To be or not to be eaten: that is the question. For many animals, the only true threat is their predator. And since communicating with them is impossible, nature has offered prey different strategies. Fleeing is one, of course, but doing so requires time. Camouflage, by disappearing into the background or imitating an inedible element – a branch or a leaf, in what biologists sweetly call the masquerade. Other species choose mimicry: taking on the appearance of another dangerous animal to ward off predators. Some moths pretend to be wasps, while some octopuses pretend to be sea snakes.
Aposematism is undoubtedly one of the most fascinating strategies. Suggested by Alfred Wallace (1823-1913) to Charles Darwin (1809-1882) in their correspondence regarding caterpillars and hornets, it concerns species communicating their toxicity as clearly as possible by sending a signal to predators, typically a bright color. This mode of defense has been adopted almost everywhere in the animal kingdom including among insects, mammals, reptiles, mollusks and amphibians.
For decades, however, its origins had remained an enigma for biologists. For a signal to be effective, it must have been preceded in its evolution by the toxic substance’s presence. Before it appears, the best strategy is therefore camouflage. So far, so good; everyone agreed. But how does a cryptic species become aposematic? “This seems like a mystery,” said Tom Sherratt, a professor of biology at Carleton University in Ottawa, Canada. “The first visible mutants of a cryptic species should be quickly detected by predators and not as easily recognized as toxic.” In essence, killed by mistake, like Polonius by Hamlet, because predators did not have enough time to learn that the colored signal was a warning signal. So how did this property spread?
An intermediary stage
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In an article published on Friday, March 17, in the journal ScienceSherratt’s team outlined a new scenario. Aposematic species may have gone through an intermediary stage in which they possess vivid but mostly hidden signals. Such species exist in nature, like catocala moths, with their cryptic forewings and bright hindwings. Or different amphibians that conceal the bright colors on their belly and show them only when in danger.
To argue their case, Sherratt and his two colleagues, Karl Loeffler-Henry of Carleton University and Changku Kang of Seoul National University, modeled the phenomenon in amphibians. They analyzed 1,106 species (14% of the total), representing 76% of families. They located them on a phylogenetic tree and classified their colored phenotype (cryptic, semi-cryptic, visible). They then established the probability of the different appearance scenarios. The result seems clear: If aposematism has evolved several times in amphibians, “it can only have appeared directly in exceptional cases,” insisted Karl Loeffler-Henry. “It required the intermediary stage of hidden colors.”
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