Why is the frog's tongue so sticky? Will it be unable to swallow with food?

By Ed Yong

Translation: Porco Rosso

Edit: You Zhiyi

Four years ago, Alexis Noel, who had just started her Ph.D. at Georgia Tech, made a strange request to a student in an anatomy class: Can they give up their tongues to her when they've finished dividing the frogs? The students agreed.

Noel holds a toad in the lab. (Image: Alexis Noel)

Predator artifact – the tongue of a frog

Noel has loved frogs since he was a child. Growing up and joining David Hu's biomechanics lab, she couldn't wait to start her research on frogs. Hu Lide was also interested in the subject. He recently visited the Atlanta Botanical Garden and saw a group of colorful poison dart frogs eating. He saw a physical miracle in the frogs' movements: when they popped their sticky tongues out of the insects, their speed and precision were astonishing. He later filmed a leopard frog with a high-speed camera and found that it could catch insects in less than 0.07 seconds — five times faster than a human blink of an eye. When its tongue hits its prey, it accelerates 12 times faster than gravity. Somehow, however, the prey was not blown away, but stuck to the tongue.

You can try designing a wet material that hits a roughly textured object at high speed and attaches itself to it. You can't design it. No one has ever made it. However, frogs demonstrate this skill every time they prey. Humans have been studying frogs' tongues since the 19th century, but until now they have not understood why they are so sticky.

The moment the giant leaf frog (Phyllomedusa bicolor) preys on (Image: Ed Yong/Youtube)

Why is it so sticky?

To answer this question, Noel collected a number of frog tongues from the anatomy class. Then she followed the great tradition of naturalists and stabbed them all with her fingers. The soft touch surprised both her and Hu Lide. "They're like silly putty for children, and when you touch them, you can't tell if it's solid or liquid," Hu Lide says, "and they're sticky, like gum you've just chewed or marshmallows that stick to your hands and can't get rid of them." ”

Noel then walked into a materials testing laboratory with a bag of bloody frog tongues and human tongues collected from the school's corpse farm. By slowly pressing a cylinder into these cut organs, she proved that the human tongue is ten times harder than the frog's tongue. Not only that, but the tongues of some frog species are the softest of all the biological materials that humans have measured. "They're as soft as human brains — but they don't pop up to catch prey," Hulid said. "Because of its softness, the frog's tongue can wrap it around the insect when it hits it, creating the largest contact surface. Kurt Schwenk of the University of Connecticut also noted: "The inertia of the frog's tongue itself allows it to elongate significantly during the ejection process, which increases the predatory distance of the frog and also improves their ability to raid insects." ”

Frogs' tongues also have shock absorbers built in — some of which are fat and muscle that cushion part of the energy on impact. These shock absorbers play a role in the recovery of the frog's tongue: the insect, although subjected to a great deal of force, does not fall off. "The tongue is like a trampoline with a pair of baseball gloves — it will stretch and contract, but it will also grab you." Hu Lide said.

Stretching experiments on frog tongues. (Image source: Ed Yong /Youtube)

"We know that leopard frogs prey on many species of insects, earthworms, and other invertebrates," says Jenna Monroy of claremont College, "they may mobilize the tongue muscles depending on the size of the prey to regulate the hardness and damping of the tongue when it touches the prey, thus ensuring a successful catch." ”

"But this soft tissue is only half the battle," Noel said. The other half is the saliva of frogs.

She and her colleagues spent a lot of time and effort scraping saliva from the collected frog tongue. "The saliva is very sticky, and every time you finish scraping, you have to scrape them off the spatula again," Hu Lide said, "and it takes half an hour to collect half a milliliter." It's like a rare treasure. ”

"In the past, people thought that frog saliva was just sticky," Noel said. But upon closer examination, she discovered that the saliva was a non-Newtonian fluid—a liquid whose properties change with the force exerted on it. For example, honey will change from solid to liquid when stirred, the mixture of cornstarch and water will become solid under beating, and ketchup will become easier to flow because of the shaking of the bottle. Saliva is like ketchup: external forces reduce its viscosity. However, also subject to external forces, the viscosity of human saliva will drop to about 1/10, and the viscosity of frog saliva will drop to about 1/100.

Thus, when the frog's tongue hits the insect, the saliva on it begins to flow freely and easily penetrates into every crevice and opening on the surface of the insect's body. And when the tongue slows down and contracts back, the saliva re-condenses into a slimy shape, which is equivalent to clenching a fist and catching the insect back.

The tongue is so sticky,

How to swallow food?

"This analysis can be used to explain many of the wonderful phenomena we observed, such as why frogs use the back half of their eyeballs to push their prey into their throats." Kiisa Nishikawa of Northern Arizona University says: Once an insect is swallowed in its mouth, the frog must get it off its tongue. Fortunately, its slime tricks only work best in the vertical direction; it is difficult to rip the insects off the tongue, and it is easier to pry them apart. The frog only needs to push the insect with something — so it uses its eyeballs. Twelve years ago, Robert Levine used X-rays to show frogs contracting their eyeballs inward when swallowing and using their eyeballs to push insects off their tongues.

Horned frogs use their eyeballs to help swallow their prey. (Image source: Ed Yong /Youtube)

The inspiration of sticky animals

Many studies of sticky animals have led to the advent of new materials. The gecko's ability to climb walls led to the development of dry adhesives. The foot filaments of mussels (the net of sticky fibers that hold them in place of the rock) inspired a viscose that could be used underwater. Worms inspired the invention of a better-performing medical tape, as well as a glue that does not bleed and can repair wounds without stitches. Who knows what inventions a frog's tongue will lead to? "Imagine a suture that hardens immediately after a quick stitch." Hu Lide said, "Or a Band-Aid that doesn't hurt when it is torn off quickly." ”

"Why should we care about these things?" Nishikawa, who studies animal mechanics, said, "Because inspiration from biology can solve practical human problems." Frogs' tongues may seem unpopular, but don't forget that antibiotics also come from bread mold. ”

Image source: http://panda390.blog.so-net.ne.jp/

Now, Noel turned his gaze to the cat's tongue again. Their surface is covered with a layer of tiny ridge spines. Noel wondered if the structure was meant to make it easier to comb his hair, remove the flesh from his bones, or for some other purpose. "I studied both domestic cats and tiger tongues," she says, "and cats and frogs are my favorite animals and are my current research subjects." It's a wonderful life. ”

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编译来源：Ed Yong, Why Frog Tongues Are So Sticky

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