Significance of two frequency bands in long distance vocal communication in the green treefrog

Abstract

VOCAL communication is crucial in the reproductive biology of the green treefrog (Hyla cinerea). The male calls at night near a suitable breeding site : the acoustic signal serves for both localisation and species recognition by the female^1–3. A typical mating call has two frequency bands of about the same relative amplitude : the lower one consists of one component of about 1 kHz; the upper one consists of one to four (usually two) components at about 2.7–3.3 kHz (ref. 2). Both frequency bands are pertinent for species discrimination at a moderate sound pressure level (SPL), corresponding to the SPL at 2–4 m from vocalising males^4,5. This report discusses the significance of the two frequency bands in communication over greater distances. Not only does the SPL decrease with distance, but also the relative amplitudes of the two spectral peaks change because excess attenuation is frequency dependent. In general, high frequencies attenuate more rapidly than do low frequencies^6,7. I found that a low frequency component of the call was more attractive to females than were high frequency components. Furthermore, in discrimination experiments females detected attenuation of a 3-kHz component relative to a 0.9-kHz component at moderate to high SPL but failed to do so at low SPL.