Auditory dynamics during echolocation in freely swimming trained and wild toothed whales

Echolocating animals rely on hearing to forage and navigate while moving through complex acoustic scenes. However, most auditory data from toothed whales come from stationary animals, limiting our understanding of how they receive and process sound during natural biosonar behavior. Here, we used suction-cup biologging tags with external electrodes to record auditory brainstem responses (ABRs) from four freely swimming, trained bottlenose dolphins and a wild harbor porpoise during prey capture. ABRs were measured in response to each individual’s outgoing clicks and returning echoes during target approach. We found that biosonar adjustments reduced ABR amplitudes for outgoing clicks, while amplitudes for returning echoes remained generally constant. Notably, faint echoes elicited response amplitudes similar to those for higher-level self-produced clicks with reduced amplitudes to their own clicks as click rate increased. These findings indicate that auditory responsivity dynamically changes during target approach and suggest an additional gain control mechanism for minimizing sensations of self-generated clicks. Tag-based ABRs close a major gap between stationary lab studies and natural behavior, offering direct insight into sensory reception under real-world conditions. This method is broadly applicable to other toothed whale species and provides a novel tool to study hearing in free-ranging animals.