Effects of Noise on Bottlenose Dolphin Communication
Communication masking is defined as a reduction in the abilities to both send and receive signals as a result of acoustic interference. Cetaceans rely heavily on acoustic signals for communication, and thus are likely to be greatly impacted by increasing levels of ocean noise. Multiple recent reviews have highlighted the need for field studies using natural stimuli that quantify masking effects; however, the challenges associated with carrying out such studies are substantial.
A well-studied resident population of approximately 160 bottlenose dolphins in waters near Sarasota, Florida, provides the ideal scenario to conduct playback experiments with free swimming dolphins to examine the potential effects of communication masking. Annual health assessments during which animals are briefly captured and released enable a combination of high-quality acoustic recordings, hearing tests, and hand placement of DTAGs (digital acoustic tags). Upon release of animals wearing DTAGs, we will play back to them recordings of natural whistles that are masked by varying amounts of anthropogenic noise. In addition, we will add a cutting edge technology to this proposed work: overhead video recording of playback trials from drones. We will examine movement responses of all animals, as well as all sounds produced before, during and after stimuli presentations. Sounds will be recorded on the tag(s), and we will also independently estimate the environmental noise field using a novel, compact three dimensional directional sound monitor, allowing measurements of sound intensity and directionality. We predict approach and vocal responses to recognizable signature whistles of closely related animals, and reduced responses to whistles that are masked to the point of being unrecognizable (although potentially still detectable). These experiments should allow us to determine the thresholds at which whistles no longer contain communicative value, while accounting for parameters such as individual hearing abilities, age, sex, and behavioral context. The proposed work represents a truly unparalleled opportunity for study of effects of noise on communication in a free-ranging cetacean.