Atmospheric and underwater propagation of bullfrog vocalisations

Male bullfrogs vocalize while partially submerged in shallow freshwater ponds. This imposes two potential propagation pathways, atmospheric and underwater, on transmission of their communication sounds. Propagation of pure tones, amplitude modulated (AM) broadband noise and natural calls was measured in air and underwater at three bullfrog breeding sites. In air, propagation losses were consistent with spherical spreading. No excess attenuation was observed for any tone frequency at any site. Both temporal envelope modulations and spectral cues are available to conspecific receivers at biologically realistic distances. The bullfrog's advertisement call is thus well adapted for transmission in air at the air/water interface. Underwater signal propagation differed at the three sites, consistent with substrate effects. Tone propagation showed the high-pass frequency window characteristic of shallow water. Broadband signals underwent propagation losses greater than expected by cylindrical spreading. Modulations of the envelope of natural calls remained discernible at distances where frequency-dependent propagation losses distorted the shape of the spectrum. Measurements of the propagation of the advertisement call emitted by a chorusing frog at the air/water interface confirm that periodicity cues embedded in the envelope are available to receivers both in air and underwater. High frequency cues available underwater overlap the maximal hearing sensitivity of larval conspecifics (tadpoles).