Of all the strange and marvelous appendages to arise in animal anatomy, the frog tongue is one of the few to meet the requirements of a Marvel Comics superpower: the “X-Men” villain named Toad boasted a 30-foot prehensile tongue with which he would do battle.
Real amphibian organs are no less deadly — if you are a cricket, anyway — and have long been objects of fascination. In 1849, pioneering neurophysiologist Augustus Waller published one of the first scientific examinations of frog tongues. The tongue, he wrote in the journal Philosophical Transactions, “acts as an agent for seizing prey by being rapidly thrown out of the mouth, and enveloping the object to be laid hold of.” More than a century later, German researchers observed horned frogs catching bugs with strikes that clocked in at 40 milliseconds.
Yet how, exactly, frogs could maintain their grip on insects during such speedy attacks was not fully understood. Scientists knew the tongues were super-adhesive; one 2014 study revealed a that frog tongue could heft objects 1.4 times the animal’s own body weight, relying on a mechanism that the Los Angeles Times likened to the glue on the back of a Post-it note. Others have compared the tongues to rolls of sticky transparent tape.
But it would not be until Alexis C. Noel, a biomechanics PhD student at the Georgia Institute of Technology, watched a video of an African bullfrog crushing digital bugs with its tongue (the pet frog was playing the mobile game “Ant Smasher,” the stuff multi-million-view YouTube clips are made of) that she began to wonder if researchers had missed a trick.
That trick turned out to be frog spit, Noel found. More specifically, frog spit can change physical properties, transforming from a glue more viscous than honey to a thinner fluid and back again.The interplay between this reversible frog saliva and extra-soft frog tongues, as Noel and her colleagues revealed Monday in the Journal of the Royal Society Interface, allows the animals to capture meals in the amount of time it takes a human brain to think of and speak a word.
The saliva currently swishing around your teeth and gums, as helpful and lubricative as it is, will not transform like frog spit. (By amphibian standards, the mammalian maw is a slobbery, torrential place. The spit a five-year-old child produces in a single day, according to one estimate, could fill a 16-ounce soda cup.) Where we have salivary glands, the frog tongue itself produces spit. Even when you cut the tongue out of a frog, as Noel did more than a dozen times as part of the new study, the organ will still ooze saliva.
Frogs do not need much spittle. Noel spent several hours wringing out frog tongues to accumulate the milliliter of frog saliva she needed to evaluate for the study. “It’s like ketchup,” Noel said of the frog saliva, in an interview with The Washington Post early Tuesday morning. Both ketchup and frog spit are what mechanical engineers describe as shear thinning fluids. As anyone who as flipped a Heinz bottle upside down knows, ketchup can be reluctant to flow even with the aid of gravity. But smack the bottom of the bottle — apply a shear force, in other words — and the ketchup, suddenly thinned, starts to move.
Frog tongues are also much softer than human tongues. “It feels like when you chew a piece of gum for too long,” Noel said, who has also investigated properties of sweat, earwax and cat tongues. “Soupy and disgusting.” The Georgia Tech researchers compared frog tongues to brains and tongues removed from human cadavers. They found that the frog tongues were slightly softer than brains and 10 times softer than human tongues, making the tongue one of the softest known biological tissues.
Having determined the biomechanical properties of the frog tongues, Noel and her colleagues then watched frogs catch food with high speed cameras. Noel broke down the attack into three steps. First, when the tongue slapped into a bug, the organ deformed and wrapped around the prey, maximizing contact area. With the force of the impact the viscous spit turned to liquid, seeping into the tiny little cracks of the insect’s shell.
Second, the tongue retracted into the mouth as the spit returned to its thick state, securing the bug in place. If a soft frog tongue was an elastic bungee cord, Noel likened the stiffer human tongue to a rope. This elasticity allows the frog to keep its prey and dampen the extreme forces of the strike. “Jump off a bridge with a rope tied around your ankle,” Noel said, “and your ankle is going to come off.”
Finally, once it had a bug in its mouth, the frog needed to dislodge its prey from the spit. To do so, the frog pressed down with its eyes. Like a fishing bobber that dipped below the surface of a pond, the frog’s eyeballs momentarily disappeared into its mouth. This sheared the bug free from the sticky tongue and down into the frog’s gullet.
The scientists collaborated with the Atlanta Botanical Garden to test seven species of exotic frogs. All of the frogs had tongue tissue that acted in a similar way, even those that were not closely related species. Noel said that recent studies of chameleons hint at the same type of mechanism; sundew plants, too, which feed on captured insects, secrete a fluid with properties like frog spit, she said.
Beyond a better understanding of frog biology, at a time when many amphibian species are threatened in the wild, Noel envisioned several possible applications of frog tongue technology. “It gets you thinking about novel types of high-speed, resealable adhesives,” she said. And though Noel admitted this was likely on the more whimsical end, perhaps there might a future in frog-like drones —such as quadcopters kited out with delivery systems capable of picking up cargo in the blink of a tongue.
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