How did prey species survive long enough to evolve warning coloration whilst living amongst predators who can better see them but have not yet learned to avoid them?
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Ranitomeya amazonica is a poison dart frog species. There is no explicit sexual dimorphism in this … [+]
Some of the world’s most spectacularly colorful animals are amphibians — especially frogs and salamanders. Many of these colorful animals are toxic or have developed some sort of chemical defense, and they rely on their brilliant colors to warn would-be predators to stay away. This form of advertising by an animal to warn its predators that it is not edible is a phenomenon known as aposematism. But how these animals become so colorful has inspired and mystified scientists for many decades.
“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”, said the co-lead author of the study, behavioral ecologist Changku Kang, an assistant professor at Seoul National University, in a statement. Professor Kang’s research is focused on understanding the evolution of animal coloration as an adaptive trait.
In an effort to solve this age-old riddle, Professor Kang collaborated with Karl Loeffler-Henry, a postdoctoral fellow at Carleton University, and his supervisor, Thomas Sherratt. Dr Loeffler-Henry uses a variety of tools including computer-based simulations to investigate how natural selection influences animal coloration.
To study how colors evolved in amphibians and the evolutionary transitions between these colors states, the research team ranked more than 1,100 species of frogs, salamanders and newts into one of five groups (Figure 1): Animals with bright colors, such as reds, yellows and blues, that make them easily visible were classified as conspicuous (“Con”). Animals at the opposite end of the coloration scale, those that are well camouflaged, were classified as cryptic (“Cry”). The middle three classifications comprise fully conspicuous (“FV”) animals that have cryptic upperparts and bright colors on their entire underbellies, such as fire-bellied toads (genus, Bombina); partially conspicuous (“PV”) animals with somewhat hidden colors in its limbs and other body areas, as seen in the taxonomic family, Hylidae. Often, these PV and FV species have defensive behaviors intended to fully display their conspicuous coloring to potential predators. The fifth group consisted of amphibian species with a conspicuous form and a cryptic form (“Poly”).
Figure 1 | Cry = cryptic; PV = (partially conspicuous venter): cryptic dorsum with conspicuous color … [+]
The use of five categories makes this study strikingly different from other studies of the evolution of animal coloration because in those studies, animals are placed in one of two categories — either conspicuous or camouflaged — which prevents us from understanding of the evolutionary complexities of animal coloration, according to Dr Loeffler-Henry.
Professor Kang and Dr Loeffler-Henry tested nine different models to reconstruct the potential evolutionary trajectories the various amphibians might have followed on their route to aposemitism. These models included when each species first developed coloration as well as toxicity as their defense strategy.
F I G U R E 2 | Diagrammatic representations of the nine models compared and tested. … [+]
Professor Kang and Dr Loeffler-Henry found that developing aposemitism is not a random event: it follows a particular evolutionary pathway. Importantly, they found that instead of evolving directly from a camouflaged state, the path to developing the brilliant colors of aposematism goes through an important transitional state (Figure 2). This transitional state is where a cryptic animal conceals its bright coloration in body parts such as the limbs or the underbelly. Threats from predators trigger a behavioral response from these animals so they would intentionally display their hidden bright colors as a visual warning signal of trouble to come. Next, the animal develops a chemical deterrent, such as warts that taste bad or a toxin that kills. Then these animals develop even brighter colors over more of their bodies.
“Aposematism has evolved independently many times in separate lineages of amphibians”, Dr Loeffler-Henry explained in a statement. “Hidden signals give an answer to how this is happening and unravel a fascinating story of how the evolutionary process took place.”
This study demonstrates the value of testing alternative and overlooked evolutionary strategies, which can advance our understanding of diverse antipredator defense strategies in the natural world. Additionally, these findings demonstrate how the cost of a trait, like being extremely colorful, may be circumvented through the evolution of intermediary phenotypes.
Source:
Karl Loeffler-Henry, Changku Kang and Thomas N. Sherratt (2023). Evolutionary transitions from camouflage to aposematism: Hidden signals play a pivotal role, Science 379(6637):1136-1140 | doi:10.1126/science.ade5156
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