Evolution of the highest sound pressure levels in vertebrate communication calls

Abstract

The loudest communication calls produced by vertebrates have been proposed to have a physical limit. However, this idea is at odds with the implicit assumption that loudness is driven by body size, where each section of an acoustic system (e.g., vocal tract, pharyngeal space, lungs) increases in size as an animal gets larger. A physical limit to loudness may exist for animals with different acoustic systems, but this has not been explored using comparative analysis. We compare the loudest communication calls across 80 vertebrate species and show the assertion of a physical limit to loudness exists for animals with particular acoustic systems but breaks down where others scale with body mass. We show that for vertebrates with air sacs, loudness scales with body mass but it approaches a physical limit. We also show that air sac location can affect loudness, where louder calls are correlated with air sacs that filter (after the sound source) as opposed to air sacs that increase air volume (before the sound source). We examine ecological and evolutionary pressures which may have led to these modifications and their influence on vocal signaling.