How well do acoustic recordings characterize properties of bee (Anthophila) floral sonication vibrations?

Floral sonication is a behaviour in which certain bees generate substrate-borne vibrations by contracting their flight muscles in order to extract pollen from poricidal anthers. Sonication vibrations, or ‘buzzes’, also contain a corresponding audible sound that results from vibrations radiating from the exoskeleton into the surrounding air. Acoustic recordings are often used as a proxy for analysis of floral sonication vibrations because they are more accessible to recording with conventional equipment such as microphones. However, the extent to which salient parameters of buzzes are reflected in the airborne components has not been experimentally verified. We examined correspondence in three properties (duration, frequency and amplitude) simultaneously recorded with acoustic and vibrational methods from freely foraging bumblebees. Duration and frequency are faithfully quantified from airborne recordings; however, two measures of acoustic amplitude (relative peak amplitude and sound pressure level) are not correlated with vibrational amplitude. Our findings validate acoustic recordings as a method to describe temporal and spectral components of floral sonication vibrations, but we caution against using acoustic measures of amplitude as proxies for the true vibrational power.