Masked hearing thresholds of harbour seals (Phoca vitulina) in air

Two wild seals, maintained in captivity in good health, were trained to swim on verbal command to a spot 27 cm above the water, and subsequently to make a L or R turn to a pure tone vs no tone. White noise was present at 80, 70, or 60 db SPL. Masked thresholds were determined by a staircase method in 2-db steps at 2, 4, 8.6, and 15 kc/s (at 15 kc/s only noise at 60 db SPL was presented). Critical bandwidths in db were calculated, all of the order of 25 db (range of to 28 db). These values are close to those reported by Terhune and Ronald (J. Acoust.Soc.Am., 1975, 58, 515-516) for seals' aerial hearing but are unexplainably lower (i.e., worse) than for seals underwater. It was calculated that in their often noisy and visually restricted natural environment, a mother would not be able to hear her pup if separated by more than 8 m. This situation was generally corroborated by field observation on the breeding grounds.