Male frogs vocalize calls to attract conspecific females as well as to announce their own territories to other male frogs. In the choruses, acoustic interaction allows the male frogs to alternate their calls with each other. Such call alternation is reported in various species of frogs including Japanese tree frogs (Hyla japonica). During call alternation, both male and female frogs are likely to discriminate calls of the male frogs because of small amount of call overlaps. Here, we show that call alternation is observed in natural choruses of male Japanese tree frogs especially between neighboring pairs. First, we demonstrate that caller positions and call timings can be estimated by a sound-imaging method. Second, the occurrence of call alternation is detected on the basis of statistical tests on phase differences of calls between respective pairs. Although our previous study revealed a global synchronization pattern in natural choruses of the male frogs, local chorus structures were not examined well. Through the observation of call alternation between specific pairs, this study suggests the existence of selective attention in the frog choruses.
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