Why Frog Tongues Are So Sticky

Four years ago, Alexis Noel, a new doctoral student at the Georgia Institute of Technology, approached a dissection class with a strange request. When they were done cutting up their frogs, she asked, could she have the tongues? They said yes.

Noel has always loved frogs, so when she later joined David Hu’s biomechanics lab, she leapt at the chance to study them. Hu was keen, too. On a recent trip to the Atlanta Botanical Garden, he had watched a group of brightly colored poison arrow frogs being fed. In their movements, he saw a marvel of physics. They would launch their sticky tongues at insects with incredible speed and precision. When he later filmed a leopard frog with a high-speed camera, he showed that it can catch insects in less than 0.07 seconds—five times faster than a human blink. And when its tongue hits, the impact knocks the target away at an acceleration 12 times that of gravity. And yet, somehow, it doesn’t fly off. It sticks.

Try to design a wet material that can hit a highly textured object at incredibly high speed and adhere. You can’t. No one has. And yet frogs perform this feat every single time they eat. People have been studying frog tongues since the 19th century, but they’ve never understood exactly what makes them so sticky.

To find out, Noel gathered tongues from the dissection class, and—in the grand tradition of naturalists—prodded them with her finger. She and Hu were astonished at how soft the tongues were. “It’s like a piece of silly putty; when you touch it, you can’t tell if it’s a solid or a fluid,” Hu says. “And they were incredibly sticky. Freshly chewed chewing gum is similar, or marshmallow fluff that you can’t get off your hands.”

Noel then went to a materials-testing lab with bloody bags full of frog tongues, and human tongues collected from an on-campus cadaver farm. By slowly pressing a cylinder into the disembodied organs, she showed that human tongues are 10 times stiffer than frog ones. Indeed, the tongues of some frog species turned out to be among the softest biological materials ever measured. “They’re as soft as the human brain—but the brain isn’t out and about grabbing things,” says Hu.When the tongue hits an insect, its superlative softness allows it to wrap around the target, giving as large a contact area as possible. It also means that “the tongue’s own inertia causes it to elongate significantly during the outward rotation, increasing the frogs’ capture range and their ability to take an insect by surprise,” says Kurt Schwenk from the University of Connecticut.

The tongue also has in-built shock absorbers—bits of fat and muscle than dampen the energy released by the impact. Those come into play when the tongue retracts: They stop the insect from peeling off, despite the large forces exerted upon it. “The tongue is like a trampoline and a baseball mitt—it stretches but also catches you,” says Hu.

“Leopard frogs are known to eat various insects, earthworms and other invertebrates,” says Jenna Monroy from Claremont Colleges. “It’s possible that depending on the size of the prey, the frog could activate the tongue muscles to adjust the stiffness and damping of the tongue at contact, guaranteeing a successful capture.”

“But the very soft tissue is just half of the story,” says Noel. The other half is the frog’s saliva.

She and her colleagues spent ages laboriously scraping the saliva from their accumulated frog tongues. “It’s highly viscous, so when you scrape, you have to scrape it off the scraper,” says Hu. “After half an hour, we got half a milliliter. It’s like the world’s most precious material.”

“Previously, people thought the saliva was just thick and sticky,” says Noel. But when she studied it, she found that it’s actually a non-Newtonian fluid—a liquid whose properties change depending on the forces applied to them. Honey goes from solid to liquid when you stir it. A mixture of cornstarch and water becomes solid if you hit it. Ketchup becomes runnier if you shake the bottle. Saliva is like ketchup: Forces makes it less viscous. But while human saliva becomes around ten times less viscous if you apply force to it, frog saliva becomes a hundred times less viscous.  

So when a frog tongue strikes an insect, its saliva flows freely and readily seeps into every crack and gap. When the tongue slows down and starts retracting, the saliva thickens again into a paste, the equivalent of a closed fist grasping the insect for the journey back.

“The analysis helps to explain many bizarre observations, like why frogs use the backs of their eyeballs to push prey down their throats,” says Kiisa Nishikawa from Northern Arizona University. When the insect’s in the frog’s mouth, the frog has to get it off its tongue. Fortunately, all of its adhesive tricks work best in the perpendicular direction—it may be really hard to pull the insect off, but it’s comparably easy to slide it off. The frog just needs something to push against the insect—so it uses its eyeballs. Twelve years ago, Robert Levine used X-ray videos to show that a frog swallows, it retracts its eyeballs inwards, and uses these to push victims off its tongue.

Many studies of sticky animals have led to the development of new materials. The wall-crawling abilities of geckos inspired the development of powerful dry adhesives. The beards of mussels—webs of sticky fibers that anchor them to rocks—inspired a glue that works underwater. Worms inspired a better kind of medical tape, and blood-resistant glues that can mend organs without sutures. Who knows what frog tongues will lead to? “Imagine sutures that you can put on really fast, and then they harden,” says Hu. “Or Band-Aids that really would hurt less when you peel them off quickly.”  

“Why do we care about this stuff?” asks Nishikawa, who also studies animal biomechanics. “Because inspiration from biology can be applied to practical human problems. Frog tongues seem pretty esoteric, but remember that antibiotics came from bread mold.”

Noel, meanwhile, is turning her attention to cat tongues. They are coated in tiny spines, and Noel wants to know whether these help the felines to groom themselves, to rip meat from bones, or something else. “I’m working with both house-cat tongues and tiger tongues,” she says. “My two favorite animals are cats and frogs and that’s what I’m studying now. It’s a good life.”