| Summary: | The evolution of whales into fully aquatic, deep-diving animals, which began over 50 million years ago, entailed a dramatic change in their sensory environment and a corresponding reorganization of their sensory systems. For hearing, this had two major consequences. (1) The physics of sound transmission from water into the inner ear required a complete redesign of the sound-transmitting structures (outer and middle ears) of their terrestrial ancestors, which were exquisitely tuned for hearing in air. (2) In the dark deep-sea environment, hearing overtook vision as the primary sense for object detection and localization at a distance. Toothed whales (Odontoceti) are crucially dependent on hearing as they use active high-frequency sounding (echolocation) to localize prey and predators and to communicate with conspecifics. The present thesis addresses aspects of both sound transmission (1) and sound localization (2) in odontocetes. The first aim was to elucidate the mechanical functioning of the middle ear as part of the transmission chain from sound-receiving head structures to the inner ear. The odontocete tympano-periotic complex (TPC) differs significantly from temporal bone complexes of terrestrial mammals. We studied 32 pairs of formaldehyde-glutaraldehyde-fixed TPCs of the short-finned pilot whale Globicephala macrorhynchus. The distribution of vibration amplitudes on the TPC was measured by Laser Doppler Vibrometry while vibrations at different frequencies were applied at a point near the exit of the acoustic nerve. The results suggest that the tympanic plate acts as a lever amplifying the force driving high-frequency vibrations (> 12 kHz) into the inner ear through the ossicular chain. The second aim was to assess whether there exists asymmetry between the left and right middle ears with respect to critical ossicular parameters, and whether this may help in localizing targets in the vertical direction. The malleus, incus and stapes on both sides were weighed with μg accuracy and their lengths as well as ...
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