MASKED AND UNMASKED PURE-TONE DETECTION THRESHOLDS OF A HARBOR SEAL LISTENING IN AIR

In-air pure tone detection thresholds of a harbour seal (Phoca vitulina) were measured using behavioural psychophysical techniques. Thresholds dropped from about 70 dB re 20-mu-Pa at 0.1 kHz to about 35 dB re 20-mu-Pa at 4 kHz and then increased to about 45 dB re 20-mu-Pa at 16 kHz. Increased sensitivities at 2 and 8 kHz, which have been reported in other pinnipeds, were not evident. In-air intensity detection thresholds averaged 32 dB above their underwater counterparts (1-16 kHz). Masking studies found the critical ratios at 0.25, 0.5, and 1 kHz to be 24, 15, and 21 dB, respectively (white noise masker). From 0.2 to 1.5 kHz, bandwidths 20 dB below the level of pure tone maskers were 0.16-0.18 kHz. Circumstantial evidence suggests the possibility that blood vascular changes associated with diving might also influence the sensitivity of the auditory systems of seals. Under optimal conditions, a pup's airborne cries may be detected by its mother at ranges of 1 km or more.