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Why Are There So Many Colors of Poisonous Frogs?
4 November 2011 2:40 pm
Hopping around in the Peruvian jungle, near the border with Brazil, is a menagerie of tiny poison dart frogs. Their wealth of colors and patterns—some have golden heads atop white-swirled bodies, others wear full-torso tattoos of black and neon-yellow stripes—act as the world's worst advertisement to predators: Don't eat me, I'm toxic. But why have so many designs evolved when a single one might do?
Evolutionary biologist Mathieu Chouteau of the University of Montreal in Canada ventured into the rainforest to find out. He was on the trail of Ranitomeya imitator, a single species of poison dart frog that comes in about 10 different patterns. That variability should be confusing for predators, he says, because the warnings are supposed to be a message to them, and it would make more sense to give them only one design to keep track of.
To figure out what was going on, Chouteau enlisted his girlfriend's help to make 3600 models of frogs, each 18 millimeters long. "It was, like, at least a month of working full-time," he says. They pressed black clay into frog-shaped molds and painted each one in one of two patterns: yellow striped or reticulated, like a giraffe, with green lines. They also made brown frogs as a nontoxic-looking control. Then Chouteau packed the frogs in his carryon baggage and flew to Peru.
The models represent the frogs that live in two different sites: one in the Amazonian lowland and one in a valley at about 500 meters above sea level. The two sites are separated by a high ridge. In one very long day at each site, Chouteau set out 900 of the frogs on leaves along narrow trails used by locals to hunt in the forest. For the next 3 days, he went back and checked them to see whether the soft clay recorded evidence of attacks by birds.
Birds mostly avoided the model that looked like the local frog, but they attacked the model that looked like the frog from the other site, Chouteau reports in the December issue of The American Naturalist. In the high valley, the land of the reticulated frogs, only 7.2% of the model frogs with the reticulated pattern were attacked, whereas 14.2% of the brown models and 26.6% of the yellow-striped models were attacked. The pattern was roughly reversed at the other site.
And that helps explain the diversity of frogs in the rainforest, Chouteau says. Different frog patterns rule at different sites, and birds keep these designs going by weeding out any frogs that deviate from the norm.
"This study shows quite nicely how, once you've got the diversity, it's stabilized," says Chris Jiggins, an evolutionary biologist at the University of Cambridge in the United Kingdom. But where the diversity comes from, he says, is "a bit of an outstanding question." It's possible that frogs with a particular pattern are somehow better suited to the environment where they live, but he thinks the differences more likely arise because of drift. Random changes in pattern get established and then keep evolving, making the frogs distinctive. "What's actually kind of surprising is the birds are really going for these frogs," he says. "You'd think, these frogs are so nasty, you wouldn't go anywhere near a poison dart frog." Maybe the rainforest is so diverse that it's always worth trying something, even something brightly colored.