Many amphibians show their potential predators with bright red, orange or yellow: I'm poisonous, don't eat me! But how could such warning colors develop evolutionarily? Why weren't the first such conspicuous individuals killed directly by predators who didn't yet know the warning message of the colors? A study now sheds light on possible mechanisms. Thus, warning colors probably evolved covertly at first, only appearing when an otherwise camouflaged animal intentionally displayed them. Only when predators got to know the colors as a deterrent were individuals with permanent warning clothing able to assert themselves.
In the course of evolution, animals have developed a variety of strategies to protect themselves from predators. Some almost blend into their surroundings, mimicking the color of the ground, leaves, or tree bark. Others, on the other hand, are visible from afar with their eye-catching patterns and bright colors. But predators know intuitively that this means a warning: the animals in question are probably poisonous, defensive or at least inedible. However, how the strategy of warning coloring, also known as aposematism, could develop evolutionarily is considered a scientific paradox. At a time when predators didn't yet understand the meaning of color, the first individuals to develop flashy colors through random mutation would have been at particularly high risk of being eaten. Thus, the characteristic could not actually have asserted itself evolutionarily.
clarification of the paradox
A team led by Karl Loeffler-Henry from Carleton University in Ottawa, Canada has now found a plausible solution to this paradox. "Our study underscores the importance of hidden color signals in the evolutionary processes that lead to diverse coloration in amphibians to deter predators," the researchers said. While previous studies on the evolution of warning colorations have typically divided analyzed species into just two groups - camouflage or warning colors - Loeffler-Henry and his colleagues also considered variants in between.
"Some species are camouflaged when at rest, but have bright color signals on hidden body parts that are only visible in certain situations, such as when fleeing or when they are in a defensive posture," explains the research team. For example, many toads are inconspicuously colored on the upper side, while their belly has warning colors in whole or in part. "These flexible signaling strategies could represent intermediate stages and therefore play a central role in evolutionary processes that produce different defense mechanisms against predators."
Development with intermediate steps
To test this hypothesis, Loeffler-Henry and his team performed phylogenetic analyzes for a total of more than 1,400 amphibian species. They divided these into five groups: In addition to fully camouflaged and conspicuously colored species, they also considered those whose abdomen has warning colors in whole or in part, and species that produce both camouflaged and conspicuous individuals. They also had information on the toxicity of 315 species, so they could also include the extent to which the warning colors actually indicate a danger and in which cases they are just bluff. Using nine different evolutionary models, the research team calculated the most likely path by which aposematism developed and what role toxicity played in this.
The result: "Our analyzes show that the evolutionary transition from camouflage to aposematism does not usually take place directly, but includes an intermediate stage, namely camouflaged species that optionally show a conspicuous coloration," the researchers say. “About 91 percent of species with optional warning coloration have chemical defenses. This suggests that their warning coloring is an honest signal of their defense rather than a bluff.” Predators thus learn to interpret the bright colors as an indication of danger, even before individuals who have abandoned their camouflage entirely in favor of warning colors prevail. Since conspicuously colored individuals are subsequently eaten less, a selection pressure can develop that leads to complete aposematism.
Consider hidden signals
"Macroevolutionary studies of animal coloration should consider these underestimated hidden signals that are widespread throughout the animal kingdom to improve our understanding of the evolution of predator defenses," the authors write. "Indeed, in many groups of animals, such as snakes, fish, and a variety of arthropods, there are species that carry conspicuous signals in hidden places. We therefore encourage follow-up studies in other taxa to assess the generality of the intermediate step hypothesis as a route to aposematism.”
Source: Karl Loeffler-Henry (Carleton University, Ottawa, Canada) et al., Science, doi: 10.1126/science.ade5156