BioEd Online

Imitation may be the sincerest form of flattery, but for some it's about survival.

Animals often avoid predators by copying the appearance of poisonous creatures. Usually the impostor tries to look like the most toxic species around, or imitates a range of toxic animals. But this is not so in the case of Ecuadorian frog Allobates zaparo. This frog chooses to mimic the less toxic of two local species.

"It runs counter to traditional models," says Molly Cummings of the University of Texas, Austin, who describes the frog's strategy in this week's Nature1.

Red for danger

The poison frogs Epipedobates bilinguis and Epipedobates parvulus share a similar warning sign: a bright red back. But the less poisonous and rarer of the two, E. bilinguis, also has yellow markings on its upper arms and thighs.

Cummings found that when A. zaparo was found in the same region as one of these poisonous species, it would imitate that one. But in areas where all three species lived, A. zaparo tended to mimic E. bilinguis.

This is odd. Mimics usually evolve to imitate the more abundant or more toxic species, says Cummings, because that normally guarantees the most protection.

Pecking order

Cummings and her team decided to investigate how the frogs fared in the face of hungry chickens, as a model of natural bird predators. Chickens that were exposed to the very toxic E. parvulus soon learned not to peck them, and avoided any frogs that looked even remotely like the noxious animal.

But chickens that were exposed just to E. bilinguis did not become so wary. They avoided E. bilinguis and any frogs with yellow leg markings, but were not put off all red-backed frogs.

If the mimic had evolved to look like the most toxic creature, it wouldn't have been protected against birds that had met only E. bilinguis, points out Cummings. "This strategy covers all its bases by being protected no matter what species a predator learned on."

The theory may stand up to more scrutiny than previous ideas, says herpetologist John Endler, who works at the University of California, Santa Barbara. The original explanation for why some snakes mimic less toxic cousins, he notes, was that the most dangerous model would end up killing the predator rather than giving it an opportunity to learn. "We now have a more satisfying explanation," he says.

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