Low-frequency hearing and masking parameters in representative seals and sea lionsa)

Despite the prevalence of low-frequency natural and human-generated noise, there are relatively few biological data describing hearing and masking in non-human mammals at frequencies below 100 Hz. Information about the auditory capabilities of mammals with high sensitivity to low-frequency sounds is needed to understand and quantify masking effects. In this study, behavioral methods were used to investigate low-frequency underwater hearing in two trained bearded seals (Erignathus barbatus) and a California sea lion (Zalophus californianus) in quiet conditions and in the presence of controlled background noise. The measured auditory parameters provide the most comprehensive low-frequency hearing data yet available for any marine mammal. Results demonstrate that seals and sea lions detect sound frequencies at least as low as 40 Hz. The seals were more sensitive-with thresholds up to 24 dB lower between 100 and 40 Hz-and may be particularly vulnerable to noise effects in this range. Critical ratio measurements and critical bandwidth estimates for these subjects, as well as measures of temporal processing capacities, can be used to evaluate low-frequency auditory masking by anthropogenic and natural noise in the marine environment. This study also offers insight into hearing and the effects of noise in other presumed low-frequency specialists, notably whales.