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Critter Chemistry

February 8, 1999


Getting More from Ants Than Just a Good Meal

Maureen Rouhi

Some frogs are a clever bunch. It appears that after having their fill of ants, the frogs make a defensive coat from toxins the ants produce.

The close similarity between alkaloids isolated from the ant Solenopsis azteca and those from the strawberry poison frog Dendrobates pumilio supports the hypothesis that alkaloids in the frog's skin are of dietary origin.

Brightly colored frogs of the family Dendrobatidae are well known for bioactive alkaloids found in their skin. But the alkaloids become depleted over time in frogs raised in captivity and their offspring have none at all. Those observations have led researchers to suspect a link between the compounds and the frogs' diet. Dramatic differences in the alkaloid profiles of frogs of the same species growing in different habitats also point to environmental influences at work.

New evidence supporting the dietary hypothesis comes in the form of two decahydroquinoline alkaloids isolated for the first time from ants. Alkaloids of this type are plentiful in skins of dendrobatid frogs and the two found in ants are very similar to two produced by the strawberry poison frog Dendrobates pumilio. Discovery of the two compounds in ants supports the idea that some frogs may be sequestering the compounds from their diet for defensive purposes.

The evidence was obtained by researchers led by John W. Daly, head of the National Institute of Diabetes & Digestive & Kidney Diseases' Laboratory of Bioorganic Chemistry, in Bethesda, Md., as part of a years-long project to elucidate the chemistry and pharmacology of the alkaloids in dendrobatid frogs. The work was done in collaboration with researchers at three Japanese institutions: Shimadzu Corp., Toyo Polymer Co., and Osaka City University; the University of Puerto Rico, San Juan; the Los Angeles County Museum of Natural History, in California; and Virginia Military Institute, Lexington [ J. Nat. Prod., 62, 5 (1999)].

The evidence is indirect, because the ants from which the alkaloids were obtained were not actually fed to the frogs. But according to Thomas F. Spande, a research chemist in Daly's group, biological studies indicate that these particular ants, collected in Puerto Rico, are very probably part of the diet of these particular frogs, collected in Costa Rica.

"The structural coincidences in uncommon structures are fairly convincing evidence that the ants are a likely source of the alkaloids in frogs," comments Jerrold Meinwald, a chemistry professor at Cornell University whose research interests include the chemistry of insect defense and communication. Rigorous demonstration of the dietary link, he notes, would require "very tedious experimental work" involving either chemical analysis of the frogs' diet or feeding experiments with ants.

The work underscores one of the basic relationships between organisms in which one species bases its defense on another, Meinwald says. "If the prey disappears, as could happen if people went after the ants with insecticide, the frog itself would become very vulnerable to attack."

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