Diminution of vocal response in treefrogs due to the presence of masking noise

Abstract

The neotropical treefrog, Eleutherodactylus coqui produces a two‐note advertisement call: the first (Co) note plays a role in spacing of calling males and the second (Qui) note serves to attract conspecific females [P.M. Narins, and R. R. Capranica, J. Comp. Physiol. 127, 1–9 (1978)]. Acoustic playback experiments were performed in which simulated Co notes were presented to isolated calling males in their natural habitat. When presented with a Co note stimulus at an appropriate level, males produced a characteristic one‐note response. This response could be totally suppressed by the simultaneous addition of broad‐band masking noise. The noise levels necessary for total suppression of the one‐note response increased with the stimulus level required to evoke a threshold response. In some cases, wide‐band noise of low to moderate levels significantly increased the one‐note response rate of a particular male. In another experiment, noise of various bandwidths geometrically centered around 1 kHz, and with total noise power held constant, was added to a Co note of fixed level and presented to a calling male. The average one‐note response rate for all males tested was an increasing function of noise bandwidth, for a constant S/N ratio at the animal. Thus the noise energy was more effective at masking when concentrated in a narrow band around the stimulus tone.