In Cope's gray treefrog (Hyla chrysoscelis), thresholds for recognizing conspecific calls are lower in temporally modulated noise backgrounds compared with unmodulated noise. The effect of modulated noise on discrimination among different conspecific calls is unknown. In quiet, females prefer calls with relatively more pulses. This study tested the hypotheses that noise impairs selectivity for longer calls and that processes akin to dip listening in modulated noise can ameliorate this impairment. In two-stimulus choice tests, female subjects were allowed to choose between an average-length call and a shorter or longer alternative. Tests were replicated at two signal levels in quiet and in the presence of chorus-shaped noise that was unmodulated, modulated by a sinusoid, or modulated by envelopes resembling natural choruses. When subjects showed a preference, it was always for the relatively longer call. Noise reduced preferences for longer calls, but the magnitude of this reduction was unrelated to whether the noise envelope was modulated or unmodulated. Together, the results are inconsistent with the hypothesis that dip listening improves a female gray treefrog's ability to select longer calls in modulated compared with unmodulated noise.
This work was supported by Grant No. R01 DC 009582 from the National Institutes of Health. The authors thank M. Linck and the Three Rivers Park District for access to study sites, D. Pereira and the Minnesota Department of Natural Resource for permission to collect frogs, N. Lee for feedback on an earlier version of the manuscript, and S. Tekmen and numerous undergraduate research assistants for help collecting and testing frogs.
I. INTRODUCTION A. Pulse-number discrimination B. Dip listening in modulated noise C. The current work II. METHODS A. Subjects B. Apparatus C. Acoustic stimuli 1. Target signals 2. Unmodulated, SAM, and CAM noises D. Behavioral testing 1. Choice tests 2. Control experiment E. Statistical analysis III. RESULTS A. Pulse-number discrimination in quiet and in noise B. Pulse-number discrimination in modulated and unmodulated noise C. Control experiment IV. DISCUSSION V. CONCLUSIONS