Hearing Abilities of Bottlenose Dolphins in Sarasota Bay
By Mandy Cook, PhD Student, University of South Florida

Bottlenose dolphins are exposed to a wide variety of natural and anthropogenic noise in their environment, and there is increasing concern that these noises may have negative effects on their hearing. Because dolphins rely heavily on acoustics to both navigate and forage, hearing losses in these animals can be especially detrimental.  As a third-year Ph.D. student at the University of South Florida in St. Petersburg with Dr. David Mann, I have been investigating the hearing abilities of free-ranging bottlenose dolphins. Dolphin hearing ranges from about 75 Hertz to over 150 kiloHertz, with peak sensitivities between 8 and 32 kiloHertz (for reference, most humans can hear from 20 Hertz to 20 kiloHertz). Variations in hearing ability do occur between individual animals, and a few studies on captive dolphins have shown that hearing abilities decrease as a function of increasing age (similar to humans). No study has examined the hearing abilities of free-ranging bottlenose dolphins; therefore, variations in hearing thresholds among individuals and variations with respect to age have not been examined.

We are measuring the hearing thresholds of bottlenose dolphins in Sarasota Bay during temporary capture-release sessions using an auditory brainstem response (ABR) protocol based on techniques used to measure hearing in human infants. Short duration tones of varying frequencies and intensities are played to the dolphins using a jawphone while each animal is on the processing boat. Sensors on the surface of the dolphin’s head measure microvolt potentials produced by the brainstem in response to the tones. The brain’s responses to the sounds are analyzed to determine each dolphin’s hearing threshold.

Data have been collected from 38 bottlenose dolphins (18 females and 20 males) ages 2-26 during capture-release sessions since June 2003. Our findings to date suggest that bottlenose dolphins exhibit a large degree of variability in their hearing abilities. A summary of these data was presented at the 148^th Meeting of the Acoustical Society of America in November, 2004. Future datasets will be collected during capture-release sessions in February and June of 2005. This full dataset will be used to determine if bottlenose dolphins in Sarasota Bay exhibit hearing losses with increasing age or if they exhibit hearing losses due to daily exposure to high levels of environmental noise, including anthropogenic sources of noise.

Support for this research has been provided by: Harbor Branch Oceanographic Institution, Inc. Protect Wild Dolphin Program, Dolphin Quest, 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.