Aposematic signals as well as body behaviours may be important anti-predator defences. Species of the genus Melanophryniscus are characterised by having toxic lipophilic alkaloids in the skin and for presenting a red ventral colouration, which can be observed when they perform the behaviour called the unken reflex. Both the reflex behaviour and the colouration pattern are described as defence mechanisms. However, there are currently no studies testing their effectiveness against predators. This study aimed to test experimentally if both ventral conspicuous colouration and the unken reflex in Melanophryniscus cambaraensis function as aposematic signals against visually oriented predators (birds). We simulated the species studied using three different clay toad models as follows: (a) in a normal position with green coloured bodies, (b) in the unken reflex position with green coloured body and extremities and (c) in the unken reflex position with a green body and red extremities. Models were distributed on a known M. cambaraensis breeding site and in the adjacent forest. More than half of the attacks on the models were from birds; however, there was no preference for any model type. Thus, just the presence of the red colour associated with the motionless unken reflex position does not seem to prevent attacks from potential predators. It is possible that the effective aposematic signal in Melanophryniscus is achieved through the unken reflex movement together with the subsequent exhibition of the warning colouration and the secretion of toxins.

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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.

In anuran amphibians, acoustic signals are fundamental mechanisms of mate recognition and mate choice, which makes frog calls a fundamental tool for anuran taxonomy. In this work, we describe the advertisement call of two species for the genus Melanophryniscus, M. cambaraensis and M. macrogranulosus and use the descriptions to try to solve a taxonomic problem between them. We collected data after heavy rains in three different sample sites in Rio Grande do Sul, Brazil, between 2012 and 2013. The advertisement call of both species is composed of two segments. It always begins with part A (about 0.44–6 seconds) composed of single modulated pulses separated by long time intervals. It is followed by part B, a long train of unmodulated pulses with short time intervals, lasting from 9 to 32.2 seconds. Principal Component Analysis (PCA) indicated some variation between temporal parameters of the two species, but Multivariate Analysis of Variance showed no significant differences. Within-individual Coefficient of Variation (CV) showed only two static parameters: pulse rate and peak frequency, both in the part B of the call. Despite intra-male variation in some acoustic parameters, it is not possible to differentiate between M. cambaraensis and M. macrogranulosus species only using bioacoustics.