X-ray video of cane toads puts whole new meaning to ‘heart in your mouth’

Have you ever been so excited it felt like your heart was in your mouth? Well, a new paper published in the journal Integrative Organismal Biology has found that cane toads take their mouth to their heart every time they eat.

The researchers used X-ray video to track for the first time the tongue movements of the cane toad (Rhinella marina) eating crickets (Gryllodes sigillatus). The results are as horrifying as you’d imagine – the toads use a complex pulley system of cartilage and muscle that travel so far down their throat it meets their heart.

“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 happens after they close their mouths was a mystery,” said first author Dr Rachel Keeffe, a biologist at Mount Holyoke College in the US.

This is partially because toads have incredibly weird structures inside their mouths. Amphibians have  a cartilaginous plate called a hyoid, which has loops and prongs attached to muscles. The hyoid plate rests on the floor of their mouths, and its function pertaining to toads’ ability to swallow prey was – until now – entirely unknown. Many frog species also have two sets of fanglike teeth on the roof of their mouth, and toothless toads have ridges along their upper palate resembling a wash board. The whole thing is bizarre.

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Despite this odd set of mouth instruments, there’s been plenty of theories trying to work out how toads deliver their food to their stomach once it’s safely in its mouth. Some scientists suggested that frogs used their tongues to directly deposit food in their throats, while others thought they pushed food along by squeezing their eyes shut.

So, to work out what was really going on the team placed cane toads in a clear box, and then, while filming them with x-ray video, they fed them crickets.

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Keeffe positioned cane toads in a plexiglass container and fed them a steady stream of crickets. Credit: Keeffee et al., 2022, In Intergrative Organismal Biology.

“We weren’t sure what was happening at first,” Keeffe said. “The whole floor of the mouth was pulled backward into the throat and the tongue along with it.”

After spending months reconstructing the movements frame by frame, the researchers discovered that once the tongue is fully extended to catch the cricket, the hyloid (the cartilaginous plate) at the bottom of the mouth lowers down until it reaches the heart. The tongue then slingshots back into the mouth and hits the hyloid.

Then the rest of the ensemble comes into play to remove the bug from the toad’s sticky tongue.

“The hyoid shoots up and presses the tongue against the roof of the mouth, after which it moves forward, essentially scraping the food off into the oesophagus,” Keeffe said.

The final ascent and scraping motion may explain the presence of ridges and fangs on the upper palate of some frogs. “If this is what all frogs are doing, then these structures are definitely playing a role in swallowing,” she added.

Of course, there’s still much more to understand yet. Cane toads are just one of thousands of species of amphibians, many with significant diversity in their feeding apparatus.

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