Bottlenose dolphins can hear at frequencies from about 75 Hertz (Hz) to more than 150,000 Hz, well beyond the range of human hearing (20-20,000 Hz). Because dolphins are exposed to a wide variety of both naturally-occurring and anthropogenic (human-caused) noise in their environment, there is concern that these noises may have negative effects on their hearing abilities. Hearing losses in these animals can be especially problematic because dolphins rely primarily on sound production and reception to navigate, find food, and to 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 auditory evoked potential (AEP) procedures 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). The jawphone makes use of the fact that dolphins hear through their lower jaw, with sounds conducted to the ear through a fat channel
associated with the jaw bone. Sensors in small suction cups placed on the dolphin’s head measured tiny electrical signals 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 5 females and 3 males, ages 3-31 years, during May’s health assessments. Overall, our findings show that 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.
In addition, this year we measured the “auditory oddball” response in a subset of dolphins, in collaboration with Dr. Peter Tyack. This test evaluates an individual’s ability to differentiate between two different sounds—one common, repeated sound, and one novel, rarely-presented sound. This paradigm has been used with human infants to study discrimination of phonetic sounds before they begin to speak. In dolphins, we hope to use the technique to learn about their ability to discriminate signature whistles. The Sarasota dolphin population is ideal for this study because of the long-term recordings of signature whistles from known individuals.
This article appeared on page 11 in the December 2015 issue of Nicks n Notches.