Hearing abilities of bottlenose dolphins in Sarasota Bay
By Mandy Cook, PhD, Portland State University, and David Mann, PhD, University of South Florida

      Bottlenose dolphins can hear from about 75 Hertz (Hz) to over 150 kiloHertz (kHz). (For reference, most humans can hear from 20 Hz to 20 kHz.) Because they are exposed to a wide variety of noise in their environment, both naturally-occurring and man-made (anthropogenic), there is concern that these noises may have negative effects on their hearing. Hearing losses in these animals can be especially damaging because dolphins rely primarily on acoustics to navigate, forage, and communicate with each other, especially in murky estuarine habitats such as Sarasota Bay.

      We measured the hearing abilities of bottlenose dolphins in Sarasota Bay using an auditory evoked potential (AEP) protocol based on techniques 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 embedded in a suction cup and attached to the lower jaw of the animal), 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 10 bottlenose dolphins (5 females and 5 males, ages 2-23 years) during May 2009 health assessments. Our findings suggest that bottlenose dolphins exhibit a large degree of variability in their hearing abilities. Overall, the bottlenose dolphins in Sarasota Bay do not exhibit increasing hearing losses 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 losses due to daily exposure to environmental noise, including anthropogenic sources of noise. There is still unexplained variability in hearing thresholds that is being investigated using the extensive data available on the lives of these dolphins.