Manatees, Bioacoustics and Boats
Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide
Finding the Sound Source
In addition to simply detecting sounds, manatees must be able to locate them. For manatees, as for other animals, the ability to localize sounds is critical to their survival. Unfortunately, the low-frequency sounds of many boats are omnidirectional and therefore, by their nature, difficult to locate. Prior to our studies, wildlife officials relied on anecdotal assumptions that manatees could readily hear as well as locate the sounds of slow-moving boats.
The objectives of the localization investigations were to measure the manatee's perceptual abilities to locate sound sources as a function of signal frequency, noise spectra, duration, projection angle and position to the left or right of the animal's head. To accomplish this, an egocentric or orientation paradigm was used, which required the subject to locate the sound source physically in space. Egocentric paradigms have traditionally relied on the subject to make simple head or body turns to indicate an orientation. For this test, manatees were required to swim to and touch the actual sound source. The test required the subjects to locate the sound source unambiguously relative to their own position in space. Whereas an otocentric (ear-centered) paradigm such as the minimal audible angle is arguably a more acute measure of sound localization, it is designed to measure a subject's ability to recognize a shift in sound location relative to an arbitrary sample reference. In the real world manatees must react by orienting themselves toward or away from the sound. Although there are significant procedural differences between egocentric and otocentric approaches, sound localization measurements using variations of both methods have resulted in remarkably consistent measurements of sound localization in animals (Brown 1994).
To conduct all the directional tests, we lined the back pool with sound-absorbing open-cell foam panels to dampen surface, side and bottom reflections. The subjects were trained to position themselves inside a stationing hoop surrounded by underwater speakers. They were trained to leave the station immediately upon hearing a sound and push on the speaker that projected the sound. The speakers were triggered electronically and rotated periodically so that the manatees could not key in on any speaker artifacts.
Localization was significantly greater at higher frequencies across all conditions. When the signals lasted longer than 200 milliseconds, localization improved, as the manatees had the opportunity to make a slight head movement to scan the sound field. Manatees have fused cervical vertebrae, which restricts quick, sharp, angular head movements; this limited mobility suggests they may require relatively longer reaction times (compared with other marine mammals) to scan and sample the environment. Both manatees were tested using real-world sounds—manatee vocalizations and boat noise, as well as narrow-band signals derived from our wavelet analysis of select manatee vocalizations.
Playback of these wavelets revealed salient features of the manatee vocalizations that are detectable below ambient levels. These "designer" signals derived from the higher harmonic frequency bands in manatee vocalizations are highly directional and easily detected by manatees against the most competitive of acoustic conditions. When it became apparent that manatees may not be able to reliably detect or locate the sounds of boats, we explored sounds that manatees could hear best. We hoped that this information could be applied at some stage to help manatees detect and localize approaching boats (see "In Search of Solutions," below). Stormy and Dundee heard and localized these signals with the same sensitivity and accuracy with which they detected manatee vocalizations.
The manatees demonstrated symmetrical localization abilities, meaning that their hearing accuracy was equivalent from the left and right sides. Sound localization was relatively poor at frequencies below 2,000 hertz but improved significantly with higher frequencies. The best directional sensitivity was at frequencies above 10,000 hertz. Manatee vocalizations and the designer wavelet signals were localized correctly 90 percent of the time, whereas the sounds of idling boats were correctly localized only 55 percent of the time. Ironically, the higher-frequency cavitation sounds produced by faster-moving boats were localized 65 to 75 percent of the time. The results demonstrate that manatees can reliably locate their own vocalizations and the frequency-modulated sounds we created with great accuracy. They can also readily locate the sounds of boat-propeller cavitation (the formation of small vacuums, or bubbles, by a rapidly rotating propeller), but have difficulty detecting low-frequency sounds and the sounds of idling boats.