Behavioral response of treefrogs to low level sound stimuli

Abstract

Acoustic avoidance behavior was demonstrated in a natural population of the neetropical treefrog Eleutherodactylus coqui. An electronically synthesized mimic of the two‐note advertisement call, or a single tone of 100‐ms duration, were both effective in eliciting a suppression in spontaneous calling during the 10 to 250 ms directly following the end of the stimulus. The duration of the call suppression zone following a 100‐ms tone was found to be frequency and level dependent. By adjusting these two parameters, it was possible to determine the threshold for acoustic avoidance at different frequencies in the range of 230 to 3420 Hz. Pooled data from 13 frogs (147 frequency‐intensity pairs) were used to construct a behavioral audiogram for this species. Tones of 605 to 2000 Hz were uniformly above threshold when presented at a level of 60 to 70 dB SPL. Below 665 Hz, threshold dropped at 14 dB per octave to a maximum sensitivity of 41 dB SPL at 230 Hz. Tones of 3420 Hz (approximately the third harmonic of the first note of the advertisement call) failed to elicit a response even at high levels (over 81 dB SPL in one case). In a second type of experiment, 1 to 2 s duration single tone stimuli were presented. These were spaced at the frog's spontaneous call interval (2 to 3 s). In every case (7 animals), the frog redistributed his calls in time such that they fell almost exclusively within the brief time window between tone bursts, thereby avoiding overlap with the tone. The average background noise level at the frog's calling site was 39 dB SPL at 500 Hz, 59 dB SPL at 1000 Hz, and 66 dB SPL at 2000 Hz. Thus the avoidance behavior is observed at stimulus levels barely exceeding the noise floor of the frog's environment.