for National Geographic News
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.
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.
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