Non-random patterns of acoustic interactions in chorusing bullfrogs.

Abstract

Male bullfrogs form choruses to vocally advertise for females and to announce territory occupation to rival males. In dense choruses, calls of individual males may temporally overlap due to the large numbers of vocalizing neighbors. Overlapping calls may be a deliberate communicative strategy, perhaps to form a more salient auditory object in order to effectively guide females to a particular location within the chorus. We used a custom-written MATLAB program to simulate the proportions of overlapping and nonoverlapping calls in five mock choruses, and then compared model output to empirical data from natural choruses of equal size. The simulation assumed that each bullfrog called independently of his neighbors according to a Poisson process so that overlapping calls occur randomly rather than through cooperation. In four of the five empirical recordings, the number of overlapping calls was significantly smaller than the averages produced by corresponding simulations, indicating that more males vocalized together in bouts than predicted by the random simulation. Empirical and simulated data also differed significantly in the remaining chorus, but in the opposite direction. These data suggest that overlapping calls may confer some communicative advantage. [Work supported by NSF Advance program.]