In 1862, a British naturalist named Henry Bates proposed that some harmless plants and animals evolve the appearance of deadly ones to protect themselves from predators. The Monarch butterfly, for example, is toxic to many birds, but the Viceroy butterflya monarch imitatoris harmless. Yet Bates never had proof for his idea. Now, nearly 140 years later, scientists have found strong support for Batesian mimicry. The new findings appear in today's issue of the journal Nature.

A key prediction of the mimicry theory is that when there is no dangerous "model" present, mimicry should fail to offer protection. To test that, David W. Pfennig of the University of North Carolina and his colleagues designed an experiment around deadly coral snakes and their harmless look-alikes, scarlet king snakes. The team built 1,200 life-size models of the two strikingly similar ringed snake species from plasticine and placed the copies both within the coral snake's natural range in the southeastern U.S. and north of it. If mimicry has shaped the appearance of the scarlet king snake (right), they reasoned, theory predicts that predators will more likely attack the king snakes in places where coral snakes are not found.

The experimental datanamely bite and scratch marks recorded on the plasticine by would-be predatorsbore this out. "Attacks were much more frequent on our ringed models in central North Carolina than they were in southern North Carolina and South Carolina, about 50 percent versus about 6 percent," Pfennig notes. "Various predators readily attacked our model scarlet king snakes, but only where no coral snakes lived." Comparable experiments in Arizona yielded the same findings.

"These are very exciting results," Pfennig says, "because they show how very strong natural selection is even in areas that are not so far away from each other."