The superfast tongue of the horned frog is also a superpowerful adhesive.
Evolutionarily speaking, it makes sense for the frogs to have an accelerating rocket of a tongue—to get the biggest smack for their effort and thus the grabbiest one. This mechanism is what lets the animals catch relatively large prey.An Argentine horned frog (Ceratophrys cranwelli) jumps on a mouse. Photograph by Fritz Rauschenbach, Corbis
Horned frogs do eat insects but also lizards, snakes, rodents, other frogs, and small birds. With a less sticky apparatus, those animals would be too heavy or could easily escape before being pulled into a frog’s jaws. (Learn about the Amazon horned frog.)
Inspiring their amphibian subjects to shoot out their tongues at glass slides (behind which a tasty cricket was visible), Thomas Kleinteich and Stanislav Gorb, who research functional morphology and biomechanics at Kiel University in Germany, measured the sticking power and contact areas of the tongues of South American horned frogs (genus Ceratophrys).
And what they learned surprised them.
Frog tongues produce on average only about one-fifteenth the adhesive strength of the feet of a gecko, another famously sticky animal, Kleinteich said. (Related: “Watery Gecko Grip Could Lead to Stickier Tape.”)
“However, in terms of prey capture, frog tongue adhesive forces are enormous—on average 1.4 times their body weight.
“Translated into human dimensions,” he said, “that would be an 80-kilogram [176-pound] person lifting 112 kilograms [246 pounds] just by using his or her tongue. And they do this within milliseconds” of making contact.
Plus, with frogs’ diverse prey, their tongues must stick as well to feathers as to cuticle—a trick that nature seems to have made sure these animals have down pat.
Frog tongues leave behind slime, making them a “wet” adhesive system. But that mucus has been long misunderstood, the authors say.
“People tend to think that the mucus acts as some sort of superglue,” said Kleinteich. “But our results suggest that less mucus results in better adhesion.”
In the study, the longer a tongue was against the glass before snapping back, the more mucus was left behind. In natural feeding events, contact time would generally be lower than it was in the experiment—where there was no prey item to pull back to swallow—supporting the theory that mucus is not the key to the grab.
The authors said that frog tongues behave similarly to so-called pressure-sensitive adhesives (a term used by material scientists for things like sticky tapes and labels).
That means the sticking power depends on how forcefully the tongue hits the glass, how big an area it covers (in frogs measured by the wet prints left behind), and how it then detaches. As with those engineered materials, “high force is needed to initiate detachment, and then lower pulling forces occur before the contact breaks,” Kleinteich explained.
So the highest impact smack of tongue on target allows for the strongest pulling forces, which means contact is broken almost immediately. And that means both less mucus and better adhesion.
Inspired by Nature
Sticky tongues aren’t just a frog thing, of course. Though the systems aren’t identical, chameleons and salamanders, for example, have a similar feeding apparatus that they can fire ballistically at distant targets. (Also see “Tarantulas Shoot Silk From Feet, Spider-Man Style.”)
Meanwhile, animals’ stickiness interests engineers for possible answers to human problems.
For instance, gecko feet (and some beetle feet) have a dry grabbing mechanism that is structural. The mechanism—actually tiny hairlike protrusions that hold with intermolecular forces—allows them to walk upside down on ceilings and such. (Also see “Gecko, Mussel Powers Combined in New Sticky Adhesive.”)
Those have already inspired some products that need to stick well but lift off quickly, such as various kinds of bandages and military “gecko gloves” for scaling walls; co-author Gorb has also worked on a beetle-inspired tape with a similar design.
The authors say that as more engineering challenges arise related to holding tight and letting go, especially in moist environments like within the human body, the frog’s tongue, with its unique and powerful wet-adhesive system, could inspire even more sticky solutions.
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