Sound Localization Abilities of Florida Manatees,Sound Localization Abilities of Florida Manatees, Trichechus manatus Trichechus manatus latirostrislatirostrisDebborah E. Colbert1, 2, 5, Dr. David Mann1, 2, Joseph C. Gaspard III1, 4, Dr. Gordon B. Bauer1, 3, Kim Dziuk1, Adrienne Cardwell1, & Dr. Roger Reep4

1Sensory Biology and Behavior Program, Mote Marine Laboratory & Aquarium 2University of South Florida 3New College of Florida

4University of Florida 5Sea Life Park by Dolphin Discovery

MethodsThe subjects were two captive-born male Florida manatees that reside at Mote Marine Laboratory and Aquarium in Sarasota, Florida. At the inception of this study Hugh was 20 years of age and Buffett was 17 years of age. Both had an extensive training history over the previous seven years and were subjects in an auditory evoked potential study.

Each subject was trained to position perpendicular to a stationing bar. When a test stimulus was played from one of the underwater speakers, the subject swam to and depressed a speaker. A correct response, touch of the speaker that emitted the sound, was followed by an acoustic secondary reinforcer and food. Subjects were called to station following incorrect responses. Sound location was determined quasi-randomly. Minimum ITI was 30 seconds. All test trials were video-recorded from an overhead camera.

IntroductionThe Florida manatee lives in an environment where recreational boats are found in high numbers and conspecifics are often out of visual range. How then do they avoid boat collisions and find conspecifics? The manatee auditory system may play a crucial role in accomplishing these challenging tasks. Typical recreational boat engines produce broad-band frequencies that range between 0.01–2 kHz, although they can reach as high as 20 kHz. Manatee vocalizations have fundamental frequencies that range from 2.5–5.9 kHz but can extend to 15 kHz (Nowacek et al., 2003 & 2004). Previous auditory studies have found that manatees can hear over 40 kHz and may be able to localize higher frequencies (Gerstein, 1999). More recent studies have indicated that manatees have rapid auditory temporal processing (Mann et al., 2005). To further investigate the localization abilities of manatees, two captive-born manatees, Hugh and Buffett, were tested on both a four-choice and an eight-choice sound localization task using different types of sound stimuli in a controlled environment.

Four Choice Experiment

Special Thanks To:

• Jann Warfield • Jay Sprinkle• Joe Lappin• Manatee Care Team

Interns • New College Students

Results: Both subjects performed well above the 25% chance level for all of the broad-band frequency conditions (Table 1). Hugh showed a drop in percentage correct as the broad-band signal durations decreased, but this result was not observed with Buffett. Both animals also performed above chance levels with the pure tone signals, but at a much lower accuracy rate than with the broad-band signals.

Results: Both subjects performed well above the 12.5% chance level for all of the frequency, duration, and decibel level conditions (Table 2). Hugh’s accuracy declined more rapidly than Buffett’s with decreases in decibel level.

DiscussionBoth experiments indicated that manatees are able to locate underwater sounds and suggest that they should be able to use sound cues for localizing boats and conspecifics in their natural habitat. Both subjects were capable of localizing the test stimuli. Front-back confusions were surprisingly few and only Hugh had difficulty localizing the test stimuli when the correct speaker was directly behind him with the 200 ms duration. Errors tended to be to the “nearest neighbor” for broad-band stimuli, but were scattered among the locations with no obvious pattern for the tonal stimuli.

Eight Choice Experiment

Design & Conditions: Testing was conducted in the center of the Shelf Area with the subject positioned at mid-water depth (0.75 m). Four test speakers were positioned to the front 180 degrees at 45o, 90o, 270o, & 315o at a distance of 105 cm (Figure 1). Three broad-band stimuli (0.2-20, 6-20, & 0.2–2 kHz) were tested at four durations (3,000, 1,000, 500, & 200 ms) and two tonal stimuli (4 & 16 kHz) were tested at 3,000 ms. All stimuli were tested at 100 dB re:1 uPa (±1.5dB).

Design & Conditions: Testing was conducted in the deeper Exhibit Area with the subject positioned at mid-water depth (1.37 m). Eight test speakers were positioned at 0o, 45o, 90o, 135o, 180o, 225o, 270o, & 315o relative to the front of the subject’s head at a distance of 3.05 m (Figure 3). Three broad-band stimuli (0.2-24, 18-24 & 0.2–1.5 kHz) were tested at two durations (3,000 & 200 ms). The 0.2-24 kHz, 3000 ms stimuli were tested at several decibel levels (120, 111, 105, 100 & 80 dB re:1 uPa [±1.5dB]).

  Frequency (kHz)

  0.2 - 20 6 - 20 0.2 - 2 4 16

Duration:  Hugh

3000 ms 93% 86% 81% 49% 32%

1000 ms 74% 71% 65%    

500 ms 71% 63% 57%    

200 ms 64% 51% 58%    

  Buffett

3000 ms 88% 82% 92% 44% 33%

1000 ms 93% 79% 92%    

500 ms 85% 92% 86%    

200 ms 93% 89% 85%    

Shelf Area

Deep Exhibit Area

3.05 m

270o

315o

225o

180o

45o

90o

135o

0o

Frequency (kHz)

0.2-24 kHz 18-24 kHz 0.2-1.5 kHz

Duration & Level: Hugh200 ms; 120 dB 55%

3000 ms; 120 dB 75% 43% 41%

3000 ms; 111 dB 72%

3000 ms; 105 dB 48%

Buffett

200 ms; 120 dB 66%

3000 ms; 120 dB 77% 60% 63%

3000 ms; 100 dB 56%

3000 ms; 80 dB 77%

Shelf Area

270o

90o

45o

315o

105cm

Deep Exhibit Area

ReferencesGerstein, E. (1999). Psychoacoustic Evaluations of the West Indian manatee (Trichechus manatus latirostris). Unpublished

Doctoral Dissertation, Florida Atlantic University, Boca Raton, FL.Mann, D., Colbert, D. E., Gaspard, J. C. III, Casper, B., Cook, M. L. H., Reep, R. L., & Bauer, G. B. (2005). Temporal resolution

of the Florida manatee (Trichechus manatus latirostris) auditory system. Journal of Comparative Physiology, 191, 903-908.Nowacek, D. P., Casper, B. M., Wells, R. W., Nowacek, S. M., & Mann, D. A. (2003). Intraspecific and geographic variation of

West Indian manatee (Trichechus manatus spp.) vocalizations. Journal of the Acoustical Society of America, 114 (1), 66-69.Nowacek, S. M., Wells, R. S., Owen, E. C. G., Speakman,T. R., Flam, R. O., & Nowacek, D. P. (2004). Florida manatees,

Trichechus manatus latirostris, respond to approaching vessels. Biological Conservation 119, 517-523.

Table 1. Overall accuracy performance per subject by frequency and duration conditions (based on 72 trials per condition).

Table 2. Overall accuracy performance per subject by frequency, duration and level conditions (based on 80 trials for the 3000 ms and 120 trials for the 200 ms conditions).

Figure 2. Selection distribution by frequency conditions (collapsed across durations for broad-band). Correct location notated by yellow circles.

Figure 1. Testing set-up for 4-choice experiment. Figure 3. Testing set-up for 8-choice experiment.

Figure 4. Selection distribution for the 0.2-24 kHz at 200 ms. Correct speaker location notated by yellow circles.

6-15 kHz All Durations

100 50 0 50 100

((45))315

270 90

100 50 0 50 100

45315

270 ((90))

100 50 0 50 100

45315

((270)) 90

100 50 0 50 100

45((315))

270 90

Percent Percent

0.2-20 kHz All Durations

100 50 0 50 100

((45))315

270 90

100 50 0 50 100

45315

270 ((90))

100 50 0 50 100

45315

((270)) 90

100 50 0 50 100

45((315))

270 90

Percent Percent

0.2 - 20 kHz 6 – 20 kHz

0.2-2 kHz All Durations

100 50 0 50 100

((45))315

270 90

100 50 0 50 100

45315

270 ((90))

100 50 0 50 100

45315

((270)) 90

100 50 0 50 100

45((315))

270 90

Percent Percent

4 kHz3000 ms

100 50 0 0 50 100

((45))315

270 90100 50 0 0 50 100

45315

270 ((90))

100 50 0 0 50 100

45315

((270)) 90100 50 0 0 50 100

45((315))

270 90

Percent Percent

16 kHz3000 ms

100 50 0 0 50 100

((45))315

270 90100 50 0 0 50 100

45315

270 ((90))

100 50 0 0 50 100

45315

((270)) 90100 50 0 0 50 100

45((315))

270 90

Percent Percent

16 kHz4 kHz

0.2 - 2 kHz

Hugh

Buffett

Percent Selected

Percent Selected

Percent Selected

45315

270 ((90))

135225

0

180

45315

270 90

135((225))

0

180

45315

((270)) 90

135225

0

180

45((315))

270 90

135225

0

180

45315

270 90

135225

((0))

180

45315

270 90

135225

0

((180))

((45))315

270 90

135225

0

180

45315

270 90

((135))225

0

180

Hugh

Buffett