Passive acoustic monitoring of bullfrog choruses: Spontaneous and evoked changes in group calling activity

Abstract

We developed a multiple-microphone array method for recording temporal and spatial interactions of groups of vocalizing male bullfrogs, and for analyzing how this chorusing behavior is perturbed by playbacks of modified frog calls. Chorusing bullfrogs were recorded over 3 nights (90 min sessions) using an array of ten MEMS microphones distributed along a 20-m sector beside a natural pond. Vocal responses were digitized at 50 kHz using Measurement Computing A-to-D boards and customized software on a Lenovo Thinkpad. Individual frogs were located by time-difference-of-arrival measurements at the array. Baseline chorus activity was recorded for 10-20 min before and after playbacks. Playbacks consisted of digitized exemplars of two natural 5-croak advertisement calls, which were manipulated by adding or subtracting spectral components or by introducing masking noise. Baseline chorus activity featured both alternation of calls, mostly between far neighbors, and overlapping of calls, mostly by near neighbors. Bullfrog evoked vocal responses were modified by playbacks of stimuli with altered spectral components, suggesting that the animals perceived these spectral modifications. The array technique indicated that responses of far neighbors were often more strongly impacted by playbacks than those of near neighbors.