The sticky, elastic tongues of amphibians have fascinated researchers for decades—the first study of frog and toad tongues was done in 1849. However, the underlying physics of this adhesive feat remained unclear. Previous studies compared frog tongues to scotch tape, but that’s not the full picture, researchers report today in the Journal of the Royal Society Interface. Yes, frog tongues are uniquely sticky (and their saliva makeup is crucial in bug capture), but they’re also very soft—10 times softer than human tongues and one of the softest known biological materials. That softness makes frog tongues more like adhesive shock absorbers than scotch tape, the researchers suggest. Here’s what happens: Say a bug is buzzing by and a frog releases its tongue. When the bug hits the frog’s tongue, the tongue wraps around the bug like a sticky bubble gum blanket and absorbs the bug’s inertia. Upon impact, the bug gets coated in saliva. Because the tongue is so soft, it can stretch out more than twice its thickness to cover more of the bug’s surface area and get its saliva into the grooves of a bug’s bumpy exoskeleton. Frog saliva is a non-Newtonian fluid, the team found, meaning it defies Newton’s law of viscosity and its flow changes under stress. So the saliva flows normally until the bug hits it, and then the saliva grips the bug as the tongue is retracted back into the frog’s mouth. But if a frog’s tongue is so sticky, how does it get its meal off its tongue and into its stomach? In a much studied phenomenon, frogs swallow with their eyes. The researchers found that when the eyes push against the bug in a motion parallel to the tongue, the saliva starts to flow easily again and down goes dinner. And although the researchers no longer think frog tongues are all that similar to scotch tape, this new finding might help them design new adhesives that stick at high speeds.
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