Spatially-mediated call pattern recognition and the cocktail party problem in treefrog choruses: can call frequency differences help during signal overlap?

Male gray treefrogs, Hyla versicolor, advertise for mates in dense assemblages characterized by high levels of noise and acoustic clutter. In pairwise interactions, males alternate pulsatile advertisement calls and so reduce call overlap to levels below that expected by chance. However, in choruses consisting of more than two males, acoustic interference increases dramatically. Moreover, males do not seem to exhibit selective attention in a way that reduces call interference among nearest neighbours. Previous research has also demonstrated that although females discriminate strongly against overlapped calls, negative effects of call overlap can be attenuated by a large angular separation between signal sources. However, call stimuli employed were identical in spectrum and so this situation differs from that likely in nature. Based on studies of ‘auditory stream segregation’ with other taxa, we hypothesized that realistic differences in the frequencies of overlapping calls could improve the ability of females to discern critical call features during overlap of separated call sources. We found that, although, under some circumstances, differences in call frequency may help females distinguish among neighbouring males giving temporally proximate calls, naturalistic spectral differences do not seem to help females perceptually separate the overlapping calls of neighbouring conspecific males.