Looking like scans of a freaky frog-shaped suitcase at airport security, the series of 3D images reveal the “frog concealment” in extraordinary detail.

There was something odd with the animal because it had bones where there shouldn't be bones

Packed like some sort of amphibian Russian doll, the frog that was eaten – highlighted by the red colour – is all neatly curled inside the predatory horned frog’s stomach. All, that is, apart from its left back leg. It is still protruding out of the oesophagus, with the foot actually in the horned frog’s mouth, lying on its tongue.

This is, quite literally, a frog with a frog in its throat.

And it’s the beginning of an intriguing journey of discovery about the life and death of a museum specimen frog whose eyes may have been too big for its belly.

This accidental discovery was made by Dr Thomas Kleinteich from the Functional Morphology and Biomechanics group at the Zoological Institute of Kiel University, Germany. He generates 3D computer models of animals using a micro-CT scanner, which works in a similar way to a medical CT scanner but is designed for investigating small objects.

“Before I even had a 3D view of the specimen, I realised in a 2D section that there was something odd with the animal because it had bones where there shouldn't be bones.

“It didn't take me long to understand that it swallowed another frog species,” he tells BBC Earth.

In theory, you could attach a horned frog to the ceiling by its tongue

South American horned frogs (the Ceratophrys genus) are part of Kleinteich’s current research focusing on tongue adhesion in amphibians – how frog and salamander tongues stick to prey. His research covers two main aspects: the first is the anatomy of the tongue, for example how it’s attached to the jaw and its surface structure. The second is the forces with which the tongue can pull items towards the mouth before the contact breaks.

“For the anatomical part, I use specimens from museum collections because zoological museums are amazing resources to get access to many different species from all over the world,” says Kleinteich.

The specimen in question, an ornate horned frog (Ceratophrys ornata), came from the Zoological Museum in Hamburg.

However, aside from its final meal, very little is known about this horned frog. The jar, which would normally have details such as the date when the frog was stored or the circumstances surrounding its death, doesn’t have any labelling.

Pac-Man frogs

Horned frogs are best known for two things: their fearless reputation and their feeding habits. If threatened, these frogs have no qualms defending themselves, leaping at their potential attacker, regardless of size, and can deliver a painful bite.

Sometimes they will pretend that their toes are worms to lure prey closer to their mouth

Their capacious mouths, which account for about half their body size, have several odontoid structures – bony, teeth-like projections – along their jaw. In South American folklore, tales describe how these frogs – known locally as escuerzo – have bitten fingers and refused to release their grip until after dark, hours later.

And they have a voracious appetite to boot. They will attempt to eat anything they judge can fit in their mouths and, it would seem, some things that can't.

All eight species of horned frogs in the Ceratophyrs genus are “sit-and-wait” ambush predators, having to bide their time until food comes to them. But some are capable of making that happen a bit more quickly.

“Sometimes they will pretend that their toes are worms to lure prey closer to their mouth,” says Dr Kleinteich, who has filmed C. cranwelli exhibiting this behaviour in captivity.

Popular in the pet trade for their decorative skins, horned frogs’ gluttonous behaviour has earned them the nickname “Pac-Man frogs” among owners, after the iconic yellow video game character with an insatiable appetite.

Their relatively unfussy diet consists of a variety of prey, including spiders; insects; crabs and worms, but also larger, vertebrate prey such as snakes; lizards; rodents and other frogs.

Kleintech’s previous studies have shown that a horned frog’s tongue can cope with pulling forces well beyond its own weight, which aids it in tackling these larger prey items.

“In theory, you could attach a horned frog to the ceiling by its tongue,” explains Kleinteich.

 

Frog forensics

So to discover that this horned frog had eaten another, smaller frog isn’t really that unusual, but there is something strange about this particular duo.

After his discovery using micro-CT scans, Kleinteich dissected the body, revealing some very interesting further information. The findings are published in the journal Salamandra.

The foot of the prey frog that was still lying in the oral cavity must have made it harder for the horned frog to breathe and such a state might be lethal 

The prey is in almost immaculate condition so digestion had not really begun, suggesting that its killer died very soon after having swallowed it. Also, Kleinteich tentatively identified the prey species as a juvenile Lithobates pipiens frog – but as there's a level of doubt he prefers to refer to it as a species of Lithobates. If it is L. pipiens then their natural range in the wild doesn’t overlap with the ornate horned frog (C. ornata), suggesting that the horned frog was a captive animal either kept as a pet or in a zoo.

Approximately 21% of the horned frog’s total volume was taken up by the prey species. It had swallowed a meal that was roughly a quarter of its own weight – that's like the average male in the UK eating 192 quarter pound burgers in one go.

So was this last gigantic meal the reason the horned frog croaked it?

Kleinteich says it “might be possible” but points out that he’s not a frog forensics expert and that without any documentation indicating where this frog came from, it’s speculation.

“I can imagine, however, that the foot of the prey frog that was still lying in the oral cavity must have made it harder for the horned frog to breathe and such a state might be lethal,” postulates Dr Kleinteich.

“However, frogs have a rather low metabolism and a substantial portion of the oxygen exchange happens over the skin. They can definitely survive for several hours with prey in their mouth. So there might have been something else [that caused its death].”

Sadly we will never know for sure if this horned frog’s eyes were too big for its belly and ultimately caused its demise. But this just goes to show how scientists are still revealing secrets from specimens lying in museums all around the world.

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