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Toxic Frogs Get Their Poison From Mites

Scott Norris
for National Geographic News
May 14, 2007
 
Tiny mites give some Central American poison frogs most of their toxic sting, researchers have discovered.

Many tropical frog species secrete compounds known as alkaloids to protect themselves from predators and prevent infections.

Scientists knew that frogs don't produce the toxic compounds themselves, but rather acquire them from their diet. For instance, poison frogs raised on a diet of fruit flies—which contain no alkaloids—quickly lose their dangerous slime. (Related: "Poison Frogs Losing Their Toxicity, Study Suggests" [November 7, 2006].)

But until recently, said Ralph Saporito of Florida International University (FIU) in Miami, "it's been a mystery what the dietary sources are."

Ants are known alkaloid producers, so previously they had been assumed to be the major source of the more than 800 known alkaloid compounds present in poison frogs.

Saporito's team, however, found that frogs in the lowlands of Panama and Costa Rica obtain most of their protective chemicals from tiny arthropods known oribatid mites—a group not previously known to produce alkaloids at all.

The findings appear in today's online edition of the journal Proceedings of the National Academy of Sciences.

Mighty Mites

Oribatid mites feed on decaying plant materials and are abundant in soil and leaf litter.

Saporito's team collected mites in traps set on the forest floor. Analysis of the mites showed that more than 80 different alkaloids were present.

At least half of the mite alkaloids were also present in the secretions of strawberry poison dart frogs collected from the same locations.

That species was chosen because it is "typical of poison frogs in general and shares many alkaloids in common with other poison frogs throughout the world," Saporito said.

The mite-produced compounds included representatives of nearly half of the 24 major chemical classes of alkaloids known from poison frogs, Saporito said.

"It is surprising that mites—rather than ants—were found to be the major source of alkaloids," commented Christopher Raxworthy of the American Museum of Natural History in New York.

"The source of many alkaloids in poison frogs have [still] not been identified," Raxworthy added. "Mites were one of the major arthropod groups requiring detailed investigation."

Unique Compounds

Alkaloids, which include caffeine and nicotine, are a highly diverse group of chemicals with many unusual properties.

Some simply taste bitter, while others are highly toxic. Many experts believe the compounds may be sources of powerful new drugs.

In addition to the alkaloids present in the poison frogs, Saporito's analysis turned up 40 previously unknown alkaloids unique to the mites.

"Many of these compounds show interesting biological activity and are structurally unprecedented in nature," said study co-author John Daly, of the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland.

Daly said that the frogs are not harmed by the toxic compounds, presumably because the alkaloids are consumed in very small amounts and become concentrated in the frogs' bodies over time.

"The frogs don't chemically transform [the alkaloids] at all, they just store them in their skin glands and then secrete them," Daly said.

The next big question to answer, FIU's Saporito said, is how the poison frogs accomplish this trick.

"Virtually nothing is known about how the frogs actually take the alkaloids from a dietary source and store them in their glands."

Saporito's research was funded in part by the National Geographic Society's Committee for Research and Exploration. (National Geographic News is part of the National Geographic Society.)

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