Preferences based on spectral differences in acoustic signals in four species of treefrogs (Anura: Hylidae).

Frogs have two inner ear organs, each tuned to a different range of frequencies. Female treefrogs (Hylidae) of three species in which males produce calls with a bimodal spectrum (Hyla chrysoscelis, H. versicolor, H. arenicolor) preferred alternatives with a bimodal spectrum to alternatives with a single high-frequency peak. By contrast, females of H. avivoca, in which males produce calls with a single, high-frequency peak, preferred synthetic calls with a single high-frequency peak to calls with a bimodal spectrum. These results are consistent with the expectations of the matched-filter hypothesis and run counter to the predictions of the pre-existing bias hypothesis. At moderate to high playback levels (85-90 dB), females of H. avivoca and of two of three mtDNA-defined lineages of H. versicolor preferred unimodal signals with a high-frequency peak to those with a low-frequency peak. Females of H. chrysoscelis, H. arenicolor and the third lineage of H. versicolor did not show a preference, indicating that receiver mechanisms may be at least as evolutionarily labile as call structure. Spectral-peak preferences of gray treefrogs from Missouri, USA were intensity-dependent. Whereas females chose low-frequency calls at 65 dB spl, there was either no preference (H. chrysoscelis) or a preference for high-frequency calls (H. versicolor) at 85 and 90 dB spl. These non-linear effects indicate that there is an increasing influence of high-frequency energy on preferences as females approach calling males, and these results serve to emphasize that playback experiments conducted at a single level may have limited generality.