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

Many organisms use conspicuous color patterns to advertise their toxicity
or unpalatability, a strategy known as aposematism. Despite the recognized
benefits of this antipredator 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 color information from dorsal and ventral
photos of 594 frog species for which chemical defense 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 color 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.