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Sound Localization Abilities of Florida
Manatees, Trichechus manatus latirostris
Debborah E. Colbert1, 2, 5, Dr. David Mann1, 2, Jos=
eph
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 So=
uth
Florida 3New College of
Florida
University of Florida Sea Life Park by Dolphin Discovery
Metho=
ds
Each subject was trained to position perpendic=
ular
to a stationing bar. When a test stimulus was played =
from o=
ne
of the underwater speakers, the subject swam to and depressed a speaker. A
correct response, touch of the speaker that emitted the sound, was followe=
d by
an acoustic secondary reinforcer and food. Subjects were called to station
following incorrect responses. Sound location was determined quasi-randoml=
y.
Minimum ITI was 30 seconds. All test trials were video-recorded from an
overhead camera.
Introduction
The 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 p=
lay 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 k=
Hz
but can extend to 15 kHz (Nowacek et al., 2003 & 2004). Previous audit=
ory
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 fur=
ther
investigate the localization abilities of manatees, two captive-born manat=
ees,
Hugh and Buffett, were tested on both a four-choice and an eight-choice so=
und
localization task using different types of sound stimuli in a controlled
environment.
Four Choice Experiment
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Special Thanks To:
<=
span
style=3D'font-family:"Times New Roman";font-size:111%'>• Jann Warfield
=
<=
span
style=3D'font-family:"Times New Roman";font-size:111%'>• Jay Sprinkle
Both
subjects performed well above the 12.5% chance level for all of the freque=
ncy,
duration, and decibel level conditions (Table 2). Hugh’s accuracy
declined more rapidly than Buffett’s with decreases in decibel level=
.
Design
& Conditions:
Design
& Conditions:
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<=
span
style=3D'font-family:"Times New Roman";font-size:111%'>• Joe Lappin
• Manatee Care Team Interns
<=
span
style=3D'font-family:"Times New Roman";font-size:111%'>• New College Students
Results:
=
;
Both
subjects performed well above the 25% chance level for all of the broad-ba=
nd
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 a=
lso
performed above chance levels with the pure tone signals, but at a much lo=
wer
accuracy rate than with the broad-band signals.
Resul=
ts:
Discu=
ssion
Both experiments indicated that manatees are able to locate
underwater sounds and suggest that they should be able to use sound cues f=
or
localizing boats and conspecifics in their natural habitat. Both subjects =
were
capable of localizing the test stimuli.&n=
bsp;
Front-back confusions were surprisingly few and only Hugh had
difficulty localizing the test stimuli when the correct speaker was direct=
ly
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
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 45, 90o, 270, &
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.&nb=
sp;
All stimuli were tested at 100 dB re:1 uPa (±1.5dB).<=
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Testing was conducted in the deeper Exhibit Ar=
ea
with the subject positioned at mid-water depth (1.37 m). Eight test speake=
rs
were positioned at 0, 45o, 90, 135<=
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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 duratio=
ns
(3,000 & 200 ms). The 0.2-24 kHz, 3000 ms stimuli were tested at sever=
al
decibel levels (120, 111, 105, 100 & 80 dB re:1 uPa [±1.5dB]).<=
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Deep Exhibit Area&=
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3.05 m
270o
315o
225o
180o
45o
90o
135o
0o
=
0.2-1.5=
kHz
56%=
3000 ms=
; 100 dB
72%=
3000 ms=
; 111 dB
63%=
41%=
48%=
3000 ms=
; 105 dB
66%=
60%=
77%=
77%=
3000 ms=
; 80 dB
43%=
75%=
55%=
Shelf A=
rea
270o
90o
45o
315o
105cm
Deep
Exhibit Area
References
Gerstein, 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 resolutio=
n of
the Florida manatee (Trichechus manatus latirostris) auditory system.<=
span
style=3D'mso-spacerun:yes'> 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 manat=
us 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, <=
span
style=3D'position:absolute;top:96.38%;left:61.01%;width:29.96%;height:1.2%=
'>Trichechus manat=
us
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 a=
nd
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. &=
#13;
=
Figure
1. Testing set-up for 4-choice
experiment.
=
Figure
3. Testing set-up for 8-choice
experiment.
Figure 4. Selecti=
on
distribution for the 0.2-24 kHz at 200 ms. Correct speaker location
notated by yellow circles. <=
/i>
0.2 - 20 kHz
6 – 20 kHz
16 kHz
4 kHz
0.2 - 2 kHz
Hugh
Buffett
Percent Selected
Percent Selected
Percent Selected
Hugh=
3;
Buffett=