Contrary to popular belief, it is not possible to swallow your own tongue. If you’re human, at least. It turns out that toads do it on purpose every time they eat.
“We know a lot about how frogs extend their tongues and how it sticks to their prey, but prior to this study, essentially everything that happened after they closed their mouths was a mystery,” said University of Texas herpetologist Rachel Keeffe. Florida.
So Keeffe and colleagues used high-speed X-ray video to find out what happened when these amphibians slammed their mouths around a meal, and the results were completely unexpected.
“We weren’t sure what was going on at first,” says Keeffe. “The entire floor of the mouth was pulled back into the throat and the tongue with it.”
It took months of careful study of the cane toads (Rijnella marina) while eating hundreds of crickets (Gryllodes sigillatus) and create 3D animations to puzzle out this bizarre input mechanism.
Frogs are known for catching their prey with quick, sticky tongues, but therein lies the problem their unusual anatomy was meant to solve: how to then pry the food from that clingy whip to send it through their guts.
From capture to swallow, the whole process takes less than two seconds, but a whole series of events happen in the toad during this short time frame.
The team attached small metal beads to the toad’s tongue so they could track the muscle’s movements in the X-ray images. As shown in the video below, the orange marker on the tip of its tongue lunges out to snag an insect, then pops back into the toad’s mouth. But it doesn’t stop there, it goes 4.5 centimeters down the throat until it almost touches the heart of the toad.
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“The average distance the tongue extends during retraction is equal to or greater than the average distance the tongue extends during extension,” the researchers said. to write in their paper, they state that the maximum protrusion of the tongue averaged 4.1 centimeters.
Here, close to their heart, the hyoid bone—a flexible cartilage plate suspended from muscle strands—snaps close to the tongue.
“The hyoid bone shoots up and presses the tongue against the roof of the mouth, then it moves forward, essentially scraping the food into the esophagus,” explains Keeffe.
The hyoid bone (which some toads also use to make clicking calls) naturally seals the floor of the mouth while the toad rests. But its connection to the tongue means it swings open when the muscle stretches, opening wide when the toad opens its mouth, ready for the tongue’s recoil.
This is likely why toads and many frogs have strange ridges or bump-like “teeth” on their palates, Keeffe and team suspect; to help de-stick these foods. The hyoid bone markers hit this area with precision in the researchers’ 3D reconstruction. The flexibility of the hyoid bone is also said to aid in its scraping job.
“Even if a toad moves the tongue in the mouth during a double swallow, the prey remains attached to the tongue during the manipulation,” Keeffe and colleagues write. This suggests that the frogs need the hyoid bone mechanism to successfully dislodge their food.
The researchers are now eager to repeat these studies to see if this tongue retraction and scraping feeding mechanism is universal across nearly 5,000 species of frogs, including a huge diversity of hyoid and tongue shapes.
This research was published in Organic biology.