Microclimate and Stream Noise Predict Geographic Divergence in the Auditory Signal of a Threatened Poison Frog

According to the acoustic adaptation hypothesis, abiotic noise generated by streams should promote the evolution of high‐pitched signals in acoustically communicating animals. The putatively selective effect is best demonstrated by studying trait–environment correlations across recently or currently diverging lineages, and when we understand the ecological interactions that underlie the selective pressures. We studied microclimate, frog distribution, abiotic noise, and the frequency of the advertisement call of the Andean poison frog Andinobates bombetes across variable environmental clines at five localities. Our general hypothesis involves three steps: microclimatic clines determine frogs’ distribution, which in turn is associated with different levels of abiotic noise that select for differentially pitched calls. Microclimate changed more abruptly with distance to streams in comparatively dry localities. There, frogs were more aggregated alongside streams and, in turn, experienced higher levels of abiotic noise. As predicted, frogs living alongside streams in dry forests called at higher frequencies, and thus attained comparable signal‐to‐noise ratios, compared with stream‐away frogs. Our data support a role for microclimate in determining frog distribution and thereby the noise conditions that promoted a divergence in a mate‐recognition signal.