ASA 126th Meeting Denver 1993 October 4-8

4aAB9. High-frequency hearing loss in old (25+ years old) male dolphins.

S. H. Ridgway D. A. Carder

Biosci. Div., Naval Command, Control and Ocean Surveillance Ctr., RDT&E Div., Code 5107B, 49620 Beluga Rd., Rm. 200, San Diego, CA 92152

Eight bottlenose dolphins (four male, four female) were studied in an acoustic response time task [Ridgway et al., J. Acoust. Soc. Am. 89, 1967--1968 (A) (1991)]. The animals were trained to station 1 m below the water surface and respond to a stimulus tone (St) delivered through a hydrophone located 1 m in front of the blowhole. St duration varied from 60 to 450 ms and frequencies were 20, 40, 50, 60, 70, 80, 100, and 120 kHz. St were 120 dB (re: 1 (mu)Pa peak to peak at 1 m) (plus or minus)24 dB in 6-dB steps. With the dolphin at 1-m depth and 1 m from the St hydrophone, the trainer started a randomly variable 3- to 20-St block. The computer selected St from a file in random sequence and interval (1.1 to 2.1 s in 0.1-s steps) and offered St via a St generator. Animal responses (ARs are the whistle or pulse train) were received by another hydrophone, digitized, and stored. Each AR file with 20 to 200 St was edited on a CRT display of a 700 ms St window. No-AR trials, noisy trials, and wrong ARs were identified. Three males (ages 25, 29, and 35) exhibited hearing disability at the four higher frequencies. One female (age 33) showed some loss above 80 kHz, but two females (age 32 and 36) responded to all frequencies as did a male (age 9) and a female (age 13).