SSC San Diego TR 1865, August 2001

Approved for public release; distribution is unlimited. (all ONR) for their support over the course of this effort. The authors also wish to thank Linda Green, Deborah Skaar, and Justine Zafran (all SAIC) for their important contributions as trainers on this project. It is a widely accepted hypothesis that echolocating dolphins (Tursiops truncatus) best receive returning acoustic signals over the panbones of the lower jaw. It is also widely assumed that those areas function as the dolphin's peripheral hearing system. Previous research, however, does not exclude the possibility of other sound reception sites and suggests that additional areas of the head may be acoustically sensitive and frequency-dependent. For this report, jawphones were used to behaviorally measure the relative hearing thresholds at four frequencies (10, 30, 60, and 90 kHz) on more than 40 sites of a dolphin's head, from the tip of the rostrum to the base of the pectoral fins. Measurements were converted into iso-sensitivity curves projected onto a two-dimensional, heuristic representation of the head area investigated. The results suggest high-frequency sensitivity along the lower jaw with greater sensitivity forward of the panbone area, sensitivity to low frequency in the area of the external auditory meatus, and an asymmetry in sensitivity favoring the left side of the head. These results may have correlates in the underlying anatomy and suggest that the dolphin's peripheral hearing system is more complex than has previously been assumed. Figures 1. Cutaway views of a typical jawphone. Transducer is embedded in a suction cup made of degassed rubber. The internal void is filled to avoid air spaces. The stems of the jawphones used in this experiment were wrapped in closed-cell neoprene .. 4. Locations tested with a jawphone are represented on the fiberglass model of a dolphin. 5 5. Estimated thresholds measured at 10, 30, 60, and 90 kHz represented in iso-sensitivity curves. Approximate positions of the eye, meatus, and panbone are indicated. They do not faithfully represent the dolphin's morphology. The lower edge of each panel corresponds to the tip of the dolphin's rostrum, the upper edge to the base of the pectoral fins, and the sides to the ventral midline .. 7. Results at 90 kHz indicate best sensitivity along the lower jaw forward of the panbone and the area of sensitivity as being larger on the left side of the head as opposed to Table 1. Threshold values are expressed in peak-to-peak …