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Size correlation of the acoustic vesicle. Relationship of the size of the acoustic vesicle (a.v.) and the wavelength (λ) of the species-specific calling song frequency, both relative to the respective body size (N = 25). Note that the analysis contained only those 25 species that exhibited an acoustic vesicle (see Additional file 2) and for which the carrier frequency of calling songs could be reliable determined based on sound recordings.
