How super sticky spit helps chameleons snag prey

A three month old chameleon.

A chameleon's dinnertime ritual seems to defy the laws of physics.

In two swift movements, the critter's tongue shoots out of its mouth with an acceleration 100 times greater than the best Formula 1 car, snags its prey, then zips back into the mouth just as fast.

"It's the equivalent of a human eating a 25-pound hamburger, and then having to transport that burger to your mouth using only your tongue," Kiisa Nishikawa, a biomechanics researcher at Northern Arizona University, told National Geographic.

Plenty of research has been dedicated to the mechanics of chameleons' speedy tongues, but that left scientists with another quandary: How do chameleons keeps their meals on their tongues during the high-speed trip back to their mouths?

The secret is spit. Syrupy, sticky spit.

In a study in the journal Nature Physics this week, physicists Pascal Damman and Fabian Brau report that the tip of a chameleon's tongue is coated in a small amount of incredibly viscous fluid that keeps the prey in place as it hurls into the creature's mouth.

"It's surprisingly simple," said Damman, a professor at the University of Mons in Belgium. But it's very effective: The spit is strong enough to hold prey that's as much as 30 per cent of the chameleon's body weight.

Initially, biologists had suggested that chameleons achieved the necessary adhesion with suction cups at the end of their tongues, or through a "Velcro" effect created by the interaction of rough patches on their tongues and their prey's skin. But neither possibility quite fit with Damman and Brau's models.

Then they started experimenting with the thick, honey-like saliva they found in the lizards' mouths. They coated the bottom of an inclined plane with the sticky stuff, then rolled a small steel bead down it. The spit was viscous enough to make the ball stop in its tracks - meaning it overcame the ball's acceleration due to gravity - which could explain how it keeps flies from falling off a chameleon's tongue as it snaps back into its mouth.

Damman and his colleagues also found that the fluid was stickier the faster the bead was falling. "If you pull the tongue in very rapidly you will increase the adhesion force," he said.

When the researchers compared the models built based on this experiment to the size of the prey found inside a chameleon's stomach, "this viscous adhesion alone is strong enough to explain the capture of all the prey," Dammon said.

To Damman's surprise, no one had studied this sticky spit before. But he predicts that scientists will find it in a host of other creatures with fast-moving tongues - frogs, toads, salamanders.

"It opens up a new perspective for biologists," he said.