An experimental assessment of the antipredatory function of green dorsal coloration in poisonous Neotropical red-bellied toads

Crypsis and aposematism are important forms of antipredatory strategies. Through cryptic coloration, animals reduce their detectability by matching the coloration of backgrounds, while through aposematic coloration, prey species signal to potential predators their unprofitability by conspicuous coloration. The efficacy of aposematism depends on a predator’s ability to identify and avoid unprofitable prey. Among amphibians, both strategies are well known for many species. Most species of red-bellied toads, Melanophryniscus spp., present conspicuous coloration with a likely aposematic function, both dorsally and ventrally, and display the behavior known as the unken reflex. However, there are a few species, like M. cambaraensis, that only present ventral conspicuous coloration. The dorsal coloration of this species is dull green, which is usually associated with camouflage. Although this species is diurnal and toxic, the exposition of red ventral coloration does not seem to serve as an aposematic signal. Here, we evaluated experimentally if the green dorsal color of M. cambaraensis functions as warning coloration for visually oriented natural predators. We conducted field predation experiments using clay models, representing M. cambaraensis (green models) and a generalized cryptic frog (brown models), to compare attack rates between treatments. The avian attacks were concentrated mainly on the anterior end, suggesting models were perceived as prey. However, attack rates were similar on green and brown frog models. Our results suggest that the green dorsal color of the species does not act as an aposematic signal, but functions as a cryptic color. Although crypsis in poisonous species remains a complex topic, under certain circumstances, the selection pressures imposed by predators may favor a cryptic coloration instead of a conspicuous coloration, even for diurnal species.