AUSTIN, Texas—Snakes faking dangerous colors to protect themselves from predators can successfully get away with this strategy — but only in areas where truly deadly snakes are found, according to researchers at The University of Texas at Austin and the University of North Carolina at Chapel Hill. The research adds new weight to the evidence for natural selection.
Their report on the attempts of harmless serpents to survive by mimicking deadly varieties has been published in the March 15 issue of the journal Nature. The researchers include Dr. Karin S. Pfennig of UT Austin’s College of Natural Sciences, a National Science Foundation Postdoctoral Fellow in Biological Informatics in the section of integrative biology.
Other researchers include Dr. David W. Pfennig, the lead author of the study and a professor in the department of biology at UNC-Chapel Hill, and William R. Harcombe, a UNC-Chapel Hill biology student.
The researchers focused on predator behavior toward extremely poisonous coral snakes, famous for their ringed markings of red, black and yellow, or red, black and white, along with their harmless imitators, the kingsnakes. The theory of Batesian mimicry holds that edible species that look like dangerous species will be protected, because predators evolve to avoid dangerous species — even without previous, real life, bad dining experiences.
"Mimicry has been used as the preeminent example of how natural selection works and why it works the way it does," said Karin Pfennig, who is married to Dr. David Pfennig. "There is very strong selection for predators to avoid snakes with these kinds of ringed patterns because they look dangerous. This is probably not based on predators having previous experience with coral snakes, because all it takes is one bite and you’re dead. You’re out of the gene pool."
The researchers predicted that the protective effect of looking like a coral snake would break down in areas where the genuinely poisonous coral snakes were absent. Under those circumstances, the process of natural selection would not operate to eliminate heedless predators. There would be no need for color-conscious predators to evolve.
To measure predator attacks, the researchers and their assistants constructed about 1,200 fake snakes composed of non-toxic plasticine threaded onto an S-shaped wire. They used caulk guns to squirt out appropriately sized tricolor snakes, striped snakes and plain brown snakes. The striped and brown snake replicas were intended as controls.
The number of attacks by carnivores was measured in 40 different locations, including 14 sites in Arizona where coral snakes were known to occur and 10 sites where they were known to be absent. Half of the 16 Carolina sites were home to real coral snakes and half were not.
"When a predator attacks one of these plasticine replicas, it leaves tooth marks. It’s a permanent record," Pfennig said. "Animals have distinctive tooth patterns so we were able to come along several weeks later and say what kind of predator it was."
The group primarily focused on the behavior of snake-eating mammalian predators, such as foxes, coyotes, raccoons and even black bear. "We probably didn’t get a lot of hawk predation because we tended to put the replicas in more forested areas," Pfennig said. She said bear attacks were rare, but they did occur both in Arizona and the Carolinas because "bears will eat just about anything."
The researchers found that, as predicted, the number of attacks on the ringed replicas gradually increased with increasing altitude or more northerly location. They said the attacks increased in areas with few or no coral snakes because predators had not evolved to recognize the patterns.
"As you move further north or move higher in altitude, coral snakes become increasingly rare — and attacks on the ringed snakes gradually increase. It’s exciting to find this kind of pattern of natural selection working and to be able to measure the gradual change as elevation or latitude changes," Pfennig said. "The study followed the predictions very closely."
Researchers said the results do not fully explain why color mimicry continues to be found in areas without coral snakes, but they said their study indeed provides experimental evidence to support the prediction that the success or failure of Batesian mimicry depends on the abundance of the creature being imitated.
Asked how to tell coral and kingsnakes apart, Pfennig said: "In North America, if red and yellow rings are next to each other, it’s a coral snake. If not, it’s a king snake." She warned against trying to get close enough to tell the difference. "A coral snake will kill an adult. It shuts down your nervous system," she said. "Coral snakes are in the same family with cobras."
Pfennig was graduated from the University of California at San Diego with a bachelor’s degree in ecology, behavior and evolution. She earned her Ph.D. in biology at the University of Illinois at Urbana-Champaign. In addition to her NSF postdoctoral fellowship at UT Austin, she was a NSF-NATO Postdoctoral Fellow in biological sciences at the University of Bristol in the United Kingdom.
For more information, contact Dr. Karin S. Pfennig at (919) 962-6958 or Caroline Ladhani at the UT Austin College of Natural Sciences at (512) 232-1075. For images, see: <www.utexas.edu/admin/opa/news/01newsreleases/nr_200103/coral2.html>