Confusion and human errors in locating synthetic frog calls: Insights into the evolution of a vocal communication signal.

Vocal communication behavior in anurans is energetically expensive and can reveal caller locations, making them vulnerable to predation. Thus, it is likely that calls have been selected to minimize energy and make signals difficult to locate. In earlier work, Jones and Ratnam (2023) suggested that differences in the acoustic receivers of anurans and mammals may be exploited to make anuran calls difficult for mammals to locate, thereby reducing some if not all predation pressure. To test some of these ideas, this study examined sound localization performance in human listeners in response to a synthetic narrowband frog call (pulsed calls of the gray treefrog Dryophytes versicolor, 28% duty-cycle) and its variations, in a dichotic listening task using interaural time differences (ITD) alone. Sounds which were easy to locate (positive control) and difficult to locate (negative control) were also tested. Localization performance in response to synthetic calls (64%) lay between those of positive control (86%) and negative control (32%, chance level). We argue that differences in performance were largely a function of call bandwidth. Among all the variations of calls that were tested (excluding controls), the synthetic call most closely resembling the gray treefrog call had the lowest call energy, the narrowest spectral bandwidth, and was the most difficult to localize. We suggest that calls may have been selected to keep their energy as low as possible by reducing their duty cycle and reducing spectral leakage to maintain narrowband characteristics.