How impersonating a poisonous frog can stop you from becoming dinner.
For her thesis, Catherine Darst spent a lot of her time carting coolers of thumb-sized frogs and crates of chickens on crowded buses across the Ecuadorian Andes. The work wasn't some complex relocation project but a study of batesian mimicry, a process by which one species of animal adopts the appearance of an unrelated species to gain protection from predators.
Collecting and analysing the study subjects proved to be a challenge for the University of Texas graduate student. “When I first got to Ecuador I barely spoke Spanish. I went where colleagues said the frogs were, but I could not find any,” she recalls. She eventually used tape-recorded frog sounds to convey what she was after to the locals, and they pointed out where the creatures could be found.
Darst focused on two species of frog living in different parts of the Amazonian rainforest: Epipedobates bilinguis in the north and E. parvulus in the south. Both flaunt red-spotted backs but have different-coloured markings on their legs to warn predators of their poisonous skin. A third species, Allobates zaparo, is not poisonous but looks very similar to E. bilinguis or E. parvulus. According to batesian theory, this innocuous frog adopted the markings of the local poisonous frog to trick predators into believing that it too is toxic.
In an area somewhere in the middle of the rainforest, where all three frogs coexist, Darst discovered that A. zaparo took on the markings of E. bilinguis alone. E. bilinguis turned out to be less toxic and less abundant than E. parvulus. So why would A. zaparo opt to mimic this species? The question drove Darst's project, and her findings are published on page 208 of this issue.
Initially, Darst wanted to examine the frogs' predators — but no one could identify what they are. Darst even spent evenings in the rainforest making 200 plasticine frog models hoping that she could identify potential predators from bite marks. But she got no identifiable marks, so she opted for another kind of model — the chicken.
Using a local professor's backyard as a lab, Darst introduced chickens to the various frogs. If a chicken encountered one of the poisonous frogs it would peck it and quickly let the foul morsel go. After about eight encounters the chicken learned not to bother.
If a chicken did this ‘training’ with the more toxic E. parvulus, it would also steer clear of the other two types of frog. But if it learned to avoid E. bilinguis, then it would avoid only those frogs with exactly the same markings, such as the A. zaparo found in the area where all three species of frog coexist. So by mimicking E. bilinguis, A. zaparo maximizes its chances of survival.
“It is like eating shellfish and getting really, really sick. You are likely to avoid all seafood after that. But if you only get a little bit sick, you might just stay away from the specific kind of shellfish that made you sick for a while,” explains Darst.