Every predator needs to catch its prey. We humans use our hands, sharks and wolves use their jaws, but a few animals like frogs use something much stranger: their tongue.
To understand just how frogs snatch their snacks, scientists made the first direct measurements of their tongues in action. They found that certain frogs can lift meals up to three times heavier than their body weight (although they probably couldn't eat them) using a sticking mechanism similar to the tacky glue on Post-It notes, according to a study published Thursday in the journal Scientific Reports.
For the experiment, the researchers recruited the horned frog, a rotund South American amphibian with devilish protrusions above its eye sockets that has proven itself a popular pet. Horned frogs prefer to hunker down in the duff and wait for their prey to wander by before snagging their victims with powerful tongues. Their stationary habits makes them easy subjects to use in studies and they are known for their voracious appetite — they have been caught tackling prey more than half their size.
Just before meal times, the scientists inserted glass plates fitted with pressure sensors into the frogs' quarters about an inch from their noses. Then they offered up tantalizing grasshoppers behind the glass. When the hungry frogs fired their "ballistic tongues" (yes, that's an actual scientific term), the sensors measured the impact force. The tongue prints they left behind gave scientists an indication of the contact area and the amount of mucus on the tongue.
The researchers found that, on average, the adhesive force of the frogs' tongues exceeded their body mass by 50%. One enthusiastic juvenile managed to slam 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 and a zoologist at Christian-Albrechts-Universität Kiel in Germany, "but I didn't really expect that the forces would be that high."
Kleinteich's observations may help explain the astonishing speed of the frogs' tongues, which dart in and out of their mouths in a matter of milliseconds.
"People always think the speed is to catch elusive prey, which makes sense," said Kleinteich. However, their results present another possible reason for fast flicking frogs: greater speed means more impact, more adhesion, and ultimately, a bigger meal.
This ability to reel in large prey would be a particular asset to a sit-and-wait species like the horned frog, said Kiisa Nishikawa, an amphibian biologist at Northern Arizona University in Flagstaff who was not involved in the study.
"One meal could be their energy budget for an entire year!" Nishikawa said. She should know: She has a horned frog at home that hasn't eaten since November when it went into hibernation for the winter.
Beyond just quantifying the stickiness of frogs' tongues, Kleinteich also wanted to understand how they work. So he searched for correlations between the impact force of the tongue, its adhesive strength, and the amount of mucus slimed on the plate to find clues about frogs' fantastic sticking mechanism.
"The common opinion is that the mucus is some sort of superglue that sticks to everything immediately," Kleinteich said. But the results of his experiment showed exactly the opposite: stronger adhesion occurred with less mucus.
Kleinteich thinks the mechanism most closely resembles pressure-sensitive adhesion, the same effect that allows sticky tape and labels to adhere to a surface and later be removed without a trace. In contrast to structural adhesives like glue — which harden to form a permanent bond — pressure-sensitive adhesives employ a substance somewhere between a solid and liquid. It must be fluid enough to form a connection between the surfaces it binds, but elastic enough to resist being ripped apart.
Like tape, the frogs' tongues had a greater adhesive strength when they hit the plate with more force. ("That's the pressure part," Kleinteich said.) The way the forces changed as the frogs pulled their tongues back from the plate was another indication.
"Imagine you have a piece of sticky tape and you're pulling on it," says Kleinteich. "You need a lot of force first to initiate a crack and then it gets less and less and less." The force of the frogs freeing their tongues from the plates mirrored this pattern.
Beyond this, Kleinteich doesn't know exactly what parts of frogs' tongues do the sticking — this is the topic of his current research. However, the frogs' version of pressure-sensitive adhesion bests our synthetic imitations in several ways.
First, Kleinteich said, frogs can catch and swallow many different kinds of prey — some with hair, some with feathers, some with spines.
"Their tongues stick to everything," Kleinteich said, something household tape infuriatingly fails to do.
Frogs' tongues also adhere instantaneously. They don't have to battle to hold onto a fly the way we struggle to hang a poster on the wall or seal closed a package.
If scientists can learn more about how frogs' tongues excel at sticking, they could put the same principles to work in products for people, Kleinteich said.
"There has been a strong focus in materials science to get inspired by biological materials," he said.
It may not happen anytime soon, but someday in the distant future, we might look back and see how frog tongues helped us lick the problem of sticky tape.