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Study of Poisonous Snakes Boosts Old Batesian Principle of Mimicry
David Williamson, University of North Carolina News Service, EurekAlert, 03/15/2001
Chapel Hill -- In 1862, British naturalist Henry Bates proposed -- but could not prove -- that over time, some animal and plant species that taste good to predators come to resemble other animals and plants that pose a danger to the hungry hunters.
By evolving in that way, the good-tasting species develop an effective defense mechanism and are more likely to survive and reproduce.
Although widely accepted and taught as early as elementary school, Batesian mimicry has remained unconfirmed. Now, however, a University of North Carolina at Chapel Hill scientist believes experiments he and others conducted with fake snakes strongly show the Englishman was right. Their report appears in the March 15 issue of the journal Nature. Authors are Dr. David W. Pfennig, associate professor of biology at UNC, undergraduate William R. Horcombe and Dr. Karen S. Pfennig, postdoctoral fellow at the University of Texas at Austin.
"What made Bates' idea so attractive and so unusual for its time was that there was no direct interaction between the dangerous species, which we call the model, and the mimic species," Pfennig said. "Instead, the evolution of mimicry was driven by the predators because if they didn't learn to avoid dangerous prey, they would not survive themselves. The ones that did survive were more likely to leave behind genes causing them to avoid danger."
The question became -- if you've got two species that look like each other -- how do you determine if it's really mimicry? he said.
"Lots of other nice studies of mimicry have been done, but we believe ours is the first to really test this critical prediction, which is that where there's no dangerous model present, then the protection from mimicry should break down," Pfennig said. "If the model that presents a risk is present, then there's very strong selective pressure to avoid the mimic."
In their experiments, he, his wife Karen and his student Horcombe relied on the striking similarity between deadly U.S. coral snakes and scarlet king snakes. Both species bear bright red, yellow and black bands, but the chief visual difference is that in the coral snake, the red and yellow rings touch, while in the scarlet king snake, a black band usually separates the lighter colors.
The researchers created 1,200 life-size models of coral and scarlet king snakes out of plasticine, a mixture of wax and modeling clay and placed the copies in the wild, both within the coral snake's natural range in the southeastern United States and north of that range in central North Carolina, where they are absent. The biologists reasoned that if mimicry were causing the king snake to resemble the coral snake, predators would be more likely to attack the former where coral snakes are not found.
And that's just what their experiments showed. Because the plasticine models retained bite and scratch marks, they became a clear record of how often predators grabbed them.
"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 vs. about 6 percent," Pfennig said. "Various predators readily attacked our model scarlet king snakes but only where no coral snakes lived."
The UNC team also conducted comparable experiments in Arizona with the same results, Pfennig said. Controls were fake "snakes" bearing either stripes rather than rings or plain brown skin. "These are very exciting results because they show how very strong natural selection is even in areas that are not so far away from each other," the scientist said.
Bates made his original observations about mimicry's effects on natural selection on similarly appearing butterflies, some of which were toxic to birds while others were not, he said. "You can get resemblances between species that have nothing to do with mimicry," Pfennig said. "Sharks and dolphins look very much alike, but those forms probably evolved millions of years ago because they were so efficient for swimming. We call that process convergent evolution."
