Frog tongues’ sticky properties

When mechanical engineering PhD student Alexis Noel and her supervisor David Hu watched a viral YouTube video of a frog playing the smartphone game ‘Ant Smasher’, Noel was struck with inspiration for a research project. Combining her love of the amphibians with the study of biomechanics in Hu’s lab, she turned her attention to the physics behind how frogs catch insects.

To catch an insect, a frog whips its tongue out at incredible speeds, hitting the insect with a force that is 12 times stronger than gravity. The whole process can take as little as 0.07 seconds. So why is it that the tongue can hold onto the insect, instead of knocking it away like a ball hit by a tennis racket?

Previously, it was thought that a frog’s saliva held onto its prey by virtue of being immensely sticky. However, Noel and her team found that the saliva is actually a non-Newtonian fluid with shear-thinning properties: ordinarily it is thick and viscous, however upon impact the saliva temporarily becomes watery and runny.

This behaviour is the result of a high concentration of long-chain proteins in the saliva. Human saliva also contains these proteins, but they are minimal compared to the frog’s saliva. Upon impact human saliva becomes 10 times more watery, while frog saliva becomes 100 times more watery.

These proteins are “what [make] saliva and snot in your nose disgusting,” according to Noel, referring to saliva’s ability to form long drips.

As well as the protein-rich saliva, frogs also benefit from having a phenomenally soft tongue which is about as soft as brain matter, 10 times softer than the human tongue, and one of the softest known biological materials.

The flexibility of the tongue allows it to wrap completely around an insect, covering it with saliva which has become runny upon impact and able to fill all the gaps and crevices in the insect’s body.

Then, when the tongue starts to slow down, the saliva returns to its thick and sticky state for the journey back to the frog’s mouth. The stretchy tongue acts like a bungee rope, pulling the insect to its doom.

Finally, in a move that can be both horrifying and amazing to those unfamiliar with herpetology, the frog retracts its eyeballs to push the insect off its tongue and down its throat.

Noel and Hu have big ideas for the application of the research. Hu suggests a new design of surgical sutures or plasters, while Noel more ambitiously dreams of drones with artificial frog tongues that can catch objects from mid-air.

Image: Henry Fournier

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