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The meta-frog: CT scan reveals frog inside another frog

A scientist examining frog samples found an unexpected surprise in one of the frogs: another frog.

Frog-eating: the hardcore edition. Dr Thomas Kleinteich, Kiel University

Frogs belonging to the genus Ceratophrys have a very particular way of catching their prey. They sit, squatting still and silent, waiting for another animal to cross their path -- then BAM! Out flicks the sticky tongue and down goes the prey into the gullet.

The frog actually has a very wide mouth: Ceratophrys have been known to consume not just the insects, worms and arachnids more commonly known to be frog food, but vertebrates such as lizards, snakes, rodents, crabs -- and other frogs.

And this one frog bit off a bit more cannibalism than she could chew (not that chewing is something frogs do with their food).

The frog-in-a-frog was discovered by Dr. Thomas Kleinteich, from the Functional Morphology and Biomechanics working group at the Zoological Institute at Kiel University. He was using a Skyscan 1172 desktop micro-CT scanner to make 3D images of frog specimens using X-ray computed tomography as part of the group's research into the stickiness of amphibian tongues.

Ceratophrys ornata.Melanie Mae Bryan, CC BY 2.0

"We have known for a while that horned frogs are able to eat very large prey," he explained.

"Last year I was able to show experimentally that horned frogs can produce extremely strong adhesive strength with their tongues, which they also need in order to catch larger prey. With the micro-CT I am able to show, for the first time, how such a large catch can even fit inside a horned frog."

The frog was a 70-millimetre long Argentine horned frog -- a Ceratophrys ornata -- preserved in alcohol. Prior to being donated to the Zoologisches Museum Hamburg, Germany, she had probably lived in captivity, either in a zoo or as a pet. However, information about her collection and locality was unavailable.

The frog she has swallowed was over half her length, 43 millimetres, and identified tentatively as a juvenile Lithobates pipiens. The identification was tentative because Lithobates pipiens' distribution does not overlap with that of Ceratophrys ornata. The former is across North America and Panama, the latter is South America. However, the pair could have intersected in a zoo, pet, or laboratory setting, so it's not entirely outside the realm of possibility.

The larger frog swallowed the smaller head-first, and its head can be clearly seen in the belly of the Ceratophrys ornata. Its left leg extends through the oesophagus of the Ceratophrys ornata and into her mouth, with its foot lying across her tongue.

Now that's what I call a frog in the throat.