A hard-to-swallow pillbug: First x-rays of frog feeding show them eating prey

A hard-to-swallow pillbug: First x-rays of frog feeding show them eating prey

image: Cane toads grow to be one of the largest frog species, reaching up to 15cm in length, making them the perfect specimen to observe the quick and tiny movements they make while eating.
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Credit: Rachel Keeffe

The phrase “swallow one’s tongue” has been around since at least the 1880s and has been repurposed in several languages ​​to mean everything from being silent to a general feeling of fear. While it’s anatomically impossible for a human to swallow its tongue, a new study shows that cane toads (Rheinlla Marina) achieve this feat every time they eat.

The authors used X-ray videography to study the toads’ feeding behavior and provided a first glimpse of what frogs do with food once it’s safe in their mouths. According to their findings, cane toads swallow their prey using a complex pulley system of cartilage and muscle that travels down their throats so far that it bumps against their hearts.

“We know a lot about how frogs stick out their tongues and how they stick to their prey, but prior to this study, essentially everything that happens after they close their mouths was a mystery,” said lead author Rachel Keeffe, who conducted the study Research during which she is working towards her PhD in the University of Florida Department of Biology.

As early as 1827, naturalists were fascinated by the eating habits of amphibians. But frogs are lightning fast at catching and consuming prey – so fast that it’s almost impossible for the human eye to track their movements. It wasn’t until the development of high-speed video in the mid-20th century that scientists could directly watch frogs’ tongues unfurl like a party horn, encasing their unfortunate targets in a sticky embrace.

Even then, scientists could only guess how they swallowed their food. Some theorized that frogs used their tongues to deposit food directly down their throats, while others suggested that they push food forward by squeezing their eyes shut. But none of these initial ideas could explain the strange structures that researchers found when dissecting frogs’ mouths.

Many species of frogs have two sets of tooth-like teeth on the roof of their mouths, and toothless toads have ridges along their upper palate that resemble a washboard. Amphibians also have a cartilaginous plate called the hyoid bone that has loops and prongs attached to the muscles, which Keeffe likens to a puppet. The hyoid plate rests on the floor of the mouth, and its function in relation to the frogs’ ability to swallow prey was entirely unknown.

To find out how frogs manipulated their food, Keeffe and her colleagues attached metal beads to key points in cane toads’ mouths. Rheinlla Marina. At up to 15cm long, cane toads are one of the largest frog species, making them the perfect specimen to observe the quick and tiny movements they make while eating.

After placing the toads in a clear observation box, Keeffe fed them a steady stream of crickets while filming them with X-rays. An initial examination of the resulting videos revealed a mechanism that was different than expected.

“At first we weren’t sure what was going on,” Keeffe said. “The whole floor of the mouth was pulled back into the throat and the tongue with it.”

Keeffe spent months meticulously reconstructing the movements in 3D animation to determine exactly how the contraption worked. She then illustrated a precise play-by-play from the still images, beginning with the moment each cricket is selected and rolled into the mouth.

Once the tongue has reached its full extension, the hyoid bone retracts into the pharynx. The tongue, which is attached directly to the hyoid bone, is then tucked back into the mouth. It’s unclear how far the hyoid can move back because its path is blocked by the frog’s heart, against which it slides up milliseconds before the tongue and attached prey smash into the cartilage pad, Keeffe explained.

Although the toad has successfully caught its prey by this point, it still faces the problem of having to detach the shaken insect from its sticky tongue. This occurs in the last feeding step, which the authors called hyoid dorsal ascension.

“The hyoid bone shoots up and presses the tongue against the roof of your mouth, after which it moves forward and essentially scrapes the food into the esophagus.”

The final climb and scratching motion could explain the presence of ridges and fangs on the upper palate of some frogs, Keeffe said. “If all frogs do that, then these structures definitely play a role in swallowing.”

The entire process takes less than two seconds from start to finish, and most of the time is spent repositioning the tongue and hyoid bone after swallowing.

Among the 7,000 known species of frogs, there is an incredible variety of feeding mechanisms, ranging from the tongues of toads that shoot out to suction feeding in aquatic caterpillar frogs. According to Keeffe, the next obvious step would be a comparative study between different species to see if cane toad feeding behavior is the norm rather than the exception.

The authors have published their study in the journal Integrative Organismal Biology.

Richard Blob of Clemson University, David Blackburn of the Florida Museum of Natural History, and Christopher Mayerl of Northern Arizona University co-authored the study.

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