Hearing abilities of bottlenose dolphins in Sarasota Bay
By Mandy Cook, PhD and David Mann, PhD, University of South Florida, St. Petersburg, FL

Bottlenose dolphins are exposed to a wide variety of noise in their environment, both naturally-occurring and anthropogenic, and there is concern that these noises may have negative effects on their hearing. Because dolphins rely primarily on acoustics to navigate, forage, and communicate with each other, hearing losses in these animals can be especially damaging. Dolphins can hear from about 75 Hertz to over 150 kiloHertz, which is a much larger hearing range than most other mammals most humans can hear from 20 Hertz to 20 kiloHertz when they are young, but this deteriorates with age).

We measured the hearing thresholds 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), and sensors 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 62 bottlenose dolphins (30 females and 32 males, ages 2-36 years) during capture-release sessions from June 2003 through June 2006. 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.

Support for this research was provided by: Harbor Branch Oceanographic Institution’s Protect Wild Dolphins Program, Dolphin Quest, NOAA Fisheries, the P.E.O. Scholar Award, the Jack Lake Endowed Fellowship, the Paul L. Getting Memorial Endowed Fellowship, the Von Rosenstiel Endowed Fellowship, and USF College of Marine Science Graduate Assistantships.