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Similar evolutionary forces have independently honed the chemistry of = poisonous frogs on different sides of the world, according to a team of = US and=20 Madagascan chemists and biologists. Poisonous frogs from South America = and=20 Madagascar, which are only very distantly related, derive similar toxins = from=20 their insect diet, the researchers report.

Frogs that secrete toxic chemicals known as alkaloids from their skin = as a=20 defence mechanism are found in South America, Australia and Madagascar. = Research=20 over the past decade has shown that most South American species of = poisonous=20 frog acquire their defensive chemicals from their insect food, = especially from=20 certain species of ants. Now, US and Madagascan researchers, led by = chemist=20 Valerie Clark from Columbia University, New York, have shown that three=20 poisonous frog species from Madagascar also obtain alkaloids from their = diet of=20 ants.

Clark=92s team used gas chromatography and mass spectrometry to = identify the=20 various alkaloids secreted by the three Madagascan frog species, and = also looked=20 for alkaloids in samples of insects taken from the Ranomafana National = Park in=20 Madagascar. They identified a total of 81 alkaloid compounds in the frog = secretions, of which 11 also showed up in the insect samples, mainly = produced by=20 one species of ant. The researchers detected seven of these alkaloid = compounds=20 in frogs and ants found in close proximity.

The findings, together with the discovery that ants made up the = largest=20 proportion (67 per cent) of the contents of the frogs=92 stomachs, are = strong=20 evidence that the Madagascan frogs derive alkaloid compounds primarily = from the=20 ants in their diet. These alkaloid compounds are very similar to those = produced=20 by South American species of poisonous frogs and ants, even though the = species=20 are not closely related. This implies that convergent evolution has led = certain=20 ant and frog species in these two different locations to develop the = same=20 ability to generate and store alkaloids.

Clark plans to use similar analytical techniques to study other = amphibians.=20 =91One project barely begun in this respect is to determine the skin = chemistry of=20 the nearly extinct Kihansi spray toad, which is endemic to a gorge in = Tanzania,=92=20 she told Chemistry World. =91We are hoping that we might = find=20 alkaloids or bufadienolides (toxic steroids), which would become another = example=20 of convergent evolution in the skin chemistry of frogs.=92 Jon=20 Evans