In many species of frogs and toads acoustic communication is the most relevant way by which correct syngamy is achieved. In these species females respond phonotactically to the typical stereotyped vocalisations produced by conspecific males. Moreover, many studies have proved that females are able to discriminate among conspecific males and select those uttering the most appealing calls. In the present work we employ the advertisement call of the European green toad Bufo viridis as a model to analyse the role of female choice in the evolution of the acoustic signals. Green toad advertisement calls are described on the basis of four acoustic properties: note and internote duration, pulse-rate and fundamental frequency. Female preferences for these same properties are analysed by means of playback experiments, in which pregnant females have to choose between two synthetic calls that differ in only one of the previously mentioned acoustic properties. The analysis of the within-bout of calling coefficient, of variation permits the distinction between highly stereotyped (pulse-rate and fundamental frequency) and highly variable (note and internote duration) acoustic properties of the advertisement calls. Females exhibit patterns of preferences that result in a stabilising or weakly directional selection on stereotyped properties and in a strong directional selection on highly variable (or dynamic) properties of the calls. The observed relationships between signal variability and the pattern of female preferences support three hypotheses: 1) different acoustic properties of the signals encode different kinds of biologically significant information; 2) since a high stereotypy of the signals (high redundant information) is known to reduce the risk of communication errors, the high stereotypy of some acoustic properties can be seen as an adaptive character connected to their function of species-specific information encoders; 3) female preferences and the different selective pressures acting upon the acoustic properties of the calls can be seen, together with some developmental constraints, as the most relevant forces in the evolution of the green toad communication system.
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