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First published online June 29, 2007
Journal of Experimental Biology 210, 2411-2418 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02790
Swimming kinematics of the Florida manatee (Trichechus manatus latirostris): hydrodynamic analysis of an undulatory mammalian swimmer
Department of Biology, West Chester University, West Chester, PA 19383, USA
* Author for correspondence (e-mail: ffish{at}wcupa.edu)
Accepted 1 May 2007
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Summary |
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p). Manatees swam at velocities of 0.06–1.14 m
s–1. Locomotion was accomplished by undulation of the body
and caudal fluke. Undulatory locomotion is a rapid and relatively high-powered
propulsive mode involved in cruising and migrating by a variety of swimmers.
Manatees displayed an undulatory swimming mode by passing a dorso-ventrally
oriented traveling wave posteriorly along the body. The propulsive wave
traveled at a higher velocity than the forward velocity of the animal. The
frequency of the propulsive cycle (f) increased linearly with
increasing swimming velocity (U). Amplitude at the tip of the caudal
fluke (A) remained constant with respect to U and was 22% of
body length. Pt increased curvilinearly with U.
The mean p, expressing the relationship of the thrust power
generated by the paddle-shaped caudal fluke to the total mechanical power, was
0.73. The maximum p was 0.82 at 0.95 m s–1.
Despite use of a primitive undulatory swimming mode and paddle-like fluke for
propulsion, the manatee is capable of swimming with a high efficiency but
lower power outputs compared with the oscillatory movements of the high-aspect
ratio flukes of cetaceans. The swimming performance of the manatee is in
accordance with its habits as an aquatic grazer that seasonally migrates over
extended distances.
Key words: manatee, swimming, Trichechus manatus, power output, efficiency
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Fish, 2000;
Fish, 2001) proposed a model
for the sequence of transitional stages in swimming mode from primitive to
derived aquatic mammals. According to Fish's model, the evolution of mammals
that swim using oscillations of pointed, wing-like caudal flukes (i.e.
cetaceans, dugong) was immediately preceded by a transitional locomotor stage
using dorso-ventral undulations of broad, flattened tail. Within the Sirenia,
manatees employ the more primitive swimming mode [dorso-ventral undulation
(Hartman, 1979)] and
propulsive morphology (Fig. 1)
compared to the related dugongs. Dugongs and cetaceans swim by caudal
oscillation, also described as thunniform or carangiform with lunate tail
(Lighthill, 1969;
Webb, 1975;
Fish, 2001). As differences
exist between the morphologies and movements of manatees and these caudal
oscillators (Fish, 2001) it
is, therefore, expected that there are corresponding differences in swimming
characteristics. Compared to the cetaceans, manatees were considered
inefficient swimmers due to their inability to reach and maintain high speeds,
a less streamlined body shape, broad paddle-like tail flukes and a lack of the
peduncular musculature necessary to generate thrust
(Hartman, 1979;
Fish, 1996).
|
;
Reynolds and Powell, 2002).
The manatee migrates between fresh, brackish and saltwater habitats; they
rarely remain in deep ocean waters
(Hartman, 1979;
Reynolds and Odell, 1991). In
one case, a male manatee tracked by a satellite transmitter tag traveled over
4800 km at 20–35 km per day (Reid,
1996).
Hartman (Hartman, 1979)
described the gross swimming movements of the Florida manatee. He noted that
the propulsive forces were produced by dorso-ventral undulation of the
horizontally flattened and rounded tail
(Fig. 1). No motion anterior to
the peduncle was observed to be involved in forward propulsion. Stroke cycle
frequencies observed (Hartman,
1979) ranged from 0.30 to 0.83 Hz depending on whether the
individuals were idling, cruising or fleeing. Manatees have been reported to
swim at speeds of 0.6–13.3 m s–1, but tend to swim in a
leisurely manner at 0.6–0.8 m s–1 and cruise at
0.8–1.9 m s–1
(Hartman, 1979;
Charnock-Wilson, 1968).
The objectives of this study were to analyze the swimming kinematics of
manatees with greater fidelity than originally described
(Hartman, 1979) and to use
this information to estimate thrust power output (Pt) and
propulsive efficiency (p). Based on previous
observations of manatee swimming and propulsive morphology, it was
hypothesized that values of Pt and p would
to be less than those for evolutionarily more derived swimmers (i.e. caudal
oscillators such as cetaceans). From an evolutionary perspective, manatees
swimming by undulation can represent functional analogues for the ancestors of
more derived, lift-based oscillatory swimmers
(Fish, 1996) and demonstrate
swimming capabilities of transitional forms.
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Materials and methods |
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The SW facility consisted of oblong pool of approximately 42 mx9 m with a maximum depth of 5.5 m and total volume of about 1.36x106 liters. Manatees were visible from an underwater viewing area that extended along the length of the pool with 7.3 mx2.5 m glass windowpanes. The water temperature in the pool was maintained at 24–25°C. All pools were filled with freshwater. The CZ manatee habitat consisted of one oblong pool with dimensions 33 mx12 m and volume of 7.2x105 liters. The maximum depth was 3.6 m. The water temperature in the exhibit was maintained at 26–27°C and the salinity of the pool was 14.36 p.p.t. The LP facility consisted of two pools with viewing windows that were approximately 3 mx2 m. Access to each facility was gained before normal hours of operation to facilitate video recording without interference from park visitors.
The staff members at SW, CZ and LP provided the morphometrics of the individual animals that are summarized in Table 1. Body length (BL, m) was the linear distance from the tip of the nose to the tip of the tail fluke. Fluke span (d, m) was the linear distance of the widest section of the tail fluke perpendicular to the central axis of the body.
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Video analysis
The animals were videotaped with a Sony Digital 8 video camera recorder
(model DCR-TRV530) that allowed for continuous recording of the gross
movements at 60 Hz. The video camera was held stationary on a tripod that was
placed at 5.5–6 m from the glass viewing window at each facility.
Lateral views of the animals were recorded as the animal normally swam
parallel to a viewing window. Animals swam at various depths and distances
from the window. Data were acquired only from those video records in which the
whole body or posterior portion of the steadily swimming manatee was in view
for at least one stroke cycle, with no apparent accelerations or deviations
from a horizontal trajectory. In most sequences, the animals were swimming
alone (i.e. the other individuals were out of view of the camera).
Sequential body and fluke positions were digitized from individual fields of videotape with a computerized video analysis system (Motus 2000; Peak Performance Technologies, Inc., Centennial, CO, USA). In some cases, video was transferred to VHS tapes from Hi-8 tapes prior to analysis. In order to digitize this footage, a Panasonic AG-7300 video recorder and Sony PVM 1341 monitor were used. The specific anatomical features that were digitized included the nose, eye, flipper tip, peduncle and fluke tip (Fig. 2). The body length, BL, for each animal served as its own scale.
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) was calculated from simple harmonic motion as
V/f.
To adjust for size differences, data were analyzed with respect to
length-specific velocity (U/BL) and length-specific
amplitude (A/BL). To maintain hydrodynamic similarity, data
were also analyzed with respect to Reynolds number (Re):
 | (1) |
Thrust power and efficiency calculations
The thrust power and propulsive efficiency were calculated using
Lighthill's `bulk momentum' model based on `slender body theory' model
(Lighthill, 1969;
Webb, 1975). The model can be
applied to anguilliform and carangiform swimmers, which includes the swimming
motions of the manatee (see Results). The advantage of using this model for
the manatee is that propulsive force is estimated from movements at a defined
trailing edge. Therefore, discrete movements of the thick body sections are
unnecessary. However, the model has assumptions including a small tail stroke
amplitude, neglecting forces of viscous origin, except for viscous drag, and a
propulsive efficiency that is always greater than 0.5
(Webb, 1975). Large amplitude
motions, like those in the manatee
(Hartman, 1979), were found to
have little effect on total power compared to small amplitude motions
(Lighthill, 1971). In
addition, corrections to propulsive efficiency for large amplitude movements
are only necessary at low efficiencies. Propulsive efficiencies in the range
of 0.75–0.9 only have a 5% error when a small amplitude is assumed for
the model (Webb, 1975).
According to Lighthill (Lighthill,
1960; Lighthill,
1969; Lighthill,
1970), for a hypothetical fish swimming with increasing small
amplitude motions, a reaction force is generated for each segment of the body.
This reaction force is proportional to the velocity of the segment and mass of
the water affected by the segment (virtual mass). Considering a fish of
constant depth, the mass of water passed along the body remains constant
between body segments. The momentum, however, increases as each body segment
increases in amplitude and velocity posteriorly. The more posterior segments
move through greater distances at a faster velocity, producing a relatively
greater force that is more aligned with the direction of movement of the
animal. Eventually, the water is shed into the wake with a momentum, which is
defined by the motions of the trailing edge of the caudal fin
(Webb, 1975). The thrust is
proportional to the rate of momentum shed into the wake and the
Pt is equal to the rate of work performed by the trailing
edge against the shed momentum.
The mean thrust power (Pt) is the power output
generated by the flukes to propel the manatee forward (total power –
kinetic energy), which is calculated as:
 | (2) |
is the
relative velocity of the tail. The virtual mass per unit length represents the
relatively large mass of fluid accelerated by the tail
(Webb, 1975) and is given as:
 | (3) |
), 
is
the density of water equal to 1000 kg m–3, and
dt is the maximum width of the tail. The relative velocity
of the flukes, , was calculated from:
 | (4) |
p) for thrust generation is:
 | (5) |
Statistical procedure
Statistical analysis of the data was performed using Statistica for Windows
(Version 5.1, Statsoft) in conjunction with Microsoft Excel 2000 for Windows.
Swimming trials for each individual were not considered to be independent.
Variation about means was expressed as ± one standard deviation (s.d.).
Regressions were computed using least-squares regression. Simple
repeated-measures ANOVA was used to determine differences in peak-to-peak
amplitude A at specific body positions. Post hoc analysis
was performed using the Scheffe test. In all statistical analyses, a
significance level of P=0.05 was maintained.
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Swimming velocity
Healthy, adult manatees swam at U ranging from 0.3 to 1.1 m
s–1 (0.1–0.4 BL s–1) (mean
0.6±0.2 m s–1; 0.2±0.1 BL
s–1). Reynolds number (Re) ranged from
7.0x105 to 2.8x106 with a mean of
1.6x106±0.5x106
(Fig. 3).
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The nose, eye, peduncle and tail fluke all followed a sinusoidal pathway
(Fig. 2). Cyclic movements at
the peduncle lead corresponding movements of the tail tip by
100.3±22.4°. of the sinusoidal wave measured in the fluke
was shorter than the body length of the healthy manatees
(/BL=0.9±0.2).
The kinematic parameters f, W, and V showed a significant
linear increase with increasing U (N=57;
P<0.001) (Fig. 4),
according to the regression equations:
 | (6) |
 | (7) |
 | (8) |
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p were based on
an average adult manatee (length=3.34 m; mass=857.8 kg; fluke span=0.86 m) and
kinematic statistics of f, W and V. Pt displayed
a curvilinearly with increasing U. The polynomial equation, which
describes the relationship between Pt and U, is:
 | (9) |
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Manatees swam with p of 0.67–0.81
(Fig. 6). p
showed a curvilinear increase with increasing U. Unlike cetaceans. no
plateau in p was displayed by manatees, although this may have
been a function of the relatively low swimming speed of the manatees in this
study.
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Nishiwaki and Marsh, 1985).
During migration, manatees may cruise at speeds of 0.8–1.89 m
s–1. Charnock-Wilson
(Charnock-Wilson, 1968)
reported that a fisherman in British Honduras had observed manatees swimming
at speeds up to 13.33 m s–1; however, as this speed is
exceptional even for more streamlined dolphins
(Fish and Rohr, 1999), the
record is questionable. A wide range of swimming velocities have been reported
(Hartman, 1979), with idling
manatees swimming at speeds of 0.56–0.83 m s–1,
cruising animals at 0.83–1.94 m s–1, and sprinting
animals at 5–6.94 m s–1. Dugongs, a close but more
derived relative of the manatee, cruise at speeds of 2.67 m
s–1 (Jarman,
1966). The speeds observed in this study are indicative of animals
that were idling and cruising but not fleeing. This was reasonable considering
that all individuals were in the confines of a pool.
The length-specific velocity for manatees in this study was 0.08–0.38
BL s–1. This range of velocity is substantially
slower than cetaceans, which swim by oscillation of a highly derived wing-like
fluke (Fish, 1998a). The
manatee's relative velocity was also slower than the sea otter
(0.47–1.08 BL s–1), which swims by undulation
of the body and hind feet (Williams,
1989).
Swimming kinematics
Hartman briefly described the kinematics of swimming in manatees
(Hartman, 1979). He did not
observe undulation of the body, and considered all movement to be confined to
the tail. He reported that during the full stroke the tail moved through an
arc of 130° (Hartman,
1979), which was substantially less than the arc of 200°
described for the fluke of a dolphin
(Slijper, 1962).
Observations of Florida manatees in this study suggest that swimming is
accomplished using an undulatory mode. Manatees have previously been listed
among caudal oscillators (Fish,
1993a; Fish,
1996). Undulatory swimming entails throwing the body and tail into
a traveling wave. The wave is propagated in the direction opposite of forward
motion of the animal. For organisms that undulate the body and tail, the
propulsive wave may be produced over different lengths of the body. Body and
tail undulators are characterized along a continuum as anguilliform,
subcarangiform, carangiform or thunniform, where the propulsive wave is
confined more posteriorly, respectively
(Breder, 1926;
Webb, 1975;
Lindsey, 1978). This
undulatory continuum has functional implications. Thunniform swimmers, such as
cetaceans, are characterized as high speed, high efficiency swimmers, whereas
lower swimming performance is associated with anguilliform and subcarangiform
swimmers.
Manatees swim using the paddle-like flukes (Figs
1,
2) with subcarangiform mode. In
this swimming mode, the body and caudal flukes are thrown into a wave with
more than one half-wavelength within the length of the body and the A
of the wave rapidly increases over the posterior half of the body
(Lindsey, 1978). Wavelength
for the manatee was determined to be equal to BL of the individual.
Subcarangiform swimmers have a fusiform body and deep caudal peduncle
(Webb, 1975; Lindsay, 1978).
Subcarangiform swimming is a rapid and relatively high-powered propulsive mode
(Webb et al., 1984). This mode
is used for durations of a few seconds to several weeks. It is involved in
behaviors such as cruising, sprinting, patrolling, station holding as well as
migrating (Fish, 1993b).
Estimates of stroke cycle frequency, f, for the swimming adult
Florida manatees in this study were within the range previously reported
(Hartman, 1979) for adult
manatees idling and cruising. Adult free-ranging manatees swim at f
of 0.3–0.33 Hz when idling, 0.4–0.6 Hz when cruising and
0.75–0.83 Hz when fleeing (Hartman,
1979). f for the Florida manatee was less than f
for several species of cetaceans (Fish,
1998b). f for the bottlenose dolphin Tursiops
truncatus, a marine mammal approximately one-third the mass of the
Florida manatee, is approximately 1–3 Hz
(Fish, 1993b). f
should be lower, however, in the manatee, which is not able to reach and
maintain higher U of the dolphin.
The linear increase of f with U for the Florida manatee
was consistent with results from previous studies of other marine mammals
using body and tail propulsion (Pyatetsky
and Kayan, 1975; Kayan and
Pyatetsky, 1977; Kayan et al.,
1978; Fish, 1993b;
Fish, 1998b). In fully aquatic
marine mammals, f is the major determinant of U. Modulation
of f as opposed to A is preferred in aquatic mammals because
it prevents excessive body distortion, which would increase overall drag and
thus decrease p (Fish et
al., 2003).
The analysis of A at several body positions for the Florida
manatee was consistent with previous studies for subcarangiform swimmers
(Bainbridge, 1958;
Webb, 1975;
Wardle and Videler, 1980;
Fish, 1993b;
Fish, 1998a;
Dewar and Graham, 1994;
Long et al., 1994;
Wassersug and Hoff, 1985).
A for subcarangiform fishes ranges from 0.04 to 0.07 BL at
the nose and 0.20 BL at the tail
(Bainbridge, 1958;
Webb, 1975;
Webb, 1986;
Webb and Keyes, 1982). For
cetaceans, A at the rostrum ranges from 0.02 BL to 0.08
BL and A ranges from 0.17 BL to 0.25 BL at
the fluke tips (Lang and Daybell,
1963; Videler and Kamermans,
1985; Fish et al.,
2003).
The small A observed at the anterior end of animals that swim
via body and tail propulsion is displaced as a result of the
movements of the tail (Fish et al.,
2003). The propulsive motions at the tail produce transverse
forces that need to be balanced at the anterior end of the body. This is
necessary to reduce drag, increase propulsive efficiency and maintain
stability. Large deviations at the rostrum will increase the added mass of the
system as the water adjacent to the body is accelerated by its movement
(Lighthill, 1971).
Additionally, transverse movements increase instability, which can potentially
cause the animal to deviate from it's chosen trajectory. The animal must
increase its energy output in order to maintain its course (Fish et al.,
2000). These recoil forces are balanced by throwing the body into at least one
complete wavelength (Webb,
1975; Blake, 1983).
As the wave travels posteriorly down the body, the opposing lateral forces are
cancelled out (Fish et al., 2000).
Thrust power output and propulsive efficiency
The Florida manatee produces less thrust power than other highly derived
marine mammals (Fig. 5). A
maximum Pt for the bottlenose dolphin exceeds 7600 W or
30.5 W kg–1 at 5.9 m s–1 (2.2 BL
s–1) (Fish,
1993b). This Pt was obtained for animals at
higher speeds than those observed for the manatees in this study. However,
when compared to the beluga Delphinapterus leucas of equivalent size
(length=3.55 m; mass=664.2 kg) and swimming speed (0.35 BL
s–1) (Fish,
1998b), the cetacean had a Pt that was 70%
greater than for the manatee. Such power outputs may reflect differences in
both physiological and morphological mechanisms for sirenians and cetaceans.
In particular, the paddle-like fluke and undulatory swimming mode of the
manatee are less effective in power generation than the oscillations of the
wing-like flukes of cetaceans.
Despite the more primitive morphology and swimming mode used by manatees,
p was higher than expected (0.67–0.81)
(Fig. 6) and consistent with
the performance of other undulatory swimmers. Animals that swim by undulating
the body and/or caudal appendage (anguilliform to carangiform) have
p of 0.45–0.85
(Webb, 1975;
Webb, 1978;
Wardle, 1975;
Wardle, 1977;
Videler and Hess, 1984;
Webb et al., 1984;
Vogel, 1994). However, the
more derived thunniform swimming mode of cetaceans and phocid seals has a
higher range for p of 0.75–0.90
(Fig. 6)
(Fish et al., 1988;
Fish, 1998b). Using
computational fluid dynamics, Schultz and Webb
(Schultz and Webb, 2002)
indicated that carangiform swimmers with a wavelength equal to one body length
were less efficient than thunniform swimmers. This result was due to the large
power consumption predicted for carangiform swimmers.
Relationship of manatees to evolution of derived swimming modes
Recent studies on swimming in aquatic mammals focused on how propulsive
modes have changed throughout evolution, resulting in more derived forms
(Fish, 1996;
Fish, 2000;
Thewissen and Fish, 1997;
Domning, 2001). The evolution
of fully aquatic mammals (cetaceans, sirenians) represents the culmination of
a series of transitional stages that have resulted in morphologies and
propulsive modes providing high swimming speeds with high energetic efficiency
(Gingerich et al., 1983;
Barnes et al., 1985;
Fish, 1996). However, unlike
cetaceans or the dugong, the manatee does not possess a high-efficiency,
wing-like caudal fluke.
Evolutionarily, the manatee has a swimming mode that occupies a position
just below the high-derived oscillatory swimming using the thunniform mode
(Fish, 1996;
Fish, 2000). The manatee
represents a transitional morphology and swimming mode to more derived
oscillatory swimmers. The undulatory mode, while providing sufficient thrust
with generally high efficiency, is limited in performance compared to use of
oscillating flukes. Furthermore, the rounded, paddle-like flukes of the
manatee are not specialized for steady high-speed swimming. Although thrust
increases with the span of a caudal propulsor, the drag on the propulsor
increases with its surface area in similar proportion to the increased thrust
(Webb, 1978). Fast swimmers,
therefore, have narrow propulsors with large spans (i.e. high aspect ratio).
Such flukes are found in dugongs Dugong dugon, which have converged
with cetaceans. Compared to the manatee, dugongs are found in more open water
habitats and undergo daily and seasonal movements
(Nishiwaki and Marsh,
1985).
The evolution of sirenians parallels that of cetaceans. Both groups evolved
over 50 millions years ago from quadrupedal terrestrial ancestors
(Thewissen and Fish, 1997;
Domning, 2001). Upon entry
into water these aquatic mammals are believed to have first used paddle
propulsion and subsequently simultaneous pelvic paddling combined with spinal
undulation (Fish, 1996). Later
loss of the hind limbs and exclusive undulatory propulsion were due to
maintenance of horizontal trim (Domning
and de Buffrénil, 1991) and reduced drag
(Fish, 1996). Propulsion
exclusively by spinal undulation required expansion of the tail flukes
oriented perpendicular to the plane of the traveling wave. Eventually, the
flukes were modified into wing-like structures for lift-based propulsion
(Fish, 1996;
Fish, 1998a). Diet does not
appear to have played a key role in the evolution of wing-like flukes as
sirenians are strictly herbivores and cetaceans are piscivores or carnivores.
Movements in pelagic habitats and high cruising speeds by the efficient
generation of thrust are presumed to be the important parameters in the
evolution of highly derived flukes and swimming modes
(Webb and de Buffrénil,
1990; Fish, 2000).
Such behaviors and morphologies are associated with foraging for patchy
resources.
Manatees have very little need for powerful steady swimming
(Domning, 1978). Unless
migrating, speed and thrust are less important to the manatee that has very
few threats outside of humans (Reynolds
and Odell, 1991; Gerstein,
2002; Reynolds and Powell,
2002; Rommel et al.,
2007). The majority of their daily activities involve feeding on
sessile aquatic plants and resting
(Hartman, 1979;
Best, 1981;
Bengston, 1983;
Etheridge et al., 1985;
Marshall et al., 2000).
Similarly, another undulatory swimming mammal is the sea otter Enhydra
lutra, which also forages on stationary prey. If the morphology and
ecology of the sea otter are compared to that of the manatee, several key
similarities emerge. Both species swim by vertical undulations of the caudal
region of the body. They both inhabit structurally complex environments. Much
like the manatee, the sea otter spends a large portion of its day resting.
Although the sea otter is a carnivore, its diet consists mainly of sedentary
or slow-moving invertebrates (Kenyon,
1969). Rather than using speed to capture elusive prey, the sea
otter maneuvers through rocky shores, barrier reefs and dense kelp to locate
food. The similar foraging ecology of these species may be associated with
similar swimming kinematics.
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Acknowledgments |
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