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

Communication plays an important role in the social lives of many organisms. Nevertheless, communication often takes place under less than ideal conditions. For most frogs, the successful coordination of reproductive behaviour depends upon the ability of females to assess and respond to the acoustic advertisement signals of males. A major problem of communicating in a frog chorus arises when the signals of neighbouring males overlap. 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. All females of one species, H. versicolor, approached a speaker broadcasting synthetic advertisement calls modelled after conspecific calls when the calls of the two species did not overlap. 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 population approached the source of synthetic conspecific calls even when these calls were overlapped by the heterospecific stimuli, indicating there may be differences between these closely related species in the 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 suggest that the effects of signal overlap in choruses on female phonotactic behaviour may be both negative and complex.