In-air and underwater hearing in the great cormorant (Phalacrocorax carbo)

The in-air and underwater hearing abilities of the great cormorant was measured using psychophysical and physiological techniques. The threshold of hearing was established at 3 frequencies in air and 4 frequencies underwater using a go-no go staircase psychophysical paradigm in an outdoor pen. For the in-water trials the bird was trained to put its head beneath the water surface and respond to any audible sound stimuli by emerging and pressing the beak on a response paddle. The ambient noise levels both in the aerial and underwater experiments indicate that the threshold of hearing was not masked at any of the investigated frequencies. The best hearing threshold in air was 45 dB re 20 µPa RMS and in water 78.9 dB re 1 µPa RMS, both at 2 kHz. Auditory brainstem response (ABR) measurements on one anesthetized bird showed an audiogram which shape resembled the one achieved by psychophysics. Laser vibrometry of the tympanum of a newly dead cormorant underwater indicated that the highest sensitivity underwater would have been expected at 1 kHz, i.e. below the frequency of best hearing as given in the psychophysical trials. This study suggests that cormorants have rather poor in-air hearing abilities compared to other similar-sized birds. The hearing abilities in water are better than what would have been expected for a purely in-air adapted ear. [Work funded by the Carlsberg Foundation.]