Intensity of female preference quantified through playback setpoints: call frequency versus call rate in midwife toads

We quantify and compare female preference for two different acoustic parameters of the male advertisement call in two species of midwife toads where females select mates based on call characteristics. The parameters compared were: call repetition rate, a highly variable parameter associated with male–male competition, and call dominant frequency, a parameter with low variability related to male size. We also tested whether intensity of female preference was similar when the alternate choices were perceived at high overall intensities (close range) or lower overall intensities (long range). We use ‘Playback setpoints’ a new experimental protocol based on two-speaker playback tests with the female placed in a ferret wheel (treadmill) that keeps her equidistant from the two speakers. The protocol measures preference as a trade-off with perceived call intensity: the ‘a priori’ preferred parameter is not broadcast initially, and the female initially walks towards the speaker emitting the inferior parameter. When the female walks towards this speaker the intensity of the opposite speaker is gradually increased until the female turns around and starts her approach towards the ‘a priori’ preferred parameter. At this point the differences in sound intensity between the speakers are measured, and the difference is considered to be a measure of female preference. Our results show that female preference for call repetition rate is significantly more intense than that for dominant frequency. Similarly, we find that female preference intensity is higher when stimuli are broadcast at higher overall intensity indicating that females are more selective at close range.