We evaluated the hypothesis that adjustments in call duration made by male Gray Treefrogs render their calls less easily masked in noise and so facilitate communication with females in choruses. We also tested whether shifts in male call duration and rate can be elicited through changes in the level of filtered background noise. We found that males increased the number of pulses in their calls while lowering call rate with elevations in noise level in a fashion similar to that reported during broadcasts of calls. In phonotaxis tests with females using unmodulated or modulated background noise and calls (10, 20, 30, or 40 pulses long) presented at either unequal or equal rates, we failed to find significant differences in noise levels at call recognition thresholds for calls of different duration. However, calls were detected more easily (i.e., noise levels at recognition thresholds were higher) when the noise background was modulated as compared to unmodulated. Our results and those of an earlier study by our laboratory indicate that changes in vocal behavior made by males of Hyla versicolor in response to changes in the calling of other males and background noise within choruses likely do not function to lessen the problems of signal detection or degradation due to interference. Accordingly, a small advantage accruing to males because of an inherent, albeit context-dependent, female preference for long calls (even at low call rates) may account for the dynamic calling behavior of male Gray Treefrogs.
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Vol. 47 • No. 2