Quirks & Quarks

What it takes for a frog to get a meal

Frogs can change the viscosity of their saliva so an insect will stick to their tongue. Then they suck in their eyeballs to help push it down their throat.
Leopard Frog and Cricket (Alexis Noel)

A frog can use its elastic tongue to secure an insect faster than a human blink of an eye.  But researchers who study frogs have been puzzled at how the insect stays on the tongue as it rockets back into the mouth, and how the frog then releases the insect in order to swallow. 

Alexis Noel, a Phd candidate in mechanical engineering at the Georgia Institute of Technology has the answer. In that same blink of an eye, the frog is able to change the viscosity of its saliva by means of a fluid property known as 'shear rate.'  The saliva on the extended tongue goes from being watery when it contacts the insect, to sticky and viscous when it pulls the insect into its mouth. Once the insect and tongue are in the mouth, the viscosity changes back to its watery state. 

This final process is enabled by a very surprising mechanism.  The frog can withdraw its eyeballs inside its head in order to help create the pressure needed to change the saliva and push the insect off the tongue and down the throat.

All of this happens in about one-third of a second!