Ancestral state, phylogenetic signal and convergence among anuran distress calls

Abstract

Past interactions between predators and prey have resulted in the emergence of numerous defensive mechanisms in the animal kingdom. Commonly reported in anurans (frogs and toads) are distress calls, which are vocalizations emitted while under attack that theoretically reduce predation risk by surprising or frightening the predator. Therefore, we would 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, and differ from advertisement calls, which are influenced by divergent selective pressures, safeguarding species recognition. Our hypothesis is that distress calls are more similar among related species than advertisement calls. In addition, distress calls are lacking in several unrelated phylogenetic groups or species. Therefore, it is unclear whether this attribute has been lost or evolved several times in the phylogeny of Anura. In order to test our hypothesis and to evaluate the evolution of distress calls, we used a combination of literature review, personal communications, and audio database assessment. Based on ancestral character estimation, we found that distress calls are an ancestral behavior in anurans, which carry phylogenetic signal. We also found more similarity comparing distress calls among species when we compared advertisement calls of the same group of species, suggesting that distress calls did not face divergent selective pressures. On the contrary, this attribute seems to have experienced convergent selection or neutral evolution. Finally, we found that body size determines the occurrence of distress calls in anurans, with smaller species lacking this behavior. We hypothesize that this may be because small body size may not allow for the production of long or loud enough calls to be effective against predators.