Treefrogs exploit temporal onset synchrony and harmonicity in forming auditory objects of vocal communication signals

Animals frequently communicate in dense social aggregations characterized by the presence of overlapping signals from multiple individuals. Receivers have to perceptually organize these overlapping signals into distinct auditory objects, each corresponding to the perceptual representation of an individual signal. All the signal components produced by the same individual should be integrated into a unitary auditory object while those produced by distinct individuals should be segregated. The principles of auditory object formation and their importance in vocal communication are less understood in non-human animals relative to humans. Here, using American green treefrogs, Hyla cinerea, we tested the hypothesis that receivers exploit the relative timing and harmonic relatedness of multiple spectral components to integrate or segregate sounds during auditory object formation. Using phonotaxis as a behavioral assay, females were given a choice between an attractive conspecific call consisting of three spectral components and a composite call consisting of the same three conspecific spectral components to which we added three spectral components from the call of a heterospecific sister species (Hyla gratiosa). The addition of heterospecific components to conspecific calls renders them less attractive. Across treatments, we manipulated the relative onset timing and harmonic relatedness of the heterospecific components in relation to the conspecific components. We predicted that asynchronous temporal onsets and inharmonic relationships between the conspecific and heterospecific components would promote their perceptual segregation. Our findings are consistent with this prediction. We discuss these findings in the light of parallel perceptual processes across animal taxa and neuroethological theories of auditory processing.