Uddannelses- og Forskningsministeriet - FNU - Comparative Vertebrate Hearing in Air and Underwater

The overall aim of this project was to uncover the basic principles of hearing adaptations in air and under water for secondarily adapted marine vertebrates. This was accomplished using both traditional and newly developed techniques to study the hearing of marine birds and mammals with psychophysical and physiological methods. A major focus has been on marine birds. Psychophysical hearing thresholds was established in air and underwater of the great cormorant (Phalacrocorax carbo). This is the first time that hearing thresholds have been measured in both media of a bird. The underwater thresholds are remarkably low, indicating that cormorants and probably also other marine birds may make use of hearing while foraging under water. This has fundamental importance for our understanding of how marine birds use the underwater soundscape and how they may be affected by anthropogenic noise. Additional physiological measurements as well as anatomical investigations on cormorants both in air and underwater indicate that their ears are adapted for both in-air and underwater hearing. We have also initiated studies of hearing in other species of marine birds. Playback experiments with gentoo (Pygoscelis papua) and rockhopper (Eudyptes sp.) penguins in Odense Zoo indicate that they also are surprisingly sensitive to underwater sounds. Currently, two king penguins (Aptenodytes patagonicus) are trained for psychophysical studies in air. Studies of underwater hearing in penguins are planned through a German collaboration, starting in June, 2018. Another major focus of this project has been to measure underwater hearing thresholds of grey seals (Haliochoerus grypus). Both masked and unmasked underwater hearing thresholds have been measured, currently being supplemented with additional data. This will result in the first audiogram and critical ratio measurements of this species. Two harbour porpoise (Phocoena phocoena) births in the enclosure at the neighbouring Fjord&Bælt made it possible to study the ontogeny of hearing of Odontocetes. The results show that porpoises have acute hearing abilities from birth, which is different from what has been found in land-based mammals. It may be that other marine mammals as well as marine birds also have such precocious hearing abilities. This raises questions on how the hearing system of secondarily water-adapted vertebrates can develop with such rapid pace compared to what is found in terrestrial species. Also, it raises concerns as to whether marine mammals and birds are more vulnerable to human-induced noise during early development than previously thought. The porpoise studies also resulted in measurements on the hearing path in this species. Most sound enter the porpoise through the lower jaw, but it is also sieving in from other areas of the head. Finally, we have also had the opportunity to study human hearing abilities in both air and underwater. Comparing psychophysical data in both media shows that humans have lessened hearing abilities in water as compared to air, as expected. Furthermore, directional hearing experiments indicated that humans have very poor directional hearing under water. Finally, by combining psychophysical hearing tests with FMRI brain scans of blind humans, we could demonstrate a considerable amount of plasticity in the sensory processing within the brain.