| Summary: | The vocal behavior of male Rana clamitans was studied during the summers of 1974-1976 near Blacksburg, Montgomery Co., Virginia to test the hypothesis that call quality varies among callers and this variation produces different responses in recipient conspecifics.
The studied call repertoire contained: a single-note call, a multiple-note call, and a growl call. The acoustical properties of the single-note calls (most frequently performed call) from 99 males were analyzed. The dominant frequency was negatively correlated (r =-.69 P < .001) with the caller's snout-vent length (SVL). Energy distribution between the call's low (200-450 Hz) and middle (500-1,000 Hz) frequency ranges shifted with the males' SVL. EIR scores (low freq. amplitude/middle freq. amplitude) were positively correlated with SVL (r = .67, P < .001). This is significant because Sachs (1964) found that green frog auditory perception of low-frequency sounds is inhibited if sufficient energy is also present in the middle-frequency range. Thus calls of small males having low EIR scores may be poorly perceived by conspecifics.
Playback of a small male call (EIR score of 0.6) and a large male call (EIR score of 2.3) to males in the field showed that males definitely responded to the EIR<sub>0.6</sub> call, discounting the speculation that small male calls are inhibitory. However, there was still a differential response by males to the two test calls; this provides evidence that the call is more complex than a mere carrier of species-specific characteristics.
When posed with the EIR<sub>0.6</sub> call (small male threat), the response of smaller adult males (SVL≤81 mm) was a strong increase in multiple-note and growl calls over their baseline levels, while larger males ( SVL > 81 mm) simply decreased their rate of single-note calling. To the EIR<sub>2.3</sub> call (large male threat), large males shifted to growl calls and approaches toward the speaker, while small males primarily responded with only a rise in multiple-note calls.
My data suggest a response sequence to the male-male behavioral repertoire of green frogs (single-note calls - multiple-note calls - growl calls - approach - physical contact). Small adult males, especially when presented with a large male test call, were significantly less likely to carry the above sequence to its final stages than were the larger males.
An elaborate apparatus was used to test female response against the EIR<sub>0.6</sub> and EIR<sub>2.3</sub> calls. However, female population density was not sufficient. The experiment should be retested under more ideal conditions as there remains a strong likelihood that reproductive females will be differentially attracted to large calling males rather than to smaller males. === M.S.
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