Toxin Takeout: Frogs borrow poison for skin from ants

After more than 30 years of research, scientists have found a source from which poison frogs can acquire a major group of chemical weapons. In a survey of possible frog foods in Panama, the toxins turned up in formicine ants, the subfamily that includes wood ants and carpenter ants.

Researchers have found dietary sources for some other types of frog toxins, but the ant analysis marks the first potential supply of a widespread and large family of alkaloids called pumiliotoxins, says John Daly of the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Md. Until Daly’s frog-diet surveys, the only known natural pumiliotoxins came from frog skin, Daly and his colleagues explain in the May 25 Proceedings of the National Academy of Sciences.

“It’s not unexpected, but it’s terrific that he’s done this,” says Michael Tyler of the University of Adelaide in Australia. Tyler’s lab analyzes a wide range of frog-skin compounds and is tracking dietary sources of frogs’ mosquito repellents.

Frog skins offer a magnificent diversity of chemicals, including 24 structural groups of some 500 alkaloids, which are organic compounds containing nitrogen. Several frog species pack a toxin more potent than that of a puffer fish, but other frogs simply deliver a vile taste. At first, researchers had guessed that the amphibians themselves manufacture the toxins, but frogs raised in captivity on toxinfree diets almost never make alkaloids. So far, the only exceptions are certain Australian Pseudophryne toadlets examined by Daly and Tyler.

Only 7 or so of the 24 classes of frog-skin toxins have a documented dietary source. The approximately 80 pumiliotoxins remained a mystery, despite their wide distribution. They turn up in tiny dendrobatids from Central and South America, Australian toadlets, Mantella frogs from Madagascar, and some toads from South America.

Daly’s group collected some 500 samples of arthropods less than a centimeter long from eight sites in Panama. The crucial samples came from Ralph Saporito, now at Florida International University in Miami, who noticed formicine ants on Heliconia plants during the rainy season. Frogs cruise these plants too, raising tadpoles in water pooling at leaf bases.

The only pumiliotoxins from the whole suite of samples came from some of these ants. The ant species containing pumiliotoxins appeared in the stomach contents of some Dendrobates pumilio poison frogs from the study sites, the researchers report.

The finding is news for ant specialists, too, says Diane W. Davidson of the University of Utah in Salt Lake City. Although formicine ants use an alkaloid pheromone, they defend themselves with formic acid. Until this report, Davidson says, she had not heard of other alkaloids in these ants. She suggests that the ants may get toxins or toxin precursors from their diet.

---