Acoustic Playback Experiment Yields Clues on How Dolphins Find Food
By Damon Gannon, PhD

Biologists have known for decades that bottlenose dolphins possess a sophisticated echolocation, or sonar, system. For example, trained dolphins can detect a three-inch metal sphere from a distance of almost 400 feet and can distinguish subtle differences in size, shape, and material. Despite their remarkable sensitivity, dolphins appear to use their echolocation sparingly in the wild.

Why don’t dolphins use their echolocation more often? One potential explanation for this incongruity is that echolocation is a costly behavior for dolphins. Producing the powerful sounds required for echolocation undoubtedly requires a lot of energy. Imagine how tiring it would be to shout at the top of your lungs all day long. But in addition to energetic costs, echolocation may also incur ecological costs by advertising the dolphin's presence to potential prey, predators, or competitors. These ecological costs are measured as reduced food consumption or increased risk of predation.

Besides echolocation, what senses might dolphins use to find food? Vision is not likely to be the primary sense for detecting prey because the visibility in coastal waters is often very low. Even in clear water, fish often conceal themselves within seagrass or other types of shelter. Mote Marine Lab’s Nélio Barros hypothesized that bottlenose dolphins find prey by listening for them. Several studies (including one by Barros and Randall Wells) have noted that most of the fish eaten by bottlenose dolphins are soniferous, or noise-making. Many species of fish communicate with sound to attract mates and to maintain group cohesion. Croakers, drums and grunts are all types of fish named for the sounds they make and they are all important food for bottlenose dolphins.

As part of my Ph.D. research at Duke University, I teamed up with Nélio Barros, Doug Nowacek (Mote Marine Lab), Andy Read (Duke University), Danielle Waples (Duke University), and Randall Wells to test whether the dolphins of Sarasota Bay respond to the calls of the prey species that they eat. We conducted experiments in which we measured their responses to recorded sounds played through an underwater speaker. There were two types of sounds that we played: fish sounds (the experimental treatment) and sounds of snapping shrimp (a common species in Sarasota Bay but one that is not eaten by dolphins; the control treatment). The fish sounds were recorded from 11 different species known to be eaten by the dolphins. Using an overhead digital video camera and a hydrophone, we measured the dolphins’ change in direction and rate of echolocation in response to the sound playbacks. Dolphins responded to fish sounds by turning toward the sound source and increasing their rate of echolocation. The sounds of snapping shrimp elicited neither a directional nor an echolocation response. From our results, we concluded that bottlenose dolphins use passive listening extensively while searching for food. Dolphins produced echolocation signals infrequently, except immediately after we played our fish sounds. Therefore, it appears that one strategy dolphins use to find prey is to use passive listening to make the initial detection, at which point they commence echolocating to track the fish’s precise movements during the pursuit and capture phases of the feeding process. Combining the findings of this project with that of another that I conducted in North Carolina’s Neuse River, it seems that dolphins use passive listening at two distinct scales. First, they appear to choose habitats based on the occurrence of fish calls. Second, once they have positioned themselves in an area where fish are abundant, they locate individual fish by listening for their calls.

Eavesdropping on fish communications can provide a foraging dolphin with much useful information, including the species of the fish, the number of individuals present, their sizes, and their locations. Such judicious use of echolocation suggests that this sensory modality does incur significant energetic or ecological costs. The fact that bottlenose dolphins use passive listening to detect prey indicates that there are risks associated with sound production for fish. Bottlenose dolphins and their prey may be engaged in an evolutionary arms race, similar to that described between bats and some insects, in which improvements in the predator’s ability to detect prey drive improvements in countermeasures employed by the prey, and vice versa.

This work raises further concerns regarding the effects of human-caused noise on the dolphins’ well being. Injuries and deaths of marine mammals caused by intense sounds, such as Navy sonar, have been getting a lot of press lately. But the common sounds that we associate with normal life in a modern world, such as boat traffic, automobile traffic, and construction noise, may have a greater effect on coastal marine mammals in the long run by interfering with their ability to hear and locate their prey. The sounds produced by fish tend to be low in frequency, generally between 200 and 2,000 Hz. This is also the frequency range in which many human-generated sounds occur. When a loud sound overlaps the frequency range of a quieter one, the quiet sound becomes masked, or indistinguishable. Florida’s dolphins inhabit one of the most urbanized coastlines in the world and noise pollution is particularly common here.

Field experiments of this sort are not often attempted with marine mammals because they are extremely difficult. But experimental studies provide much clearer information than do observational ones. This project was made possible by the skill of the SDRP research staff and by the high-quality tools with which they can work. And thanks to SDRP’s long-term research program with the dolphins of Sarasota Bay, we were able to gain much greater insight than would have been possible had we worked in another location. There is no other population of dolphins in the world where the sexes, ages, pedigrees, health statuses, reproductive histories, and association patterns of so many individuals are known.

This research received funding support from the Taylor Foundation, the Oak Fund, the U.S. Environmental Protection Agency, and DBRI. My co-authors and I presented the results of this research at the 15th Biennial Conference on the Biology of Marine Mammals in Greensboro, NC in December. We are also preparing a manuscript for publication in a peer-reviewed scientific journal.