How do tree density and body size influence acoustic signals in Amazonian nurse frogs?

The Acoustic Adaptation Hypothesis (AAH) predicts that acoustic signals emitted at sites with greater vegetation density should have spectral and temporal characteristics that increase signal transmission, but there is a pleiotropism related to body size: large animals produce signals with lower frequency. We used 238 advertisement calls of 34 populations of Amazonian nurse frogs from two Amazonian rainforests with different vegetation density to test if tree density influences the evolution of acoustic parameters. We used PGLS to test for relationships between acoustic traits and phenotypic, environmental and geographic predictors. Spectral and temporal features of calls have an allometric relationship with body size. We found a novel quadratic relationship between note duration and body size. The allometric relationship between dominant frequency and body size and a direct effect of tree density indicates that the evolutionary trajectories of Amazonian nurse frogs follow a general macro-evolutionary pattern as in birds. The temporal features of calls have opposite evolutionary trajectories to those predicted by AAH; frogs from lower tree density environments emit longer notes and have higher note rates than those from denser-tree environments. Subtle differences between Amazonian forest types can drive acoustic diversification of temporal and spectral features of calls at micro-evolutionary scales.