As in many other anurans, breeding males of the European green toad (Bufo viridis complex) produce two different types of acoustic signals: the advertisement call and the release call. The advertisement call is a long-range vocalisation that elicits positive phonotactic responses in receptive females and either aggressive or negative phonotactic responses in conspecific males. The release call is a short-range vocalisation and it is produced in two different contexts: by a paired male during his attempts to kick away a competitor; and by a single male when he is clasped by a second individual as a consequence of a fight or of an erroneous mating attempt. In the present work we analyse the release and advertisement calls of 222 individuals recorded from 22 populations distributed over a large portion of the green toad range (from the Italian peninsula, Corsica and Sardinia southward to Morocco and Israel, and eastward to Kyrgyzstan and Kazakhstan). We address the question of the relationships between function and structure of signals and we ask to what extent differences in function might explain the differences in signal structure and their pattern of variation. We compare the two types of calls on the basis of their acoustic properties and on the basis of the variation that these properties show at individual and populational levels. The advertisement and release calls differ markedly with respect to most of their acoustic properties (call duration, intensity, number and duration of pulses, fundamental frequency) and these differences might be often explained as the effect of their different functions. Despite these differences, however, the morpho-physiological constraints (temperature and body size) affect the signal temporal and spectral properties in a similar way, producing qualitatively consistent patterns of variation.
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