Poison dart frog mimics gain when birds learn to stay away
Catherine Darst and Molly Cummings report their findings 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.
Photo: David Cannatella.
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.
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.