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Poison Frog Uses Less-Toxic Looks to Survive, Study Finds

John Roach
for National Geographic News
March 8, 2006
 
Many animals avoid being eaten by copying the appearance of their
poisonous neighbors. But when it comes to deciding whose looks to mimic,
an Amazonian poison frog is teaching biologists a new lesson about this
evolutionary trick.

Instead of copying its most poisonous and numerous neighbors, a nontoxic species of poison frog in Ecuador has been found to get better protection from predators by looking like a less abundant frog that packs a less toxic punch.

The finding may make scientists rethink the laws of Batesian mimicry, as the copycat safety strategy is known, said Catherine Darst, a graduate student in biology at the University of Texas at Austin.

"We found another way that Batesian mimicry works," she said.

According to Darst, the way predators learn to avoid toxic prey can drive the evolution of the copycats' color patterns.

The more toxic frogs are so nasty that predators quickly learn to avoid anything that remotely resembles them, she explained. But predators that eat the less toxic frogs learn to only avoid that species and its exact mimics.

Since all the frogs look similar, the mimics can afford an imperfect match with the more toxic species and gain the advantage of looking just like the less toxic frog, Darst said.

She and colleague Molly Cummings, a University of Texas biology professor, report the discovery in tomorrow's issue of the science journal Nature.

David Pfennig, who studies snake mimics at the University of North Carolina at Chapel Hill, said the findings may seem counterintuitive, but they "make sense."

"These results are likely general to other systems as well," he said.

"There should be a larger cone of protection around more toxic species," which gives mimics room to evolve new color patterns.

Copycat Protection

Darst and Cummings based their discovery on a study of three poison frog species: the highly toxic Epipedobates parvulus, the related but less noxious E. bilinguis, and the nontoxic mimic Allobates zaparo.

All three species live in the Amazon rain forest of Ecuador (map) and bear a similar pattern of red dots on their backs to warn predators of their toxicity.

Where only the more toxic frog lives, in the southern rain forest, the mimic copies the more toxic frog. The mimic copies the less toxic frog only where the less toxic species is found, in the northern rain forest.

However, where the two brightly colored toxic frogs overlap, Darst and Cummings found the mimic only copies the less toxic species.

Evolutionary theory suggests that the nontoxic frogs would be better protected if they copied the more poisonous frog, or the more abundant frog, or if they diversified to look like both toxic species, Darst said.

"So, like, what is going on?" she asked.

To find out, she performed a series of predator-learning experiments with domestic chicks collected from villages outside of Quito, Ecuador.

In the experiments, the chicks were presented with different combinations of poison frogs, to determine which species the birds would learn to avoid eating.

The chicks quickly learned to avoid the highly toxic frogs during the sessions. Subsequently the birds avoided all similar-looking frogs, even those that weren't exact mimics.

But when the chicks were given the less toxic frogs alone, they learned to avoid only exact mimics of the less poisonous species.

"By resembling the less toxic model, the mimic gets protection from predation no matter which model species the predator learns with," Darst said.

Pfennig, the University of North Carolina biologist, added that this aversion to mimics has been seen in other species as the result of genetics.

For example, snake predators cannot afford learning to avoid toxic coral snakes, he said. One mistake and they're dead. As a result, snake predators are genetically predisposed to avoid coral snakes, which thus affords mimics protection, he explained.

Evolution Evolves

Batesian mimicry is named after famed English naturalist Henry Bates, who first described mimicry over a century ago.

Bates was a contemporary of Charles Darwin, who is widely credited as the grandfather of evolutionary theory.

According to Pfennig, Darst and Cummings' discovery demonstrates that there are "still a lot of outstanding questions" for biologists to ponder about evolution.

"We think we understand something really well, but if we plumb more deeply, there is more to this story than initially we thought was the case," he said.

"This [study] is a nice extension of Batesian theory."

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