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Poison dart frog mimics gain when birds learn to stay away

March 09, 2006

Texas biologists studying a poison dart frog mimicry complex find that predator learning can effect the evolution of mimicry
AUSTIN, Texas-Studying neotropical poison dart frogs, biologists at the University of Texas at Austin uncovered a new way that the frog species can evolve to look similar, and it hinges on the way predators learn to avoid the toxic, brightly colored amphibians.

In the Mar. 8 issue of Nature, Catherine Darst and Molly Cummings show that a harmless, colorful frog living in the Amazonian rainforest gets protected from predators not by mimicking its most poisonous neighbor, but by looking like a frog who's poison packs less punch.

The Texas biologists studied three species of poison dart frogs-one highly toxic species, one less toxic species and one harmless species. All live in the same area and are brightly colored, which warns predators that they may be poisonous.

In a series of predator learning experiments, the researchers found that the frogs' predators-in this case birds-learned to avoid anything remotely resembling the most toxic species.

"What we found is that predators are using stimulus generalization, which is a really old psychology theory," says Darst, graduate student in integrative biology. "When they learned on the more toxic frog, they generalized."

The harmless frogs can look like the less toxic poison frogs without losing any protection from predators.

The result is surprising, because mimicry theory predicts that when all three frogs occur in the same forest, the mimics would look like the more toxic frog species, the more abundant of the toxic frogs, or look like both the more and less toxic species.

"We've uncovered a new mechanism involved in mimicry processes," says Cummings, assistant professor of integrative biology. "A mimic species can actually become a different color pattern if it can enjoy the protection of the predator generalization brought on by more toxic species in the community."

Taking predator avoidance learning into account, says Cummings, you can successfully predict a specific direction of mimicry in evolution.

In studying the toxicity and abundance of all three species, Darst found that the harmless frog mimics either toxic frog where they live separately. In the north, the mimic looks like the less toxic species; in the south, it looks like the more toxic species.

Where all three live together, she unexpectedly found that the mimic looks like the less toxic, less abundant species.

"That is totally bizarre," says Darst. "The whole point of mimicry is to gain protection from predation."

Darst performed predator experiments using domestic chicks she collected from villages outside of Quito, Ecuador. The chicks quickly learned to avoid the highly toxic frogs during training sessions and subsequently avoided all other similar frogs, even those that weren't exact mimics.

The birds' learned avoidance and generalization helps explain how the harmless mimic frog can be more abundant than its model. (It's generally believed that if a mimic outnumbers its model, the system would break down because predators would stop associating color patterns with toxicity.)

If the mimics looked like the more toxic species, they might fall prey to predators who learned on the less toxic frog. By mimicking the less toxic frog, the mimic covers its bases. It gains protection from predators that have tasted both the more and less toxic frogs in the area.

When predators learn to generalize based on the most toxic species, Cummings says that mimics can actually be freed to evolve new color patterns and that this could also explain why poison frogs are so diverse.

"When predators generalize, it actually allows mimics to avoid the penalty of novelty," says Cummings. "This could be allowing diversity to take hold."

The frogs Darst and Cummings studied happen to live in the same forest basin where the famous naturalist Henry Bates first described mimicry in butterflies over 100 years ago. His theory, known as Batesian mimicry, describes how edible species can gain protection from predators by looking like toxic species.

University of Texas at Austin