The evolution of conspicuousness in frogs: When to signal toxicity?

Many organisms use conspicuous colour patterns to advertise their toxicity or unpalatability, a strategy known as aposematism. Despite the recognized benefits of this anti-predator tactic, not all chemically defended species exhibit warning coloration. Here, we use a comparative approach to investigate which factors predict the evolution of conspicuousness in frogs, a group in which conspicuous coloration and toxicity have evolved multiple times. We extracted colour information from dorsal and ventral photos of 594 frog species for which chemical defence information was available. Our results show that chemically defended and diurnal species have higher internal chromatic contrast, both ventrally and dorsally, than chemically undefended and/or nocturnal species. Among species that are chemically defended, conspicuous coloration is more likely to occur if species are diurnal. Our results also suggest that the evolution of conspicuous colour is more likely to occur in chemically defended prey with smaller body size. We discuss potential explanations for this association and suggest that prey profitability (related to body size) could be an important force driving the macroevolution of warning signals.