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Chameleon Spit Is a Wonder of Physics

As if the 'ballistic' tongue wasn't enough.
June 20, 2016, 3:00pm

Chameleons may be the source material for many a stuffed toy and a series of really pretty strange beer commercials, but, make no mistake, when it comes to predatory behavior, they're complete assassins. The chameleon tongue is a wonder of evolutionary engineering, enabling these old world lizards to hunt opportunistically—waiting, waiting, and then, zap. The tongue is deployed in a blur of slime, retrieving prey from up to a third of the chameleon's body weight and from distances of over twice its body length. As such, the chameleon can essentially hunt without moving.

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How chameleons actually accomplish this remains something of a mystery. The "ballistic projection" of the tongue is only part (a fascinating part) of the story—the chameleon still has to reel its prey back in to be chomped upon. It does this thanks to an extremely sticky tongue, obviously, but how this stickiness is actually implemented is of great interest to biologists and physicists. Now, according to a paper published Monday in Nature Physics by Pascal Damman and colleagues at the Université de Mons in Belgium, we may have some answers. It's all in the spit.

More specifically, it's all in the viscosity of the spit, which is about 400 times that of human spit. Given the right conditions, it can even behave more like an elastic solid than a proper liquid, however sticky. This is key.

Part of the problem in characterizing said spit is that it's produced only in very small amounts by glands on the chameleon's tongue. Damman and co. were nonetheless able to measure the substance's viscosity by tricking a chameleon into whipping its tongue out at a glass microscope slide. The tongue slapped into the slide rather than whatever prey the chameleon thought it was after and this slide was taken immediately to the next phase of the experiment. Here, the spit-coated slide was employed as a tiny ramp down which the physicists rolled tiny metal beads. The angle of the ramp required for the beads to not get stuck in the chameleon spit was then translated into a measure of viscosity. This seems intuitive enough.

"This unexpectedly large mucus viscosity strongly suggests that the prey sticks to the chameleon's tongue through viscous adhesion," Damman writes. "However, knowing the value of this quantity is not sufficient to determine the adhesion strength. The strain/shear rate in the fluid film, which is only significant during the retraction phase, should also be known to determine the magnitude of the viscous forces. For this purpose, we introduce a dynamical model for the retraction phase."

This brings us to the second part of the group's research: the model. What Damman and co. found is that once the chameleon tongue contacts its prey, e.g. some bug, and the tongue and prey initially become stuck together via the liquid spit, the sudden retraction of the tongue back into the lizard's mouth causes the spit to suddenly increase in thickness. As the tongue accelerates backward, this suddenly firm adhesive is enough to keep the prey stuck to the tongue despite this sharp acceleration.

"Viscous adhesion alone is therefore largely sufficient to allow the capture of very large prey," the study authors note. "Actually, the adhesive mechanism appears to be outsized with respect to the usual prey found in stomach contents. However, this outstanding adhesion strength allows chameleons to occasionally capture birds, lizards or mammals, when they have the opportunity."

The results raise some interesting questions about other animals known to use their tongues to capture prey, some of which (some frogs and toads, for example) don't necessarily use the same method of ballistic projection. Is highly optimized sticky spit the norm among tongue predators? Or did the chameleon just get lucky?