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News

Biologists discover that frogs evolved complex color warnings multiple times

University Of Texas At Austin : 01 October, 2003  (Technical Article)
If Kermit were a brightly colored poison frog, the Muppet character might sing a different tune than
“The fact that this happened several times in one little group of frogs means it must be a good evolutionary way of doing this,” said David Cannatella, an assistant professor of integrative biology in the College of Natural Sciences. Cannatella is a co-author for the study, which will be published online the week of Monday, Oct. 6, by the Proceedings of the National Academy of Sciences.

Biologists had thought warning coloration and toxic skin had only evolved once within this family of frogs because of the initial downside to producing the combination (called aposematism) and the difficulty of synthesizing these biochemically complex toxins. A snake or other predator may eat several aposematic prey before associating the coloration and taste, so that early prey lose their lives by standing out visually. Special mechanisms also may be needed to protect Monarch butterflies and other aposematic animals from the harmful toxins they ingest and deposit in their skin.

To determine how common aposematism is, graduate research assistant Juan Carlos Santos, who is lead author of the paper, caught more than 38 frog species in Amazonian and Chocoan tropical forests near Quito, Ecuador, and obtained them from sources in Central America and the Caribbean. At the time, the Ecuadorian native was an undergraduate in the laboratory of Dr. Luis Coloma, the second author of the paper, who is at the Pontifical Catholic University of Ecuador.

Because different tropical frogs can look nearly identical, Santos cataloged where frogs were found, their calls and other behaviors. He also noted what they ate, determining that several South American species in the main family of frogs that was studied dined primarily on specific mites or ants that harbor alkaloid chemicals used to synthesize the toxins. “You see these ants moving around the forest, and you see the colorful frogs chasing them, and the frogs are always there in the same area,” Santos said.

Meanwhile, Santos noted, the less colorful, or cryptic, frogs eat almost anything that fits into their mouths. They also rapidly disappear when humans come near, whereas their colorful counterparts move slower.

“This suggests that the costs to the colorful frogs of acquiring toxins are outweighed by the benefit of being out in the middle of the forest and having predators avoid you,” Santos said.

Details of the evolutionary relationship of aposematic frogs in the family Dendrobatidae were uncovered after Santos began graduate work in Cannatella’s lab in 2002. Santos analyzed genetic material from 38 species in that family and six species from other families that lacked aposematism. In particular, he compared the sequence of three genes that serve as templates for creating molecules called RNA. The genes came from vessels inside the frogs’ cells called mitochondria, whose genetic material is a good marker of evolutionary changes.

In some cases, the researchers found that different groups of highly colorful Dendrobatidae frogs were most closely related to cryptic family members. Using computer simulation, the pattern of shared genetic sequences among the species was analyzed to map out their relatedness on a phylogenetic tree, and the tree compared to that expected if aposematism developed one or more times during evolution. In addition to recurring several times in evolution, the genetic data revealed that millions of years often occurred between some instances of aposematism, although Cannatella noted that the exact gaps are difficult to measure.

“We could clearly show that aposematism evolved four to five times, and that we are pretty confident in those results,” Cannatella said.

Editor’s note: Both Cannatella and Santos, who is a native Spanish speaker, are available for interviews. Images of frog species studied also are available by contacting Barbra Rodriguez at brodriguez@mail.utexas.edu.

This research was supported by grants from the National Science Foundation.
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