Dynamic properties of the advertisement calls of gray tree frogs: patterns of variability and female choice

Abstract

We assessed the potential for several acoustic properties of the advertisement calls of male gray tree frogs to affect relative mating success by relating patterns of variation in these properties to minimum differences required to elicit female choice. Dynamic properties (pulse number, PN; call rate, CR; and duty cycle, DC, the ratio of call duration to call period) varied much more within bouts of calling than a static property (dominant frequency, DF) but nevertheless exhibited significant between male variation in three of four breeding seasons. Many multiply recorded males consistently produced calls with values substantially above or below mean values of males recorded on the same nights. Nightly ranges of variation in PN and CR were often greater than the minimum differences required to elicit female preferences in the laboratory. In most experiments, females chose high-PN or fast-CR calls over low-PN or slow-CR alternatives, respectively, even if the preferred stimuli were farther away or 6-10 dB lower in sound pressure level (SPL), provided that differences in PN or CR were 100%. Consistent with these results, females did not always choose the closer of two calling males in the field. Nightly ranges of variation in DF rarely equaled the minimum difference required to elicit SPL independent preferences. Females preferred a stimulus of high-PN and slow-CR over an alternative of low-PN or fast-CR with the same acoustic on-time; in two experiments, females chose calls of high-PN over low-PN alternatives even though the playback of the high-PN call was interrupted and the low-PN call was broadcast continuously. Thus, female preferences were not merely based on the total time of acoustic stimulation. Responses of females tested twice in the same experiment suggest that phenotypic variation in preference was limited in our study populations.