The effect of call amplitude on male spacing in choruses of barking treefrogs, Hyla gratiosa

The amplitude of signals has been shown to play an important role in regulating spacing in aggregations of acoustically signalling insects and amphibians. This conclusion is based primarily on experiments in which calls are played back to resident males, thereby mimicking the signals of arriving males. However, such experiments investigate only one side of the interaction between resident and arriving males. We examined the effect of the amplitude of resident calls on the spacing of arriving male barking treefrogs, Hyla gratiosa, by placing two speakers, one playing advertisement calls 6 dB more intense than the other, at the chorus site and beginning playback before males arrived at the chorus. Distances between the speaker and the nearest male were greater for the speaker broadcasting calls at the higher amplitude, but this effect occurred only in low-density choruses. These results, combined with theoretical considerations, raise the possibility that the immediate benefit to males of producing high-amplitude signals may differ at different chorus densities.