Stabilizing and directional female choice for male calls in the European green toad

We analysed variation in three temporal (pulse rate, call and intercall duration) and one spectral (fundamental frequency) acoustic properties of the green toad's, Bufo viridis, advertisement call, at different levels, from the single bout of an individual to between populations. Independent of the level, pulse rate and fundamental frequency were less variable than intercall and call durations. The former were classified as static, the latter as dynamic properties of the signal. We analysed the effects of body size and temperature, assumed to represent, respectively, morphological and physiological constraints on the signals. Static properties were under strong constraints, either morphologically (fundamental frequency) or physiologically (pulse rate), whereas no significant effects of size and temperature were observed on dynamic properties. We used two-choice discrimination experiments to investigate females' preferences for call properties. Females preferred extreme to mean values of the dynamic properties, even when these fell above the typical range of variation of the population. In contrast, females had weak preferences for mean values, and for lower than average compared with higher than average values, of static properties. We discuss the hypothesis that static and dynamic call properties may convey different information to females: static properties may be important for species recognition, whereas dynamic properties may be important for mate selection.1998 The Association for the Study of Animal Behaviour