Donald A. Carder

Demonstration of adaptation in beluga whale echolocation signals

The echolocation signals of the same beluga whale (Delphinapterus leucas) were measured first in San Diego Bay, and later in Kaneohe Bay, Oahu, Hawaii. The ambient noise level in Kaneohe Bay is typically 12-17 dB greater than in San Diego Bay. The whale demonstrated the adaptiveness of its biosonar by shifting to higher frequencies and intensities after it was moved to Kaneohe. In San Diego, the animal emitted echolocation signals with peak frequencies between 40 and 60 kHz, and bandwidths between 15 and 25 kHz. In Kaneohe, the whale shifted its signals approximately an octave higher in frequencies with peak frequencies between 100 and 120 kHz, and bandwidths between 20 and 40 kHz. Signal intensities measured in Kaneohe were up to 18 dB higher than in San Diego. The data collected represent the first quantitative evidence of the adaptive capability of a cetacean biosonar system.

Temporary shift in masked hearing thresholds in odontocetes after exposure to single underwater impulses from a seismic watergun

A behavioral response paradigm was used to measure masked underwater hearing thresholds in a bottlenose dolphin (Tursiops truncatus) and a white whale (Delphinapterus leucas) before and after exposure to single underwater impulsive sounds produced from a seismic watergun. Pre- and postexposure thresholds were compared to determine if a temporary shift in masked hearing thresholds (MTTS), defined as a 6-dB or larger increase in postexposure thresholds, occurred. Hearing thresholds were measured at 0.4, 4, and 30 kHz. MTTSs of 7 and 6 dB were observed in the white whale at 0.4 and 30 kHz, respectively, approximately 2 min following exposure to single impulses with peak pressures of 160 kPa, peak-to-peak pressures of 226 dB re 1 microPa, and total energy fluxes of 186 dB re 1 microPa2 x s. Thresholds returned to within 2 dB of the preexposure value approximately 4 min after exposure. No MTTS was observed in the dolphin at the highest exposure conditions: 207 kPa peak pressure, 228 dB re 1 microPa peak-to-peak pressure, and 188 dB re 1 microPa2 x s total energy flux.

Temporary shift in masked hearing thresholds of bottlenose dolphins, Tursiops truncatus, and white whales, Delphinapterus leucas, after exposure to intense tones

A behavioral response paradigm was used to measure masked underwater hearing thresholds in five bottlenose dolphins and two white whales before and immediately after exposure to intense 1-s tones at 0.4, 3, 10, 20, and 75 kHz. The resulting levels of fatiguing stimuli necessary to induce 6 dB or larger masked temporary threshold shifts (MTTSs) were generally between 192 and 201 dB re: 1 microPa. The exceptions occurred at 75 kHz, where one dolphin exhibited an MTTS after exposure at 182 dB re: 1 microPa and the other dolphin did not show any shift after exposure to maximum levels of 193 dB re: 1 microPa, and at 0.4 kHz, where no subjects exhibited shifts at levels up to 193 dB re: 1 microPa. The shifts occurred most often at frequencies above the fatiguing stimulus. Dolphins began to exhibit altered behavior at levels of 178-193 dB re: 1 microPa and above; white whales displayed altered behavior at 180-196 dB re: 1 microPa and above. At the conclusion of the study all thresholds were at baseline values. These data confirm that cetaceans are susceptible to temporary threshold shifts (TTS) and that small levels of TTS may be fully recovered.

Temporary threshold shift in bottlenose dolphins (Tursiops truncatus) exposed to mid-frequency tones.

A behavioral response paradigm was used to measure hearing thresholds in bottlenose dolphins before and after exposure to 3 kHz tones with sound exposure levels (SELs) from 100 to 203 dB re 1 microPa2 s. Experiments were conducted in a relatively quiet pool with ambient noise levels below 55 dB re 1 microPa2/Hz at frequencies above 1 kHz. Experiments 1 and 2 featured 1-s exposures with hearing tested at 4.5 and 3 kHz, respectively. Experiment 3 featured 2-, 4-, and 8-s exposures with hearing tested at 4.5 kHz. For experiment 2, there were no significant differences between control and exposure sessions. For experiments 1 and 3, exposures with SEL=197 dB re 1 microPa2 s and SEL > or = 195 dB re 1 microPa2 s, respectively, resulted in significantly higher TTS4 than control sessions. For experiment 3 at SEL= 195 dB re 1 microPa2 s, the mean TTS4 was 2.8 dB. These data are consistent with prior studies of TTS in dolphins exposed to pure tones and octave band noise and suggest that a SEL of 195 dB re 1 microPa2 s is a reasonable threshold for the onset of TTS in dolphins and white whales exposed to midfrequency tones.

Hearing deficits measured in some Tursiops truncatus, and discovery of a deaf/mute dolphin

Eight bottlenose dolphins Tursiops truncatus (four male, four female) were trained to respond to 100-ms tones. Three male dolphins (ages 23, 26, and 34) exhibited hearing disability at four higher frequencies-70, 80, 100, and 120 kHz even at 111-135 dB re: 1 microPa. Two females (ages 32 and 35) responded to all frequencies as did a male (age 7) and a female (age 11). One female (age 33) responded to all tones at 80 kHz and below; however, she failed to respond at 100 or 120 kHz. One young female dolphin (age 9) exhibited no perception of sound to behavioral or electrophysiological tests. This young female was not only deaf, but mute. The dolphin was monitored periodically by hydrophone and daily by trainers (by ear in air) for 7 years until she was age 16. The animal never whistled or made echolocation pulses or made burst pulse sounds as other dolphins do.

Nasal Pressure and Sound Production in an Echolocating White Whale, Delphinapterus leucas

At the Jersey conference on Animal Sonar Systems in 1979 we gave strong evidence that dolphins produce sound in the nasal system rather than in the larynx as most mammals do (Ridgway et al. 1980). With electromyography (EMG), we studied the activity of laryngeal muscles and nasal muscles, making comparisons between the two groups of muscles during sound production. Certain muscles of the nasal system were active during all dolphin whistles and click trains while muscles of the larynx were active during respiration but not during sound production. Perhaps more importantly, we measured pressure in the nasal cavities and in the trachea adjacent to the larynx. During sound production, intranasal pressure increased markedly but intratracheal pressure remained unchanged. Subsequently, Amundin and Andersen (1983) replicated the EMG and pressure monitoring aspects of our study in Tursiops and Phocoena.

Auditory filter shapes for the bottlenose dolphin (Tursiops truncatus) and the white whale (Delphinapterus leucas) derived with notched noise

Auditory filter shapes were estimated in two bottlenose dolphins (Tursiops truncatus) and one white whale (Delphinapterus leucas) using a behavioral response paradigm and notched noise. Masked thresholds were measured at 20 and 30 kHz. Masking noise was centered at the test tone and had a bandwidth of 1.5 times the tone frequency. Half-notch width to center frequency ratios were 0, 0.125, 0.25, 0.375, and 0.5. Noise spectral density levels were 90 and 105 dB re: 1 microPa2/Hz. Filter shapes were approximated using a roex(p,r) function; the parameters p and r were found by fitting the integral of the roex(p,r) function to the measured threshold data. Mean equivalent rectangular bandwidths (ERBs) calculated from the filter shapes were 11.8 and 17.1% of the center frequency at 20 and 30 kHz, respectively, for the dolphins and 9.1 and 15.3% of the center frequency at 20 and 30 kHz, respectively, for the white whale. Filter shapes were broader at 30 kHz and 105 dB re: 1 microPa2/Hz masking noise. The results are in general agreement with previous estimates of ERBs in Tursiops obtained with a behavioral response paradigm.

Temporary threshold shift in a bottlenose dolphin (Tursiops truncatus) exposed to intermittent tonesa)

Temporary threshold shift (TTS) was measured in a bottlenose dolphin exposed to a sequence of four 3-kHz tones with durations of 16 s and sound pressure levels (SPLs) of 192 dB re 1 μPa. The tones were separated by 224 s of silence, resulting in duty cycle of approximately 7%. The resulting growth and recovery of TTS were compared to experimentally measured TTS in the same subject exposed to single, continuous tones with similar SPLs. The data confirm the potential for accumulation of TTS across multiple exposures and for recovery of hearing during the quiet intervals between exposures. The degree to which various models could predict the growth of TTS across multiple exposures was also examined.

Growth and recovery of temporary threshold shift at 3 kHz in bottlenose dolphins: Experimental data and mathematical modelsa)

Measurements of temporary threshold shift (TTS) in marine mammals have become important components in developing safe exposure guidelines for animals exposed to intense human-generated underwater noise; however, existing marine mammal TTS data are somewhat limited in that they have typically induced small amounts of TTS. This paper presents experimental data for the growth and recovery of larger amounts of TTS (up to 23 dB) in two bottlenose dolphins (Tursiops truncatus). Exposures consisted of 3-kHz tones with durations from 4 to 128 s and sound pressure levels from 100 to 200 dB re 1 μPa. The resulting TTS data were combined with existing data from two additional dolphins to develop mathematical models for the growth and recovery of TTS. TTS growth was modeled as the product of functions of exposure duration and sound pressure level. TTS recovery was modeled using a double exponential function of the TTS at 4-min post-exposure and the recovery time.

TACTILE SENSITIVITY, SOMATOSENSORY RESPONSES, SKIN VIBRATIONS, AND THE SKIN SURFACE RIDGES OF THE BOTTLE­ NOSE DOLPHIN, TURSIOPS TRUNCATUS

Kramer (1960, 1977) was the first to suggest that dolphins have a compliant skin that enhances their hydrody-namic performance by damping incipient turbulence. Kramer also was the first to develop a synthetic vessel coating based upon dolphin skin. However, his coating contained no mechanism for active vibration or for other adjustments to changing boundary layer conditions. Lang (1966) reviewed the earlier work on dolphin hydrodynamics and evaluated the various theories. Concerning the idea that dolphins might actively change their skin surface to reduce hydrodynamic drag, he stated: “An alternate explanation for low drag with regard to cetaceans is that they actively adjust the flexibility and movement of their skin to damp out the microscopic disturbances in the laminar boundary layer. Betchov showed that the laminar flow might be extended indefinitely by this means.”