Müllerian mimicry is a classic example of evolution by natural selection: When two or more foul-tasting species share the same habitat, they tend to warn potential predators using the same colors. The phenomenon makes sense; because it's easier to learn about danger from a single warning system (the reason why traffic lights are the same colors the world over), distasteful organisms share the cost of "educating" their predators. But strangely, nobody had ever bothered to prove that they actually benefit from looking the same. Now, müllerian mimicry has passed its first real-world test with flying colors.
In the rain forests of Ecuador, ecologist Durrell Kapan from the University of British Colombia in Vancouver studied three very similar Heliconius butterflies that tend to live in separate areas. While still caterpillars, these insects incorporate noxious chemicals from the plants they eat, making them unpalatable to birds even after they metamorphose into butterflies. One of the species, Heliconius sapho, has white markings on its wings; another, H. eleuchia, has yellow markings, while the third, H. cydno, comes in two forms: with yellow or with white wing-bands. Kapan captured 117 cydno butterflies, wrote numbers on their wings, then released them in places where either sapho or eleuchia were abundant.
Stalking the butterflies in camouflage gear, Kapan tracked them for up to 2 weeks after their release. He found that yellow cydno were indeed better protected in areas where the local birds were used to avoiding the similarly colored eleuchia. Likewise, white cydno survived longer where it enjoyed protection from the white sapho. Birds even went to great lengths to avoid the butterflies. "One time, I saw a flycatcher make a complete loop around a Heliconius cydno to eat a moth," says Kapan, who is now at the University of Puerto Rico at Rio Piedras. In contrast, looking different was dangerous. Survival rates of the cydno butterflies that stood out from their neighbors fell by as much as 64%, Kapan reports in the 19 January issue of Nature.
"I think it is a great study," says evolutionary biologist James Mallet, who studies Heliconius mimicry at University College London. "Mimicry theory seems plausible, but experiments to show its existence in nature have been rarer than hen's teeth."