Spatial release from masking of aerial tones in a California sea lion ( Zalophys   californianus )

Spatial release from masking (SRM) occurs when a signal and masker are spatially separated, resulting in improvement of signal detection relative to when they are spatially co-located. Sea lions forage in the water, breed on land, produce airborne vocalizations that are associated with social and reproductive activities, and possess highly reduced pinnae. In this study, SRM was measured at 1, 8, and 16 kHz in a California sea lion who had to detect an aerial tone in the presence of an octave band of white noise centered at the tone frequency. Testing was conducted in a hemi-anechoic chamber. While the masker always occurred in front of the subject (0 deg), the tone occurred at 0, 45, or 90 deg in the horizontal plane. Absolute thresholds were also measured at these angles to account for differences in hearing sensitivity based on source azimuth. Masked thresholds were lower by as much as 12 dB at 1 kHz when the signal and masker were separated by 90 deg. These results were compared with those of a harbor seal who, like all true seals, naturally lacks pinnae. Performance differences between the two subjects at the highest frequency likely reflect variations in pinna anatomy.