Auditory brainstem response hearing measurements in free‐ranging bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) rely on sound for communication, navigation, and foraging. Both natural and anthropogenic noise in the marine environment could mask the ability of wild dolphins to detect sounds, and chronic noise exposure could cause permanent hearing loss. The hearing abilities of a wild population of bottlenose dolphins in Sarasota Bay, FL are being investigated to determine whether they suffer hearing losses in comparison to animals living in quieter environments. This study is the first to measure the hearing sensitivity of a large population of wild dolphins that are exposed to significant levels of noise. Data on hearing sensitivities at frequencies used for acoustic communication (5–20 kHz) and echolocation (20–100 kHz) are reported. Hearing sensitivity was measured in the field using the non‐invasive auditory brainstem response (ABR) procedure. ABR responses were evoked by the presentation of amplitude‐modulated (AM) tones (carrier frequencies of 5, 10, 20, 30, 60, and 80 kHz) through a jawphone. The tones were modulated at 600 Hz, which elicited a robust envelope following response. A rapid ABR procedure was employed so that an entire audiogram could be obtained in approximately 30 min. This study also provides baseline data for longitudinal hearing studies in known individuals.