The cormorant ear – an adaptation to underwater hearing?

Depending on species, diving birds may spend 2-20 minutes under water during a single foraging dive when they may reach depths ranging from one to several hundred meters. Surprisingly little is known about avian underwater hearing despite the fact that several hundred species dive for food. We do not know how sensitive their underwater hearing is or even if they respond to underwater sound. Consequently, we do not know if anthropogenic sound affects their hearing during dives. To help filling this gap we measured the audiograms of cormorants (Phalacrocorax carbo sinensis) and studied the ear anatomy. Wild-caught fledglings were anesthetized and their auditory brainstem response (ABR) to clicks and tone bursts was measured, first in an anechoic box in air and then in a large water-filled tank with their head and neck submerged 10 cm below the surface. The overall shape of air audiograms was similar to that reported for birds of the same size in air. The bandwidth and slopes of the audiograms were similar in air and water. However, in air the highest sensitivity was found at 1-2 kHz, whereas it was displaced towards lower frequencies under water. These results suggest that cormorants have rather poor in-air hearing compared to similar-sized birds. Their underwater hearing sensitivity, however, is higher than what would have been expected for purely air-adapted ears. A possible reason for the poor in-air sensitivity is the special ear anatomy with the central eardrum shaped as a rigid piston, which is reminiscent of the turtle ear.