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Hearing and whistling in the deep sea: depth influences whistle spectra but does not attenuate hearing by white whales (Delphinapterus leucas) (Odontoceti, Cetacea)

Sam H. Ridgway^1,*, Donald A. Carder¹, Tricia Kamolnick², Robert R. Smith², Carolyn E. Schlundt² and Wesley R. Elsberry³

¹ Marine Mammal Program, D35, PLBS, 53560 Hull Street, San Diego, CA 92152-5001, USA,
² Science Applications International Corporation, San Diego, CA 92110, USA and
³Marine Acoustics Laboratory, Texas A&M University at Galveston, Galveston, TX 77551, USA

View larger version (162K): [in a new window]   Fig. 1. Radiograph of the nasal and auditory areas of the head of a white whale adult (Delphinapterus leucas) that died of natural causes (not one of the subjects of this report). The path of the Eustachian tube to the nasal cavity is traced on one side. Scale bar, 5 cm.

View larger version (23K): [in a new window]   Fig. 2. A comparison of nasal cavity pressure and hydrophone recording during clicking (A) and whistling (B) for white whale MUK. For this recording, the animal was stationed at a depth of 1 m in San Diego Bay. The upper trace in each part shows pressure rising in the nasal cavity before the animal produces sound. The trace in B illustrates nasal pressure in response to a 500 ms tone. In response to the tone, the whale whistles approximately 400 ms after the suprathreshold tone is presented. The rise in nasal cavity pressure begins approximately 200 ms after the onset of the tone (not shown). Soon (220 ms) after the pressure rise begins, the whistle is recorded. Whistling requires considerably more pressure than clicks of similar amplitude (see also Ridgway and Carder, 1988). Because the click train is longer in duration, the axes are different for the two recordings (A and B).

View larger version (21K): [in a new window]   Fig. 3. Up to 40 tones were presented to the whale during a dive. This example timing scheme shows 15 tones presented together with 12 responses (hits, H) and three misses (M) during the last 86 s of one dive. Together with each test tone, the computer digitized a 2 s sampling window. After the last correct response, the trainer sounded a 500 ms bridge sweeping up in frequency and amplitude, signaling the whale to return to the surface for a reward of several fish. HTP, hearing test platform.

View larger version (41K): [in a new window]   Fig. 4. (A) The boat contained the computer and other equipment together with necessary battery power and cable storage. The hearing test platform (HTP) was suspended by a Kevlar line and by an electrical cable. The cable carried power for lights, video, preamplifier, projector and hydrophone lines. A projector below the HTP delivered tones. A hydrophone in front of the whale station received the projected tones, the whale whistles and background noise. A video camera above the HTP allowed us to observe the whale to ensure correct positioning. (B) An amplitude/time series of a typical whistle employed by each whale as a response to tones it heard. MUK’s whistle lasted approximately 100 ms, while NOC’s whistle lasted 300–400 ms. (C) Typical spectra of whistles (all levels are received levels). The only response from NOC at 300 m was a pulse train, and this was not included in the whistle spectral analysis.

View larger version (122K): [in a new window]   Fig. 5. On a cue, the whale dived from the surface (A) to the hearing test platform (HTP) and stationed (B) to listen for tones from the projector 2 m below the HTP. The whale responded to a series of tones projected at random intervals (see Fig. 3) until the trainer signaled the end of the test dive by sounding a frequency-swept bridge tone (BSt). On hearing the BSt, the whale returned to the surface (C) for a fish reward.

View larger version (15K): [in a new window]   Fig. 6. Drawing illustrating the geometry of the location of the sound projector relative to the whale’s ears and the receiving hydrophone. Note that the projector was 2 m below the platform (HTP), 2.24 m from the whale’s ears and 2.24 m from the receiving hydrophone. The receiving hydrophone was also 2 m from the whale’s blowhole. All values reported here are received levels at the location of the receiving hydrophone or the whale’s ears.

View larger version (22K): [in a new window]   Fig. 7. Spectra of two response whistles that were higher in amplitude than the usual responses seen in Fig. 4C. Note that the 5 m whistle spectra show a higher low-frequency amplitude and much lower high-frequency amplitude than the 300 m whistle.

View larger version (108K): [in a new window]   Fig. 8. Illustration of a sagittal section through the right nasal cavity of a white whale. P, phonic lips; V, vestibular sac; B, blowhole position; F, fatty melon tissue; A, right anterior nasofrontal sac; N, nasal cavity where air is pressurized by muscle action for sound production.

View larger version (95K): [in a new window]   Fig. 9. A dorsal view showing the large vestibular sacs (V) on either side of the blowhole (B). F, fatty melon tissue; M, muscle. The vestibular sacs are important reservoirs holding air for recirculation for sound production, especially during dives. Scale bar, 5 cm.