Amphibious hearing in a diving bird, the great cormorant (Phalacrocorax carbo sinensis)

Abstract

Diving birds spend up to several minutes underwater during pursuit-dive foraging. To find and capture prey, like fish and squid, they probably need several senses in addition to vision. Cormorants, very efficient predators of fishes, have unexpectedly low visual acuity underwater. So, underwater hearing may be an important sense, as for other diving animals. We measured auditory thresholds and eardrum vibrations in air and underwater of the great cormorant (Phalacrocorax carbo sinensis). Wild-caught cormorant fledglings were anesthetized, and their auditory brainstem response (ABR) and eardrum vibrations to clicks and tone bursts were measured, first in an anechoic box in air and then in a large water-filled tank, with their head and ears submerged 10 cm below the surface. Both the ABR-response waveshape and latency, as well as the ABR-thresholds, measured in units of sound pressure, were similar in air and water. The best average sound pressure sensitivity was found at 1 kHz, both in air (53 dB re. 20 µPa) and underwater (58 dB re. 20 µPa). When thresholds were compared in units of intensity, however, the sensitivity underwater was higher than in air. Eardrum vibration amplitudes in both media reflected the ABR-threshold curves. These results suggest that cormorants have in-air hearing abilities comparable to similar-sized diving birds, and that their underwater hearing sensitivity is at least as good as their aerial sensitivity. This together with the morphology of the outer ear (collapsible meatus) and middle ear (thickened eardrum), suggest that cormorants may have anatomical and physiological adaptations for amphibious hearing.