Photograph by Joel Sartore, National Geographic

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Pretty poison: a frog of the Mantella genus, found only on Madagascar (file picture).

Photograph by Joel Sartore, National Geographic

Why Some Poison Frogs Taste Bittersweet When Licked

Dangerous Madagascan amphibians sweeten poison with sugar, study finds.

It's a discovery perhaps only a frog-licking scientist could make: Some toxic frogs secrete sugars and bile acids in addition to their poisons, a new study says. (See your frog pictures.)

For about 50 years researchers have skinned poison frogs and ground up the tissue to study its chemistry. The practice, however, was focused almost entirely on extracting the toxins, which can have pharmaceutical applications.

Now, alongside her father, electrical engineer William Clark, herpetologist Valerie Clark—an unabashed frog lover and occasional frog licker—has co-created and used an electro-stimulation device to help extract chemicals out of skin glands without killing the frogs.

"Skinning [was] a standard practice, but in the last couple of decades, improvements in technology have skyrocketed," said Clark, a former grantee of the National Geographic Society's Committee for Research and Exploration. (The Society owns National Geographic News.)

"Our sensitivity to detect and analyze compounds is much greater, so we can do a lot more with a lot less."

For frogs of the Mantella genus of Madagascar, the device helped isolate bile acids and sugars never before seen on frog skins.

The chemicals had been "lost in the mush" of ground-up skins in other expeditions, said Clark, who just completed her Ph.D. at Ireland's Queens University Belfast. (Video: Meet the Frog Licker.)

The chemical analyses suggest bile acids and sugars exist in roughly the same amounts but outnumber the mass of poison alkaloids by roughly ten to one on the frogs.

Mystery Cocktail

It's still a mystery why the skins should secrete sugars and bile.

Clark suspects the sugars—obtained via the ants the frogs eat—may have a protective function in the amphibians' damp, moldy environments. (Related: "Toxic Frogs Get Their Poison From Mites.")

"During times of war, soldiers have taken sugar and rubbed it into their wounds to prevent infection," Clark said. And in fact, Boston University researchers recently said they'd found evidence that sugar may boost antibiotics' effectiveness against bacterial infections.

As for the bile acids, she imagines they may help explain the frogs' immunity to the ant-borne poisons, as well as how the poisons get from ingested ants to the frogs' skins without harming the amphibians.

"One day I think what's going to be shown is that bile acids help transport alkaloids to skin glands and protect the frogs," she said.

"Your body has bile acids that help clear drugs. They move them to the kidneys so you can get rid of them," she added.

"In the frogs the acids may be shuttling [the toxins] out to the skin."

Bitter Pill

Physiologist Alan Hofmann, who has studied bile acids for about 50 years, thinks the discovery of bile acids on poison-frog skin is a first.

"Ordinarily they're kept in the digestive tract," said Hofmann, of the University of California, San Diego, who wasn't part of the new study.

The only other animals known to excrete bile acids are lampreys, and those fish seem to use the acids as pheromones, a form of chemical communication.

In poison frogs, the bile may be just one more weapon in an already toxic arsenal, Hofmann said. Bile acids are "terribly bitter," so they may  help make the frog unappetizing to predators.

The study of tropical poison-frog skin secretions was published online January 30 in the Journal of Natural Products.