Beyond the river: underlying determinants of population acoustic signal variability in Amazonian direct-developing Allobates (Anura: Dendrobatoidea)

The multidimensional nature of animal signals makes acoustic traits potentially subject to different determinants. The Amazonian frogs Allobates nidicola and Allobates masniger have an allopatric distribution, occurring along the left and right sides of the Madeira River, respectively. These are two sister, phenotypically similar species whose eggs are deposited and develop entirely in a terrestrial nest. In this study, we analyzed 2,000 advertisement calls recorded from ten localities across Central Amazonia, in order to understand the role of determinants of acoustic signal variability at the population and species levels. We assessed, through nested analyses of variance, the differentiation of six characters of this sexual signal among populations and between interfluves. Moreover, we measured the degree of variability and the extent of temperature- and body size-induced plasticity in call traits. We also tested for isolation-by-distance effects in phenotypic differentiation through Mantel tests. Coefficients of variation were higher among than within populations for all call measurements. Spectral call properties were more distinctive than temporal traits among populations and species. Advertisement call traits showed strong temperature-induced plasticity (e.g., 45 % of the variation in note duration). In contrast, the effects of body size were restricted to frequency-related characters. The river barrier effect was significant among all the acoustic variables analyzed even after controlling for male body size. Geography (sampling locality) and body size also jointly affected call variability. No correlation between geographical and acoustic distances among populations was observed, suggesting that local stabilizing selective pressures have an important role in the evolution of call differentiation.