Harmless frogs gain protection by mimicking toxic species

Harmless frogs gain protection by mimicking toxic species
University of Texas at Austin
March 13, 2006

When predators learn to avoid a highly toxic frog, they generalize, and this allows a harmless frog to mimic and be more abundant than a frog whose poison packs less punch, biologists at The University of Texas at Austin studying poison dart frogs in the Amazon have discovered.

Catherine Darst and Molly Cummings report their findings in the March 8 issue of Nature.

“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 predator avoidance generalization brought on by more toxic species in the community.”

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 they may be poisonous.

The complex of frogs Darst and Cummings studied in the Ecuadorian rainforest. Their work is published in the March 8 issue of Nature. Top: the harmless mimic, Allobates zaparo. Middle: Epipedobates bilinguis, the toxic poison dart frog that A. zaparo mimics when all three frogs occur together. Bottom: E. parvalus, the more abundant, more toxic poison dart frog. Images courtesy of David Cannatella. RELATED ARTICLES Study discovers why poison dart frogs are toxic Poison poison dart frogs are small, colorful frogs found in the tropical forests of Central and South America. The brilliant coloration of these amphibians warns predators of their extraordinary toxicity — the golden poison frog (Phyllobates terribilis) of Colombia is said to be lethal if held in one’s hand. Frogs may help in fight against HIV A new weapon in the battle against HIV may come from an unusual source — a small tropical frog. Investigators at Vanderbilt University Medical Center reported this month in the Journal of Virology that compounds secreted by frog skin are potent blockers of HIV infection. The findings could lead to topical treatments for preventing HIV transmission, and they reinforce the value of preserving the Earth’s biodiversity. Climate change is killing frogs finds new research The dramatic global decline of amphibians may be directly connected to global warming warns a new study published in the journal Nature. Looking at a group of frogs found in biodiversity hotspots in Central and South America, scientists found links between higher temperatures and frog extinctions caused by a skin fungus. The infectious skin disease—a type of chytrid fungus (Batrachochytrium dendrobatidis)—is now found in frog populations around the world and is the main suspect in the rapid disappearance of amphibians.

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.

“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.”

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.

The birds’ learned avoidance and generalization helps explain how the harmless mimic frog studied by Darst and Cummings can be more abundant than its model.

If the mimics looked like the more toxic species, they could 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 either the more or less toxic frogs in the area.

Taking predator avoidance learning into account, says Cummings, you can successfully predict a specific direction of mimicry in evolution. She says that when predators learn to generalize based on the most toxic species, 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 more than 100 years ago. His theory, known as Batesian mimicry, describes how edible species can gain protection from predators by looking like toxic species.

This is a modified news release from the University of Texas at Austin.