Abstract

Eight (4 male, 4 female) bottlenose dolphins, Tursiops truncatus, were studied in an acoustic response time task. Three male dolphins (ages 25, 29, and 35) exhibited hearing disability at four higher frequencies -- 70, 80, 100, and 120 kHz even at 120 dB (1 N/M² ) to 144 dB re 1 upa. One female (age 33) showed slight loss above 80kHz, but two females (age 32 and 36) responded to all frequencies as did a male (age 9) and a female (age 13).

One young female dolphin (age 9) exhibited no perception of sound to behavioral or electrophysiological tests. The deaf/mute dolphin was monitored for seven years and never whistled or made echolocation pulses or made burst pulse sounds as other dolphins always do. In our observations of more than 200 bottlenose dolphins, this is the first completely deaf dolphin we have encountered. We observed this animal over a seven year period. Her consistent failure to produce any normal dolphin sounds suggests that deafness occurred before the dolphin was collected from the Mississippi sound at the age of 5 years and probably before or near the time of birth.