Effects of heterospecific call overlap on the phonotactic behaviour of grey treefrogs

We investigated the effects of overlap between the advertisement calls of two closely related and often-sympatric species of grey treefrogs on female phonotactic behaviour and signal selectivity. The tetraploid H. versicolor and the diploid H. chrysoscelis have advertisement calls consisting of trains of stereotyped pulses that are similar in their spectral composition. All females of H. versicolor from a population sympatric with H. chrysoscelis approached a speaker broadcasting synthetic advertisement calls modelled after conspecific calls when the calls of the two species alternated from speakers with a 90° angular separation with respect to the release point. Surprisingly, the same females almost exclusively approached the source of synthetic calls modelled after the heterospecific advertisement calls when these calls completely overlapped the conspecific stimuli. Female H. chrysoscelis from the same sympatric population approached the source of synthetic conspecific calls even when these calls were overlapped by the heterospecific stimuli, indicating that females in these closely related species may differ in their susceptibility to the negative consequences of call overlap. When female H. versicolor were presented with conspecific advertisement calls that were completely overlapped by those of H. chrysoscelis, many females failed to approach either signal source. Response times of females that did respond were also longer, and the direction of phonotaxis was shifted in the direction of the heterospecific signal source. These negative influences of acoustic interference were reduced or eliminated as the degree of overlap between the calls of the two species was reduced. The results demonstrate that the effects of signal overlap on female phonotactic behaviour may be complex.