech.

The frog whips its tongue back and forth with impressive force and speed. So how is it that crickets, flies, and other freshly caught prey don’t just fall off?

Lead author Alexis Noel, a Ph.D student in mechanical engineering at Georgia Tech, and her colleagues brought five leopard frogs into the lab and set them up in front of super-high-speed cameras. Then, they dangled crickets in the air near the frogs and captured the resulting tongue-strikes on film.

Next, the team analyzed two kinds of samples: frog tongue tissue and frog spit. They tested the tongue tissue for stretch, softness, and compression, and the spit for viscosity and elasticity. Using this new data, they then created a computer simulation that predicted how both the tongue and the spit would behave when unleashed by a hungry frog.

As it turns out, Noel said in a statement, a frog’s tongue is a lot like a bungee cord: “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.”

But its spit is another story altogether. The simulation results showed that a frog’s spit transforms as its tongue is moving. “There are actually three phases,” Noel said. "When the tongue first hits the insect, the saliva is almost like water and fills all the bug'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. The saliva turns watery again when the insect is sheared off inside the mouth."

This adaptation is great for frogs. It might also be good for us, noted Noel’s co-author David Hu. "Most adhesives that have been created are stiff, especially tape," he said. “Frog tongues can attach and reattach with soft, special properties that are extremely stickier than typical materials. Perhaps this technology could be used for new Band-Aids."