ABSTRACT

Prior efforts to characterize the capabilities of the vibrissal system in seals have yielded conflicting results. Here, we measured the sensitivity of the vibrissal system of a harbor seal (Phoca vitulina) to directly coupled sinusoidal stimuli delivered by a vibrating plate. A trained seal was tested in a psychophysical paradigm to determine the smallest velocity that was detectable at nine frequencies ranging from 10 to 1000 Hz. The stimulus plate was driven by a vibration shaker and the velocity of the plate at each frequency–amplitude combination was calibrated with a laser vibrometer. To prevent cueing from other sensory stimuli, the seal was fitted with a blindfold and headphones playing broadband masking noise. The seal was sensitive to vibrations across the range of frequencies tested, with best sensitivity of 0.09 mm s⁻¹ at 80 Hz. Velocity thresholds as a function of frequency showed a characteristic U-shaped curve with decreasing sensitivity below 20 Hz and above 250 Hz. To ground-truth the experimental setup, four human subjects were tested in the same paradigm using their thumb to contact the vibrating plate. Threshold measurements for the humans were similar to those of the seal, demonstrating comparable tactile sensitivity for their structurally different mechanoreceptive systems. The thresholds measured for the harbor seal in this study were about 100 times more sensitive than previous in-air measures of vibrissal sensitivity for this species. The results were similar to those reported by others for the detection of waterborne vibrations, but show an extended range of frequency sensitivity.