Abstract:

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: 1(mu)Pa] 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. Carder, J. Acoust. Soc. Am. 101, 590--594 (1997)]. The data reported in this paper suggest both uni- and bilateral hearing losses in the male which may be the result of age, impairment of the auditory system, or both.