The underwater hearing sensitivity of a two-year-old harbor porpoise was measured using a standard psycho-acoustic technique under very low ambient noise conditions. Auditory sensitivity was measured for narrow-band sweeps with center frequencies between 0.125 and 150 kHz. The audiogram was U-shaped; range of best hearing (within 10 dB of maximum sensitivity) was from 13 to ~140 kHz. Maximum sensitivity (threshold ~39 dB re 1 μPa) occurred at 125 kHz, at the peak frequency of echolocation pulses produced by harbor porpoises. Reduced sensitivity occurred at 32 and 63 kHz. Sensitivity fell by ~10 dB per octave below 16 kHz, and declined sharply above 125 kHz. Apart from this individual’s ca. 10 dB higher sensitivity at 0.250 kHz, ca. 10 dB lower sensitivity at 32 kHz, and ca. 59 dB lower sensitivity at 150 kHz, his audiogram is very similar to that of porpoise ID no. 02 (Kastelein et al., 2010), the subject of many hearing and behavioral response studies. Both animals and porpoise ID no. PpSH047 (Kastelein et al., 2002) have similar hearing sensitivity that is likely to be representative for harbor porpoises in general. Decisions based on information from previous harbor porpoise studies by Kastelein et al. do not need to be modified.
This report was presented at a symposium at Naturalis in Leiden on September 8, 2015. A complete report containing brief abstracts of all studies presented at the symposium can be found here.