Hearing in the sea otter (Enhydra lutris): auditory profiles for an amphibious marine carnivore

In this study we examine the auditory capabili- ties of the sea otter (Enhydra lutris), an amphibious marine mammal that remains virtually unstudied with respect to its sensory biology. We trained an adult male sea otter to perform a psychophysical task in an acoustic chamber and at an underwater apparatus. Aerial and underwater audio- grams were constructed from detection thresholds for nar- rowband signals measured in quiet conditions at frequen- cies from 0.125-40 kHz. Aerial hearing thresholds were also measured in the presence of octave-band masking noise centered at eight signal frequencies (0.25-22.6 kHz) so that critical ratios could be determined. The aerial audiogram of the sea otter resembled that of sea lions and showed a reduction in low-frequency sensitivity relative to terrestrial mustelids. Best sensitivity was −1 dB re 20 µPa at 8 kHz. Under water, hearing sensitivity was significantly reduced when compared to sea lions and other pinniped species, demonstrating that sea otter hearing is primarily adapted to receive airborne sounds. Critical ratios were more than 10 dB higher than those measured for pinni- peds, suggesting that sea otters are less efficient than other marine carnivores at extracting acoustic signals from back- ground noise, especially at frequencies below 2 kHz.