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Cane toads practically lick their own hearts as they eat. Their tongues stretch farther down their throats than they extend to catch prey.

Many frog species have tongues that are powerfully sticky and yank prey into their mouths. The muscular and skeletal dance that allows the frog to extend its tongue and ensnare prey is relatively well-understood, says at the University of Florida.

鈥淏ut we actually know nothing about what happens to the inside of a frog when it closes its mouth,鈥� he says.

To get an inside look at how frogs swallow their prey, Blackburn and his colleagues looked to cane toads (Rhinella marina). Hefty and voracious, the species seemed a good choice for some mealtime XROMM analysis 鈥� a 3D X-ray video technique for tracking the fast movements of bones and cartilage.

In three toads, the team surgically implanted nearly two dozen small tantalum markers in the skull, jaw, tongue, chest bones and hyoid apparatus 鈥� a cartilaginous sling shaped like a shallow dish that supports the tongue and throat muscles. The team recorded the high-speed movements of the toads鈥� bones and muscles, noting how the markers moved, as the toads fed on crickets in the lab.

The first glimpses of the footage were a shock. After the toad鈥檚 mouth shut, the team saw the tongue recoil deep down the throat.

鈥淲e thought something was wrong with the videos,鈥� says , now at Mount Holyoke College in Massachusetts. 鈥淲e thought that we didn鈥檛 do something correctly, because we were like 鈥榯here鈥檚 no way this tongue marker is actually behind the back of the skull鈥�.鈥�

As the toad pulls its tongue back into its mouth, the hyoid 鈥� which sits below the tongue and between the oesophagus and the front of the neck 鈥� jerks backwards and down, sliding along the length of the oesophagus. Once it slips further than the back of the toad鈥檚 head, the tongue tip races down the throat and intercepts the now flexed and cup-like basin of the cartilage plate as it presses up into the wall of the oesophagus. Blackburn likens the motion to a baseball slapping into a glove.

On average, the tongue stretches further backwards into the body than it does forwards when capturing prey, says Blackburn. The tongue 鈥� stretching some 4.5 centimetres down the throat 鈥� and hyoid manage to bump into the heart, which sits just to the rear of the toad鈥檚 chest bones, past the animal鈥檚 extremely short neck. The organ is spared direct contact thanks to a wall of tissue.

鈥淚t looks like it would hurt,鈥� says Keeffe. 鈥淸The tongue is] coming back so far.鈥�

To pry the prey off the tongue, the hyoid then pushes and scrapes the tongue against the back of the throat.

at the University of Connecticut, who was not involved in the study, wonders if the toads are exploiting the viscoelastic properties of tongue mucus, where the substance becomes less sticky under scraping shear forces.

There are thousands of frog and toad species, with a diversity of tongue types and diets. Determining if and how these species differ when swallowing prey is a key next step, say the researchers.

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