ater-adapted cetaceans, tissue conduction is the primary route of sound conduction to the middle ear. The isolation of the bullae shows an adaptation for tissue conducted sound. The lower jaw contains fat that is closely associated with the impedance of seawater. The lower jawbone of most odontocetes becomes broadened and quite thin posteriorly, and the fat forms an oval shape that closely corresponds to the area of minimum thickness of the jaw. This fat body leads directly to the bulla, producing a sound path to the ear structures located deep within the head. Paired and single air sacs are scattered throughout the skull, which serve to channel these tissue-conducted sounds (Popov & Supin, 1991). Other than this description, there are still more studies needed to determine the function of the middle ear and the type of bone conduction that occurs within the bulla. Due to detailed audiograms, dolphins have been shown to have the ability to detect high-frequency sounds. In an experiment by Johnson (1966) as cited in Schusterman et al. (1986), sine-wave sounds ranging in frequency from 75 Hz to 150 Hz were presented to a bottle-nosed dolphin. The animal was trained to swim in a stationary area within a stall and to watch for a light to come on. Following the light presentation a sound was sometimes presented. If the dolphin heard the sound, its task was to leave the area and push a lever. Sound intensity levels were varied by a staircase method of 1, 2, or 3 dB steps. The resulting audiogram, compared to the human aerial audiogram, showed that at regions of best sensitivity for each, thresholds for human and dolphin are quite similar, but separated by about 50 kHz in frequency, showing that the animals inner ear function is very similar to a human. The experiments done on dolphin auditory functions have generally shown a finely adapted sound reception system. This would be expected due to the highly adapted echolocation ability of the bot...
