Bottlenose dolphins can hear from about 75 Hertz (Hz, or cycles per second) to more than 150,000 Hz, well above the range of human hearing (20-20,000 Hz). Because they are exposed to a wide variety of both naturally occurring and anthropogenic (human-caused) noise in their environment, there is concern that some of the louder noises may have negative effects on their hearing abilities. Hearing loss in these animals can be especially damaging because dolphins rely primarily on sound production and reception to navigate, forage, and communicate with each other.
We measured the hearing abilities of bottlenose dolphins in Sarasota Bay using an auditory evoked potential (AEP) technique, similar to that used to measure hearing in human infants. Short duration tones of varying frequencies and sound levels were played to the dolphins using a jawphone (a speaker in a suction cup attached to the lower jaw of the animal during health assessments), which takes advantage of the lower-jaw sound conduction pathway in these animals. Sensors in suction cups on the surface of the dolphin’s head measured microvolt potentials produced by the brain in response to the tones. The brain’s responses to the sounds were then analyzed to determine each dolphin’s hearing abilities.
Data were collected from four bottlenose dolphins (2 females and 2 males, ages 12-21 years) during May 2016 health assessments. Overall, our findings from conducting hearing tests over nine years show that the bottlenose dolphins in Sarasota Bay do not exhibit increasing hearing loss with increasing age, nor are male dolphins more likely than female dolphins to have a hearing deficit. Also, these dolphins do not exhibit substantial hearing loss due to daily exposure to environmental noise, including anthropogenic sources of noise. Continued testing in the years to come will allow us to assess long-term changes in hearing of individual dolphins.
This article appeared on page 10 of the 2017 SDRP Annual Report, Nicks n Notches.