Photograph by George Grall, National Geographic Creative
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A Cuyaba dwarf frog bloats its rump to frighten predators with false eyes.

Photograph by George Grall, National Geographic Creative

Why This Frog’s Poisonous, Inflatable Backside Is Staring at You

This South American frog’s inflatable rear end puts on quite a show for predators.

Humans are forever trying to get their rear ends to look either smaller or bigger, but nothing we do—nothing—can make our cabooses as amazing as that of the Cuyaba dwarf frog, or Physalaemus nattereri.

Yet this frog’s glamorous hindquarters are meant to repel attention, not attract it.

We looked into why such a wonderfully weird amphibian is left alone, by both people and predators.

The Puffy Stare

There are about 123 known species of frogs worldwide with warning colors, known as aposematic coloration, on the back or underside of their bodies, says João Tonini, a postdoctoral fellow at Harvard University and collaborator on Projeto Bromeligenous, a project studying the relationship between Brazil's frogs and bromeliad plants.

In some species, such as deadly poison arrow frogs, bright warning colors tell predators, “I’m dangerous, don’t play with me,” says Arturo Muñoz Saravia, a doctoral student at the University of Ghent in Belgium and coordinator of the Bolivian Amphibian Initiative.

In others, a harmless creature may aim to trick predators into thinking it’s dangerous.

Either way, one means of looking more intimidating is to sport big spots that look like eyes. That’s the tactic of the Cuyaba dwarf frog—native to Bolivia, Brazil, Argentina, and Paraguay—and the Chilean four-eyed frog (Pleurodema thaul) in the same family, Tonini says.

These Tiny Frogs Are Some of the Most Toxic Animals in the World Scientists believe poison dart frogs assimilate plant poisons which are carried by their prey. Poison dart frogs raised in captivity and isolated from insects in their native habitat never develop poison.

The eyespots help to distract from the frogs’ heads and inflate their bodies, sending predators the message, “I’m a bigger animal than you think, so don’t eat me!” Muñoz Saravia says.

When it’s puffed up, the tiny frog may look like a good-sized snake to its predators, which include birds, snakes, coatis, or the fringe-lipped or frog-eating bat. (Related: Surprising Photo: Toad Eats Bat.)

If the predator decides to strike anyway, these frogs have a Plan B: poison. As it turns out, they’re not just bluffing with their warnings.

“Below the eyespots are large toxic macroglands,” Tonini says, which may leave telltale milky-white secretions on the black skin directly beneath the eyespots. There’s enough toxin in one frog to kill 150 mice.

But these frogs are deadly only to smaller predators, not to humans—though you probably wouldn’t enjoy the sting if you were to pick one up and then rub your eyes, says Muñoz Saravia. The real gift of the toxin is that it allows the frog to escape with its life.

Its nasty taste causes a predator to leave the frog alone, and if it gets enough of the toxin, wooziness and sickness follow, which means the frog is going to have time to escape, Muñoz Saravia says.

A Little Luck

The small size of Cuyaba dwarf frogs is also helpful in their escape, Muñoz Saravia says, and may keep them from harm in another way.

At a time when many frog species are endangered, these funky frogs are currently doing relatively well population-wise, despite some decrease in their numbers. according to the International Union for the Conservation of Nature. (Related: Ground Zero of Amphibian ‘Apocalypse’ Finally Found.)

The Cuyaba frogs breed quickly (you can hear their mating calls here and see one of its cousin’s plaintive calls here), and their small size makes them hard to find. Saravia thinks it’s also helpful to their numbers that they’re not in the pet trade.

“They’re small and brown,” he says, and thus not as interesting to pet owners as their poison arrow cousins.

Sometimes it pays to keep your talents hidden until needed. And if showing off an inflatable, poisonous, eye-spotted backside isn’t a talent, we don’t know what is.

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