Origin and evolution of the distress calls in anurans

Past interactions between preys and its predators resulted in the emergence of numerous defensive mechanisms in the animal kingdom. Commonly observed in anurans are distress calls, which are vocalizations emitted while under subjugation that reduces predation risk by apparently surprising or frightening the predator. We expect that the distress call is effective against a wide range of predators. In order to be effective against different predators, distress calls from different species must share common acoustic properties, differently for what is reported for advertisement calls, which is influenced by divergent selective pressures related to species recognition. Hence, our hypothesis is that distress calls are more similar between different species, than advertisement calls. Besides this, distress calls lacks in several unrelated phylogenetic groups or species. Therefore, we don’t know if it has been lost or evolved several times in the Anura tree of life. In order to test our hypothesis and to better describe the evolution of distress calls, we used a combination of literature review, personal communications, and audio database assessment, gathering distress call recordings for 46 anuran species. Based on ancestral character estimation we found that distress calls are an ancestral behavior in anurans, even occurring in ‘mute’ species. Besides that, we found more similarity comparing distress calls among species than when we compared advertisement calls of the same set of species. Therefore, we suggest that distress calls did not undergo divergent selective pressure, it rather could be under convergent pressure or neutral evolution. Finally, we found that body size determines the occurrence of distress calls in anurans, once smaller species lacks such behavior, as they may not produce long or loud enough calls to be effective against predators. Our analyses improve the understanding of the evolution of anuran defensive strategies and may fundament subsequent studies.