Manatees, Bioacoustics and Boats
Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide
The Manatee Hearing Test
In 1991 we initiated experiments with two captive-born manatees, Stormy and Dundee, at the Lowry Park Zoo in Tampa. Our first objective was to define an audiogram—that is, to map the absolute hearing abilities of these subjects under very quiet conditions. The audiogram or hearing curve is a graph that demonstrates the overall range of frequencies an individual can hear as well as the subject's sensitivity within this range. An audiogram plots the intensity of a signal at its minimal detection threshold. The resultant plot for most mammals is U-shaped, with the lowest thresholds depicting the greatest sensitivity. The highest thresholds (areas of least sensitivity) are found at the low and high ends of the frequency range, where greater intensity or volume is necessary to reach detection thresholds.
Before we began testing manatees' hearing and making acoustic measurements of their habitats and boat noise, most of the wildlife biologists and managers charged with protecting manatees assumed the animals could readily hear boats but were just too slow or not smart enough to learn to avoid watercraft. Earlier electrophysiological measurements conducted by Ted Bullock, Tom O'Shea and John McClune in 1982 and anatomical measurements of dead manatees reported in 1992 by Darlene Ketten, Dan Odell and Darryl Domning had suggested that manatees heard best at low frequencies, in the 1,000- to 5,000-hertz range, and therefore could readily detect the sounds of boats. However, since hearing is a perceptual phenomenon, the most accurate way to find out what an animal can truly hear is to ask it. Hence the behavioral audiogram is recognized as the definitive measurement of hearing.
These tests required tremendous commitments of time, patience and resources. Perhaps the greatest challenges lay in training manatees to understand the task and then keeping each of them motivated throughout sessions so that they could eventually complete the thousands of trials necessary to define their hearing. It took approximately one year to prepare both subjects for the tests—and thousands of monkey chow biscuits, along with a great deal of imagination and luck, to keep them interested throughout the subsequent years of testing. The demands of training during the day and testing at night required that Laura and I literally live in the zoo. Aside from the new discoveries and significant research findings, the most remarkable result of all is that after more than five years of living in a 19-foot trailer, without a working bathroom, in the back of the zoo, we are still married.
Being the first to train manatees for psychoacoustic testing, we didn't know their overall visual acuity, nor did we know which modality or weighted combination of modalities manatees might rely on the most. Therefore, I constructed the hearing test using a forced two-choice paradigm with two response paddles that were distinctly different both visually and tactually—one paddle was smooth, with a striped black-and-white pattern, the other solid white with a rough surface and a distinctly different-shaped end made of intersecting pipe sections. Both manatees were trained to position themselves inside a listening station (a hoop) where an underwater microphone, or hydrophone, recorded the signals sent to them. They were to stay in the hoop, listen and wait for a strobe light to flash. After the light flashed they could leave the hoop and select the striped paddle if they heard a sound ("yes"), or the solid white one if they did not detect a sound ("no"). These tests were repeated for many different types of sounds, including boat noise against various sound levels typical of wild ambient conditions.
We used a conventional staircase method of double-blind signal presentations, starting with very loud acoustic levels (at which the manatee would choose the "tone" paddle), stepping down the signal amplitude until the animal chose the "no tone" paddle and then stepping it back up again. Hundreds of trials were required to establish the threshold for each frequency point along the curve. The resulting audiograms for the two manatees were very similar.
Stormy and Dundee proved to be excellent test subjects. Their hearing may also be better than most manatees', since they are young, captive-born animals who have spent their lives in relatively quiet environments with minimal risk of hearing damage from continuous exposure to high noise levels. The ambient noise levels in captivity are significantly quieter than those recorded in the wild. Furthermore, these individuals were highly motivated and conditioned to listen for the slightest changes in the sound field. Wild manatees might not be expected to be as focused and attentive to acoustic subtleties as finely as our subjects were specifically trained to do. It is probable, therefore, that the hearing abilities exhibited by Stormy and Dundee are more acute than those of the population at large.
As the audiogram (Figure 3, left) illustrates, manatees have a functional hearing range from 400 to 46,000 hertz. Their peak sensitivity actually lies between 16,000 and 18,000 hertz, and not 1,000 to 5,000 hertz as previously thought. Below 16,000 hertz sensitivity decreases approximately 10 decibels per octave, and below 2,000 hertz it drops precipitously (20 decibels per octave) until functional hearing ends at 400 hertz. Unfortunately the dominant sounds produced by most boats and ships are below 1,000 hertz; these lower frequencies fall outside or overlap the lower fringe of the manatees' hearing range. The audiogram suggests that even in quiet conditions, manatees would have difficulty detecting these sounds at acoustic levels less than 90 or 100 decibels. (All underwater sound levels here are given against a standard underwater reference acoustic pressure of 1 micropascal.)