Randall L. Brill

Assessment of dolphin (Tursiops truncatus) auditory sensitivity and hearing loss using jawphones

Devices known as jawphones have previously been used to measure interaural time and intensity discrimination in dolphins. This study introduces their use for measuring hearing sensitivity in dolphins. Auditory thresholds were measured behaviorally against natural background noise for two bottlenose dolphins (Tursiops truncatus); a 14-year-old female and a 33-year-old male. Stimuli were delivered to each ear independently by placing jawphones directly over the pan bone of the dolphins lower jaw, the assumed site of best reception. The shape of the female dolphins auditory functions, including comparison measurements made in the free field, favorably matches that of the accepted standard audiogram for the species. Thresholds previously measured for the male dolphin at 26 years of age indicated a sensitivity difference between the ears of 2-3 dB between 4-10 kHz, which was considered unremarkable at the time. Thresholds for the male dolphin reported in this study suggest a high-frequency loss compared to the standard audiogram. Both of the males ears have lost sensitivity to frequencies above 55 kHz and the right ear is 16-33 dB less sensitive than the left ear over the 10-40 kHz range, suggesting that males of the species may lose sensitivity as a function of age. The results of this study support the use of jawphones for the measurement of dolphin auditory sensitivity.

The effects of attenuating returning echolocation signals at the lower jaw of a dolphin (Tursiops truncatus)

Recordings were made duringan echolocation experiment in order to observe any changes in the parameters of outgoing signals as a function of the deliberate attenuation of returning signals at the lower jaw of a dolphin (Tursiops truncatus). A dolphin was conditioned to perform a discrimination task while wearing either of two rubber hoods covering its lower jaw in addition to performing the same task while wearing no hood. A control hood, made of gasless neoprene, allowed returning acoustic signals to pass to the lower jaw. An experimental hood, made of closed-cell neoprene, significantly attenuated such signals. As previously reported (Brill et al., 1988), the dolphin’s ability to echolocate was significantly hindered while wearing the experimental hood. The acoustical data we recorded during that experiment suggest that the use of the hoods did not affect the dolphin’s ability to emit useful echolocation signals. The dolphin appeared to vary the temporal parameters of its emitted signals in terms of low...

The Jaw-Hearing Dolphin: Preliminary Behavioral and Acoustical Evidence

Two decades ago, Norris (1964, 1968) proposed that the lower jaw of the dolphin was the primary pathway to the tympanoperiotic bone for returning acoustic signals during echolocation. Unlike that of terrestrial mammals, the lower jaw in odontocetes is hollow and filled with a fatty material that extends beyond the pan bones to attach to the tympanic bulla. This material has been found to contain lipids which contribute to its ability to transmit sound (Varanasi and Malins, 1971; 1972). Electrophysiological studies have indicated that acoustical stimuli presented to the lower jaw evoke significant responses in the auditory system of the dolphin (Bullock et al., 1968; McCormick et al., 1970; 1980). Bullock et al. (1968) further reported that foam rubber or paper placed over the lower jaw to block acoustical stimuli significantly attenuated responses. Several investigators have considered the jaw-hearing hypothesis and its possible role, at least in part, in the findings of their acoustical experiments with dolphins as well (Renaud and Popper, 1975; Au, Floyd, and Haun, 1978; Au and Moore, 1984). Other than a limited attempt to hinder a dolphin’s use of its lower jaw which remained inconclusive (Norris, 1974), what has lacked in the evaluation of this theory is behavioral and acoustical evidence gained from a living animal actively echolocating under controlled conditions.

Mapping acoustic sensitivity about the dolphin’s head: A look at the peripheral hearing system

The hypothesis that echolocating dolphins best receive acoustic signals over the pan bones of the lower jaw is widely accepted. Studies in echolocation and hearing have assumed that those areas serve as the dolphin’s peripheral hearing system. The research that established that model, however, does not exclude other potential sound reception sites and suggests that additional areas of the head may be acoustically sensitive and perhaps frequency dependent. Using jawphones, relative hearing thresholds for representative frequencies (10, 30, 60, and 90 kHz) were behaviorally measured at over 40 sites on a dolphin’s head. Iso‐sensitivity curves were constructed and projected onto the image of a dolphin’s head based on these measurements. The results suggest sensitivity to high frequency along the lower jaw with greater sensitivity forward of the pan bone area, sensitivity to low frequency around the external auditory meatus, and an acoustic asymmetry with greater sensitivity favoring the right side of the hea...

Binaural hearing in dolphins

Binaural hearing is an advantage of having two ears. Human benefits are evident in a 3‐dB threshold difference, the ability to localize sound sources in space, and the ability to isolate primary sounds from corresponding echoes. The binaural capabilities of dolphins are relatively unexplored. Studies show that their localization of pure tones underwater is mediated by the same mechanisms observed in terrestrial mammals. Behavioral evidence from free‐field localization studies supports reliance on time and intensity cues. Two studies have examined binaural hearing in dolphins using contact hydrophones to isolate the hearing mechanisms. They provided masking level differences (MLDs) comparable to humans and interaural time and intensity difference thresholds that were better than any recorded for terrestrial mammals. Neurophsyiological studies using evoked potentials investigated interaural sensitivity and intensity differences as a function of multiple frequency stimuli presented at various angles around t...

Evidence of hearing loss in an Atlantic bottlenose dolphin (Tursiopstruncatus)

Auditory thresholds were behaviorally measured for two Atlantic bottlenose dolphins (Tursiops truncatus); a 14‐year‐old female, and a 33‐year‐old male. Stimuli were delivered directly to the lateral sides of the lower jaw via jawphones as opposed to free‐field broadcasts. The female’s audiogram clearly reflects the standard for this species [C. S. Johnson, in Marine Bio‐Acoustics, edited by W. N. Tavolga, pp. 247–260 (1967)]. Previous thresholds for the male measured at age 26 indicated a hearing loss in the left ear of approximately 2 to 3 dB [re: 1μPa] between 4 to 10 kHz, which were considered unremarkable. At age 33, the same male demonstrates distinctive losses. The right ear shows a 16–33‐dB loss over 10–40 kHz, the range of best sensitivity. Above 55 kHz, the right ear is 2–3 dB more sensitive than the left. Both ears then decline to an upper frequency cutoff of approximately 70 kHz below the standard 120 kHz. Hearing losses due to age have been reported for this species [S. H. Ridgway and D. A. Ca...

Evidence for an acoustical pathway to the inner ear through the lower jaw for an echolocating dolphin (Tursiops truncatus)

A dolphin (Tursiops truncatus) was conditioned to perform a discrimination task, by means of echolocation, in a “Go/No‐go” paradigm while stationed in an underwater hoop and wearing suction cups over its eyes. To investigate the hypothesis that the fat‐filled lower jaw of odontocete cetaceans provides an acoustical pathway to the inner ear, the dolphin was additionally required to perform the task while wearing either of two rubber hoods designed to cover its lower jaw. One hood, constructed from nonfoamed neoprene, allowed returning acoustic signals to pass, while the other hood, constructed from closed‐cell neoprene, substantially attenuated such signals. The dolphins performance was significantly hindered while wearing the attenuating hood (p < 0.001, X2) as opposed to its wearing the hood made of nonfoamed neoprene or no hood at all. Acoustical data tape recorded during the experiment indicate that, with few exceptions, the spectral peaks of the dolphins outgoing echolocation signals averaged betwee...