Frogs are well known for their super-fast, super-sticky tongues, but the science of frogs' tongues is actually rather scant. What makes them so sticky? Precisely how quick and powerful are they? These are questions that researchers at Christian-Albrechts-Universität Kiel in Germany thought it was fine time to get answered. Their study reveals for the first time some astonishing facts about nature's most prestigious lickers, reports the Los Angeles Times.
For their experiments, the scientists got themselves some rotund South American horned frogs, a popular pet species due to the fiendish "horns" they sport above their eye sockets. The species makes for a particularly good study subject because it is an ambush predator, preferring to hunker down and patiently wait for its prey to stumble within tongue-range. It also has a voracious appetite, and is known to tackle prey more than half its size.
Transparent glass plates fitted with pressure sensors were placed in the frogs' enclosures during feeding time, separating the frogs from their prey. When the amphibians flicked their tongues, the impact on the glass recorded things like impact power, speed and stickiness.
Remarkably, researchers found that, on average, the adhesive force of the frogs’ tongues exceeded their body mass by 50 percent. The most powerful impact recorded in the study belonged to a juvenile frog who, perhaps in an effort to prove his mettle, slammed his tongue into the plate with 3.4 times the force of his own weight.
“I knew these frogs could eat large things,” said Thomas Kleinteich, lead author of the study, “but I didn’t really expect that the forces would be that high.”
Researchers found that the remarkable speed of frogs' tongues does not just help them snatch up fast-moving prey. Greater speed also means more impact, more adhesion, and the ability to haul in a bigger meal.
The study also discovered some anti-intuitive facts about what makes frogs' tongues so sticky. “The common opinion is that the mucus is some sort of superglue that sticks to everything immediately,” Kleinteich said. But actually stronger adhesion occurs when there is less mucus.
Kleinteich likened the sticking mechanism to that of the tacky glue on Post-It notes. Post-Its can be applied and then removed without leaving a sticky, adhesive trace. Similarly, a frog wants to snare its prey without getting its tongue permanently stuck to whatever it gets flicked at (especially when there turns out to be a glass plate in the way!)
"Imagine you have a piece of sticky tape and you’re pulling on it,” explained Kleinteich. “You need a lot of force first to initiate a crack and then it gets less and less and less.”
Frogs freed their tongues from the plates in a similar fashion. Kleinteich surmises that, like with Post-It notes, frogs' tongues employ a pressure-sensitive adhesive that is fluid enough to form a connection when it binds, but still elastic enough to resist being ripped apart when the frog wants to disengage with a surface.
This is all still theoretical, however. The study was unable to identify exactly what parts of frogs’ tongues do the sticking; Kleinteich is still working on figuring out those details. Even so, the findings are remarkable: the frogs’ version of pressure-sensitive adhesion bests our synthetic imitations in many ways.
“Their tongues stick to everything,” said Kleinteich, which is more than can be said of typical household tape. The tongues also adhere instantaneously upon impact.
The hope is that further research will lead to improvements in adhesive technology for human purposes, such as improving the quality of that infuriating household tape.