Deep in the South American rainforest lives one of the brightest colored amphibians in the world: the poison dart frog. But don’t take their dazzling appearance as an invitation to come closer. These hues are a warning that they are not worth attacking. Touching the skin of a poison dart frog—among the most toxic animals in the world—can induce nausea, swelling, and paralysis, even in humans.
Predators to these frogs have learned to see bright shades as poisonous or unappetizing. But associating color with danger takes time, leaving evolutionary biologists with a paradoxical problem. How did prey species survive long enough to evolve colors as warning signals while living among predators who can better spot them and had not yet learned to avoid them?
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One theory is that colorful warning signals, also known as aposematism, evolved indirectly and through gradual stages. A study published today in the journal Science suggests some creatures permanently adapted to vivid skins, by first using hidden signals to give predators plenty of time to learn that these colors should be treated with caution. These colors were often concealed in their bellies or undersides.
“If you’re the first conspicuous individual in a chemically defended lineage, it will be very difficult for that mutation to take hold in the population, because predators have no way of knowing your coloration is associated with chemical defense,” says Changku Kang, an assistant professor at Seoul National University in South Korea and study coauthor.
To study how colors evolved in animals, the study authors ranked over 1,000 species of frogs and salamanders into five groups. Most studies on the evolution of animal coloration try to place animals in one of two categories—either conspicuous or camouflaged—which limits the complex understanding of animal coloration, says Karl Loeffler-Henry, a postdoctoral fellow at Carleton University in Canada who served as lead study author.
Instead, this research looked at those two extremes as well as other adaptations animals could have developed. Animals with colors that make them stand out like reds, yellows, and bright blue skins were classified as conspicuous. Animals that developed colors meant to camouflage into the environment were cryptic. Partially conspicuous animals were those with colors that were somewhat hidden in limbs and other body areas. Fully conspicuous critters had bright colors fully tucked away in their underbellies and other hidden regions. Amphibians with both cryptic and conspicuous forms were placed under the polymorphism category.
The biologists then tested nine different evolutionary models to reconstruct the evolutionary pathways their ancestors took, including when they developed both aposematism and toxicity as a defense strategy.
The path to aposematism was not a straight line. Instead of evolving directly from a camouflage strategy, aposematism had an important transitional state. “The conditions favoring the evolution of this type of coloration are likely to be much less restrictive, and yet they provide a clear pathway to the evolution of overall bright coloration,” says Kyle Summers, an evolutionary biologist at East Carolina University who was not involved in the study. When animals first developed vivid pigments, they initially concealed them in body parts such as the limbs or the underbelly.
If threatened, these animals would lift up their limbs or bodies to expose the colors that eventually served as warnings to predators who naively ate these creatures. “Aposematism has evolved independently many times in separate lineages of amphibians,” explains Loeffler-Henry. “Hidden signals give an answer to how this is happening and unravel a fascinating story of how the evolutionary process took place.”
“The study offers a novel solution to the long-standing paradox of evolution of conspicuous antipredator warning signals,” adds Alice Exnerová, an assistant professor of zoology at Charles University in Prague who was also not affiliated with the research. What’s more, she says the findings show the value of exploring alternative and overlooked evolutionary strategies, which can advance our understanding of diverse antipredator defense strategies in the natural world.