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Abstract

In the present paper we study the pattern of variation in call intensity in a natural population of the European green toad (Bufo viridis), and we analyse females preferences for this property by means of playback experiments. Although call sound pressure level (SPL) shows little within-bout variation, we found significant positive correlation between call SPL and fundamental frequency: on average, an increase of 6 dB SPL produces a 100-Hz increase in fundamental frequency. When females were given a choice between two calls differing by 10 and 6 dB SPL, they significantly preferred the loudest call, whereas they did not discriminate between calls differing by 3 dB. To test whether a 3-dB difference reflects a sensory or behavioural limit, we carried out three experiments where females were given a choice between two calls differing in both duration (4 s and 6 s, respectively) and SPL (3 dB). In all three experiments, the longer call attracted a larger number of females, but the 3-dB difference did not show any significant effect. Finally, we investigated the relationships between call intensity and frequency on female preferences. Previous experiments showed size-dependent preferences for calls with lower-than-average frequencies (1.3 kHz) over calls with higher-than-average frequencies (1.6 kHz). Here, we show that this weak preference is abruptly reversed when the highest-pitched call is broadcast at an intensity 6 dB higher than the alternative.