Binaural hearing in dolphins

Abstract

Binaural hearing is an advantage of having two ears. Human benefits are evident in a 3-dB threshold difference, the ability to localize sound sources in space, and the ability to isolate primary sounds from corresponding echoes. The binaural capabilities of dolphins are relatively unexplored. Studies show that their localization of pure tones underwater is mediated by the same mechanisms observed in terrestrial mammals. Behavioral evidence from free-field localization studies supports reliance on time and intensity cues. Two studies have examined binaural hearing in dolphins using contact hydrophones to isolate the hearing mechanisms. They provided masking level differences (MLDs) comparable to humans and interaural time and intensity difference thresholds that were better than any recorded for terrestrial mammals. Neurophsyiological studies using evoked potentials investigated interaural sensitivity and intensity differences as a function of multiple frequency stimuli presented at various angles around the head of a dolphin. Recent behavioral studies have mapped the acoustic sensitivity around the head and lower jaw. Those results suggest greater sensitivity forward of the panbone, thought to be the site of best reception, and an asymmetry in sensitivity that may be analogous to that observed in other animals such as the barn owl.