2749
The Journal of Experimental Biology 202, 2749–2761 (1999)
Printed in Great Britain © The Company of Biologists Limited 1999
JEB2379
THE DIVING PHYSIOLOGY OF BOTTLENOSE DOLPHINS (TURSIOPS TRUNCATUS)
II. BIOMECHANICS AND CHANGES IN BUOYANCY AT DEPTH
RANDOLPH C. SKROVAN1, T. M. WILLIAMS1,*, P. S. BERRY2, P. W. MOORE3 AND R. W. DAVIS4
1Department of Biology, Earth and Marine Science Building, A-316, University of California, Santa Cruz, CA 95064,
USA, 2The Dolphin Experience, PO Box F42433, Freeport, Grand Bahama Island, Bahamas, 3SPAWAR Systems
Center, D35, 53560 Hull Street, San Diego, CA 92152-6506, USA and 4Department of Marine Biology, Texas A&M
University, Galveston, TX 77553, USA
*Author for correspondence (e-mail: williams@darwin.ucsc.edu)
Accepted 26 July; published on WWW 30 September 1999
Summary
During diving, marine mammals must balance the
diving animals. The results showed that dolphins used a
conservation of limited oxygen reserves with the metabolic
variety of swimming gaits that correlated with acceleration.
costs of swimming exercise. As a result, energetically
The percentage of time spent gliding during the descent
efficient modes of locomotion provide an advantage during
phase of dives increased with depth. Glide distances ranged
periods of submergence and will presumably increase in
from 7.1±1.9 m for 16 m dives to 43.6±7.0 m (means ±
importance as the animals perform progressively longer
S.E.M.) for 100 m dives. These gliding patterns were
dives. To determine the effect of a limited oxygen supply
attributed to changes in buoyancy associated with lung
on locomotor performance, we compared the kinematics
compression at depth. By incorporating prolonged glide
and behavior of swimming and diving bottlenose dolphins.
periods, the bottlenose dolphin realized a theoretical
Adult bottlenose dolphins (Tursiops truncatus) were trained
10–21 % energetic savings in the cost of a 100 m dive in
to swim horizontally near the water surface or submerged
comparison with dives based on neutral buoyancy models.
at 5 m and to dive to depths ranging from 12 to 112 m.
Thus, modifying locomotor patterns to account for physical
Swimming kinematics (preferred swimming mode, stroke
changes with depth appears to be one mechanism that
frequency and duration of glides) were monitored using
enables diving mammals with limited oxygen stores to
submersible video cameras (Sony Hi-8) held by SCUBA
extend the duration of a dive.
divers or attached to a pack on the dorsal fin of the animal.
Drag and buoyant forces were calculated from patterns of
Key words: dolphin, Tursiops truncatus, diving, biomechanics,
deceleration for horizontally swimming and vertically
buoyancy.
Introduction
Animals demonstrate a wide variety of mechanical,
morphological and behavioral adaptations that promote
locomotor efficiency and subsequently reduce the overall
cost of activity. Terrestrial animals systematically change
gaits, moving from walk to trot to gallop as speed increases
(Heglund et al., 1974; Taylor, 1978; Hoyt and Taylor, 1981;
Magana et al., 1997). During walking, these animals conserve
energy by alternately storing and releasing gravitational
potential energy as they rotate over a stiffened limb. With the
change to a gallop, bound or hop, elastic elements in the limbs
and spine may be used to store and recover energy as
the speed of a terrestrial animal increases (Taylor, 1978;
Williams, 1983).
Similarly, aquatic vertebrates use a variety of strategies to
reduce the cost of locomotion. Burst-and-glide swimming and
schooling, observed in many species of fish, promote energy
conservation (Fish et al., 1991). The low drag associated with
gliding periods during interrupted forms of swimming appears
to compensate for the increased effort of re-acceleration during
burst phases (Blake, 1983). Schooling behavior also reduces
the energetic cost for individuals moving in groups by
decreasing the relative drag encountered by the trailing
swimmers (Breder, 1965; Belyayev and Zuyev, 1969; Weihs,
1973; Abrahams and Colgan, 1985).
Locomotor efficiency is especially important for aquatic
mammals during diving. When submerged, marine mammals
must balance the energetic demands of exercise with the
conservation of a limited oxygen store (Castellini, 1985;
Castellini et al., 1985). High locomotor costs will presumably
lead to the termination of a dive as oxygen reserves are quickly
depleted. Conversely, locomotor efficiency may be manifest as
an increase in time available at depth for locating and catching
prey or for predator avoidance (Williams, 1996).
Most studies addressing locomotor performance in marine
2750 R. C. SKROVAN AND OTHERS
mammals have focused on transit-swimming animals moving
near the water surface (Fish, 1992). For example, the routine
speeds (Lang and Norris, 1966; Lang, 1974; Würsig and
Würsig, 1979; Williams et al., 1992), boundary flow
characteristics (Rohr et al., 1998) and burst performances (Hui,
1987) of bottlenose dolphins have been reported. In
comparison, there is little information concerning the
underwater performance capabilities and limitations of these
animals. In view of this, the present study examined the
behavior and biomechanics of diving bottlenose dolphins
(Tursiops truncatus) and the effect of a limited oxygen supply
on underwater performance. Factors contributing to locomotor
efficiency during submergence were investigated. To assess
the relationships between swimming mode, stroke mechanics
and dive depth, we videotaped dolphins during (1) horizontal
swimming near the water surface, (2) horizontal swimming at
depth, and (3) vertical diving to depths ranging from 12 to
112 m. Digital analyses of the video sequences were used to
define locomotor modes and their pattern of use, glide duration
and rates of acceleration and deceleration. These data, in
combination with information from time/depth and velocity
recorders, allowed changes in drag and buoyancy due to depth
to be determined. The results of this study indicate that
dolphins exploit changes in buoyancy associated with pressure
changes at depth. By incorporating prolonged glide periods
during descent, dolphins can reduce the period of active
stroking and related energetic costs. Thus, glide performance
by dolphins provides one important mechanism for conserving
limited oxygen stores during submergence.
Materials and methods
Animals
Six trained Atlantic bottlenose dolphins (Tursiops truncatus)
were used in this study (Table 1). All animals were housed in
net pens (>15 m×15 m×4 m deep) connected to the open
ocean. The dolphins were fed a diet of capelin and herring
supplemented with multivitamins (Sea Tabs™, vitamin C, B12 and B complex). Five of the animals were housed at the
Dolphin Experience (Grand Bahama Island, Bahamas). Two
adult male dolphins (B1, B2) and one adult female (B3) were
used during uninstrumented dives to 12–14 m and for
horizontal swims on the water surface and while submerged.
A second female (B4) and an immature male (B5) were also
used for surface swimming tests. Deep dives and horizontal
submerged swims were performed by an adult male dolphin
(S6) wearing an instrument package. This animal was housed
at the U.S. Navy SPAWAR Systems Center (San Diego,
California, USA).
Experimental design
The swimming mechanics and behavior of dolphins were
examined under four conditions: (1) steady-state horizontal
swimming near the water surface, (2) horizontal swimming
at depths of 5–14 m, (3) shallow dives to 12 m without
instrumentation on the dolphins, and (4) deep dives ranging
Table 1. Age and morphological dimensions of the bottlenose
dolphins used in this study
Animal
B1
B2
B3
B4
B5
S6
Age
(years)
Length
(cm)
Fluke span
(cm)
Mass
(kg)
Location
13
13
13
13
2
16
249
254
237
233
188
236
64
72
68
61
N/A
66
236
227
173
177
82
177
Bahamas
Bahamas
Bahamas
Bahamas
Bahamas
San Diego
Age was estimated from body length and duration in captivity.
Body length is the straight-line distance from the tip of the rostrum
to the fluke notch.
N/A, not applicable.
from 16 to 112 m with the animal wearing an instrumentation
package. Horizontal swimming, both near the surface and at
depth, was used to evaluate the swimming mode of dolphins
in the absence of changes in buoyancy. Deep dives of
16–112 m allowed a comparison of locomotor behaviors as
buoyancy and demands on oxygen reserves changed with the
depth and duration of the dive. Swimming and gliding
performance of dolphins with and without instrumentation
were also compared to determine the potential effects of the
instrument package on the locomotor behavior and
hydrodynamic drag of the animals.
Horizontal swimming
The kinematics of bottlenose dolphins swimming near the
water surface was recorded using a hand-held video camera
(Sony Hi-8, model CCD TR400). Dolphins were videotaped
while swimming alongside a 17 foot Boston Whaler traveling
at either 1.5 or 3.7 m s−1. Boat speed was controlled by
maintaining the outboard motor at constant revs min−1 with the
same motor trim for all runs. Speed associated with each
revs min−1 was determined by videotaping the boat’s passage
past fixed points a measured distance apart. Video sequences
of the fixed points were digitized, and speed was determined
using a motion-analysis system (Peak Performance
Technologies, Inc.; Englewood, CO, USA). Trainers
maintained the position of the dolphins abeam of the boat
outside the stern and bow wakes. Analyses were limited to
video segments in which the dolphins remained clear of
interfering wakes and were stationary relative to the moving
boat.
Horizontal swimming was also examined for submerged
dolphins moving between two trainers at a depth of 14 m or
between stationary targets at approximately 5 m depth. During
the 14 m trials, movements of the dolphins were recorded by a
SCUBA diver with a hand-held video camera in a submersible
housing (Stingray, Inc.). The camera was held in a stationary
position perpendicular to the swimming path of the animals.
Progress across the field of view was converted to speed
(m s−1) using the motion-analysis system described above.
Biomechanics of diving dolphins 2751
Images were digitized and calibrated against the measured
length of the dolphin. To account for extraneous movements
of the camera, a fixed point on the sea floor within the field of
view was digitized using Peak Performance software
(Englewood, CO, USA). Movement vectors of the fixed point
were then subtracted from movement vectors of the dolphin.
In addition to these trials, fluke movements of a dolphin
swimming horizontally at approximately 5 m depth were
recorded for an animal wearing a submersible video/instrument
package (described below). Horizontal swim paths at depth
ranged from 10 to 100 m in length.
Shallow dives
Straight-line dives to less than 16 m in depth were recorded
in the Bahamas by a SCUBA diver using a hand-held video
camera in a submersible housing (described above). On each
experimental day, two dolphins followed a motor boat to an
open ocean site 1 mile (1.61 km) offshore. Sites ranged from
12 to 16 m in depth with a sand bottom. Dolphins were trained
to dive between the boat and a trainer stationed at depth. The
animals chose their rates of ascent and descent, surface interval
between dives and bottom time. Each session was recorded by
a diver positioned perpendicular to the movements of the
dolphins and as far back as visibility allowed. Depending on
the distance from the subject, the field of view for the camera
was 7–14 m. A field of view of 14 m allowed the entire ascent
and descent of the dolphins to be monitored without panning
the camera. When necessitated by surge, a monopod was used
to stabilize the camera. To control for inadvertent camera
movement, a stationary reference point was digitized, and its
movement vector was subtracted from the dolphin’s track.
Measurements of fluke movement and velocities were not
sensitive to camera range because each video sequence was
calibrated against the measured length of the dolphin.
Deep dives
Fluke movements during deep (16–112 m) diving were
recorded by a submersible video camera worn by the dolphin.
A saddle platform containing the camera and instrumentation
was custom-fitted to the dorsal fin of one dolphin, S6. The
dolphin was trained for 6 weeks prior to the experiments to
swim and dive while wearing the instrument package. This onboard system enabled us to examine the fluke movements of
the dolphins at depths exceeding 100 m, which were outside
the range of SCUBA divers.
The instrument package included a time/depth recorder,
velocity meter, camera head (See-snake) surrounded by blue
light-emitting diodes and video recorder. The camera head was
directed backwards to record the stroke activity of the
dolphin’s fluke. Video sequences and dive variables were
synchronized using custom-designed software (Pisces Design;
San Diego, CA, USA). The instrument package and platform
were neutrally buoyant and constructed of non-compressible
materials to maintain neutrality at depth. The mass of the
package was 14 kg, representing 8 % of the dolphin’s mass.
Because the package was neutrally buoyant, there was no
additional weight for the dolphin to bear. However, its mass
affected the acceleration of the dolphin. Details of the camera
and instrument package are described by Davis et al. (1999).
The stall speed and accuracy of the velocity meter, as well
as the accuracy of the time/depth recorder, were determined
prior to deployment. The minimum recording (stall) speed of
the velocity meter was measured by towing the instrument
package attached to a fusiform shape through an annular water
trough (Scripps Institute of Oceanography, La Jolla, CA,
USA). In addition, the velocity meter was self-calibrated on
the diving dolphin by plotting the velocity of the animal
against the rate of depth change (S. Blackwell, personal
communication). The latter method provides accurate
calibration of the velocity meter if any portion of a dive is near
vertical. Observations from the surface and video recordings
indicate that this condition was met in the present study. The
depth sensor was calibrated before and after the experimental
period. The accuracy of the depth sensor was tested on a
pressure station at 0–1500 psi (0–10.4 MPa) and was found to
be linear over the test range (r2=0.99) with a mean standard
deviation of ±0.2 %. Depth and velocity were recorded at 1 s
intervals throughout the dives.
Ten dives to 16 m were conducted inside San Diego Bay,
CA, USA. During these trials, the dolphin followed a boat
(Boston Whaler, 21 foot) to the dive site, where the instrument
package was placed on the dolphin and secured using a strap.
An acoustic pinger attached to a video camera was lowered to
16 m. The camera was cabled to a monitor on the boat and used
to confirm the animal’s arrival at depth. Following a signal
from the trainer, the dolphin submerged to the pinger. On
arrival, the acoustic signal was turned off and the animal
returned immediately to the boat. A rest period of at least 1 min
was provided between dives. The mean rest period was 46±23 s
before the dolphin voluntarily began diving.
Eighteen dives of 50–112 m were conducted in the open
ocean approximately 5 miles (8.1 km) off the coast of San
Diego, CA, USA. To avoid fatigue during these deep diving
tests, the dolphin was transported by boat to the dive site,
where it was immediately returned to the water. The instrument
package was placed on the dolphin, and the acoustic pinger
was lowered to the test depth (50 or 100 m). Testing procedures
were as described for 16 m dives. Recovery periods averaged
2.5 min between dives, during which the respiratory rate of the
animal was monitored. Respiratory rate was determined by
counting the number of breaths taken during the first minute
immediately following the dive (Williams et al., 1999).
Analysis
The swimming mode and kinematics of uninstrumented
dolphins were determined from video sequences from the
hand-held camera using a motion-analysis system (Peak
Performance Technologies, Inc.; Englewood, CO, USA). Each
swimming or diving segment was converted to digital format.
Anatomical points of interest (for details, see Fig. 1A) were
manually digitized for 1–60 images per second of video
recording. The acceleration, deceleration, angular acceleration
2752 R. C. SKROVAN AND OTHERS
and speed of each point were then computed. In addition,
stroke amplitude and the distance traveled by the dolphins
while stroking or gliding were assessed for each video
sequence. Changes in the amplitude (as a proportion of total
body length) of the anatomical points were calibrated against
the measured length of each dolphin.
Video images from the instrument package worn by the
dolphin on deep dives were copied onto VHS tapes with data
overlay from the time/depth recorder and velocity meter. The
annotated video recording was analyzed ‘frame by frame’ for
patterns in swimming mode and type of stroke. Strokes were
categorized as large, medium, small or gliding according to the
arc swept by the fluke. Stroke type was correlated to changes
in depth, speed and acceleration of the dolphin.
Drag and buoyant forces were determined from videotaped
sequences of horizontally swimming or vertically diving
dolphins, respectively. Total body drag was calculated by
multiplying the measured rate of deceleration of horizontally
gliding dolphins by the total mass decelerated. Deceleration
was determined from the change in speed at 1 s intervals and
averaged over the glide period. The mass of dolphin S6 was
177 kg, and the mass of the instrument package was 14 kg.
Because accelerating a body within a fluid also involves
accelerating the surrounding fluid (Daniel, 1984; Lovvorn et
al., 1991; Vogel, 1981), we accounted for the mass of the
entrained water moving with the dolphin. This is equivalent
to the mass of water displaced by the animal multiplied by
the coefficient of added mass (0.06 for a prolate spheroid of
fineness ratio 5.0; Vogel, 1981). On the basis of this
calculation, the mass of the entrained water was 11 kg, and
the total mass of the instrumented animal moving through the
water was 202 kg. It is likely that the coefficient of added
mass used in these calculations is conservative for a dolphinshaped body and that the actual added mass may be greater
because of water entrained by body contours or fins.
Calculations based on a less-streamlined shape (i.e. a fineness
ratio of 4.0) result in only a 2.1 % increase in the predicted
total mass of the instrumented animal. Such a difference
would not significantly alter our calculations for body drag
and buoyant force.
Buoyancy in diving dolphins was calculated from the
differences in deceleration between vertical and horizontal
glides. The changes in buoyancy were ascribed to changes in
volume with depth due to the compression of air spaces by
water pressure. For dolphins, the lungs represent an important,
compressible air space. Air in the lungs imparts a buoyant force
equal to the amount of water displaced according to
Archimedes’ principle (Giancoli, 1984). During diving,
pressure increases by 1 atm (98.1 kPa) for every 10 m increase
in depth (Heine, 1995). Because volume varies inversely with
pressure, the lung volume of the dolphins will decrease with
depth. On the basis of these principles, the change in air
volume of the lungs is described by:
VD = VS/(1 + 0.1h) ,
(1)
where VD is air volume in liters at depth, VS is the air volume
in liters at the surface, and h is depth in meters. The buoyant
force at any depth can be determined for the dolphin from lung
volume added to the buoyant force of its body. The resulting
equation is:
BD = VDg + BB ,
(2)
where BD is the buoyant force in newtons at depth, VD is the
air volume in liters at depth from equation 1, g represents the
acceleration due to gravity (9.8 m s−2) and BB is buoyancy in
newtons of the dolphin’s body without air (−33.2 N for dolphin
S6; see equation 7). Note that the air volume in liters is
equivalent to the mass of the displaced water in kilograms.
During vertical glide sequences, upward buoyant forces oppose
the downward pull of gravity. The resultant force will hereafter
be referred to as a positive buoyant force when the net force is
upwards and as a negative buoyant force when the net force is
downwards.
Effects of instrumentation on dolphin performance
Previous studies indicate that the addition of recording
instruments may alter the performance of an aquatic animal by
increasing drag and by adding inertial mass (Wilson et al.,
1986; Boyd et al., 1997). The total frontal area of the
instrument package in the present study represented
approximately 22 % of the dolphin’s frontal area. The
instruments were evenly divided between each side of the
dolphin, with the front end of the instruments being tapered to
minimize drag. We determined the changes in total body drag
of the dolphins due to the instrument package by comparing
horizontal glide deceleration for instrumented and
uninstrumented animals. Behavioral and mechanical effects of
instrument drag were also assessed by comparing the stroke
type, stroke frequency and speed of instrumented and
uninstrumented dolphins. Data for uninstrumented dolphins
were obtained from digital analysis of video recordings taken
by a SCUBA diver. For the instrumented dolphin, data were
obtained from video recordings as well as from velocity and
depth recorders in the instrument package.
Statistics
Linear and curvilinear regressions were determined from
least-squares methods using Sigma Plot (Jandel Scientific,
1995). Sigma Stat software (Jandel Scientific, 1995) and Zar
(1974) were used for t-tests of paired data. Sums-of-squares
analyses for curves were calculated using SuperAnova
software. Values for significance were set at P<0.05. Means
are reported ±1 S.E.M.
Results
Swimming gaits of bottlenose dolphins
Similar to previous reports (Videler and Kamermans, 1985;
Fish and Hui, 1991), we found that the entire body of the
dolphin oscillates as it swims. An undulatory wave progresses
behind the dorsal fin down the peduncle to the fluke hinge and
finally to the fluke tip (Fig. 1). The dorsal fin moves out of
phase with the rostrum and fluke. Maximum upward excursion
Biomechanics of diving dolphins 2753
frequency of these movements were dependent on the speed
and power requirements of the animal (Table 2). Both the
present study and that of Fish (1993) found no change in fluke
amplitude during steady swimming over the range of test
speeds. Three different patterns or gaits were observed.
A
Large-amplitude strokes
The largest stroke amplitudes (representing 20–50 % of
body length) occurred at the start of horizontal swims and at
the beginning of the descent and ascent phases of dives. The
amplitude of these strokes exceeded the range reported for
steadily swimming dolphins in an aquarium pool at speeds
ranging from 1.2 to 6.0 m s−1 (Fish, 1993). The use of this gait
corresponded to the periods of greatest acceleration
(3.5–4.7 m s−2). The amplitudes for all body segments were
larger than those observed during steady swimming. The
greatest change in amplitude occurred at the rostrum and was
four times that of steady swimming. In comparison, the fluke
and dorsal fin regions more than doubled their amplitude
during periods of acceleration, while the mid-peduncle region
showed the least change (Table 2).
As a result of the methodology, only the movements of the
flukes could be recorded during deep dives or horizontal swims
at depths exceeding 14 m. Large-amplitude strokes were used
during the initial 1–2 s of horizontal swims and initial descents
of dives. The period for large-amplitude stroking increased up
to 5 s during the initial ascent from 50 and 100 m dives.
B
Position (m)
1.2
1.0
0.8
0.6
0.4
3.5
3.7
3.9
4.1
Time (s)
4.3
4.5
4.7
Fig. 1. Video image (A) and range of movement (B) of four
anatomical sites during a single stroke for a bottlenose dolphin
swimming horizontally next to a boat at 1.5 m s−1. Colored squares in
the picture correspond to the line colors illustrating the movements
for each site. Note that the dorsal fin (dark blue) reaches its
maximum excursion first, followed sequentially by the peduncle
(red), the fluke hinge (green) and finally the fluke tip (pink).
of the fluke tip occurs as the dorsal fin is at the bottom of its
cycle; the reverse occurs on the downstroke. Each anatomical
site differed in range of movement. The amplitude and
Medium-amplitude strokes
Medium-amplitude strokes (approximately 20 % of body
length) occurred during steady-state swimming at
1.5–3.7 m s−1. Motion of the head was reduced in comparison
with that occurring in association with large-amplitude strokes.
The arc of the rostrum covered only 5 % of body length during
medium-amplitude stroking. Similarly, the dorsal fin showed
comparatively smaller amplitude movements.
There was a significant (P=0.05) increase in the frequency
of medium-amplitude strokes with speed during steady
swimming over the range 0.6–3.7 m s−1 (Fig. 2A). Dolphins
Table 2. Primary locomotor modes of swimming and diving bottlenose dolphins
Gait
Use
Large-amplitude
Acceleration
Medium-amplitude
Cruising
Glide
Energy conservation
Amplitude
(% body length)
R:D:F
Duration
Stroke frequency
(Hz)
Brief
(1–5 s)
>0.43×speed
(1.5 to >3 Hz)
20:10:40
Declines rapidly
as speed increases
Extended
(1 s to >1 min)
0.43×speed
(0.5–3 Hz)
5:5:20
Dependent on dive depth
(1–50 s)
0
0:0:0
Stroke frequency increased linearly with speed during steady swimming (see text). During periods of acceleration, stroke frequency was
higher than indicated by this relationship.
Relative changes in stroke amplitudes are given for the rostrum (R), dorsal fin (D) and fluke (F) for each gait.
2754 R. C. SKROVAN AND OTHERS
Stroke frequency (Hz)
3
Glides
Dolphins incorporated short (3–14 m) and long (>14 m)
glide sequences during activity. Short glides occurred at the
end of every ascent or descent, as the dolphin came to a stop
or changed direction. Ascent glides ranged from 6 to 14 m in
distance traveled (mean 9.3±2.5 m, N=28) and showed a mean
deceleration of 0.07±0.12 m s−2 (N=10). Uninstrumented
dolphins also demonstrated short periods of gliding associated
with burst-and-glide propulsion during both horizontal
submerged swimming and diving. These resulted in brief
periods of deceleration before stroking resumed. The
instrumented dolphin limited burst-and-glide propulsion to
diving periods. We attribute the absence of burst-and-glide
activity during horizontal swimming to the added drag of the
instrument package. Long-distance gliding was an important
component of the descent phase of dives for all dolphins. Glide
distance varied with depth as described below.
A
2
1
0
B
Distance per stroke (m)
5
4
3
2
1
0
0
1
2
Speed (m s-1)
3
4
Fig. 2. Stroke frequency (A) and distance traveled per stroke (B) in
relation to swimming speed for bottlenose dolphins. Data for
instrumented (filled circles) and uninstrumented (open circles)
dolphins are compared. Solid and dashed lines denote the leastsquares linear regressions through the data points. Regressions for
stroke frequency show a significant difference between the
instrumented and uninstrumented animals (P<0.05). The distance
moved per stroke was independent of speed in both groups of
dolphins. Equations for the regression lines are given in the text.
with and without instrumentation showed linear increases in
stroke frequency (f) with speed (v), but differed in the
magnitude of the response. The regression for uninstrumented
dolphins was:
f = 0.43v
(3)
(N=30, r2=0.90, P<0.05), where stroke frequency is in
strokes s−1 (Hz) and speed is in m s−1. The stroke frequency of
the instrumented dolphin was approximately 27 % higher at
comparable speeds to the uninstrumented dolphins and was
described by the equation:
f = 0.54v
(4)
(N=41, r2=0.73, P<0.05). The distance traveled per stroke
(Fig. 2B) did not change significantly with speed over the
range tested for either the instrumented (r2=0.008, N=41) or
uninstrumented (r2=0.01, N=30) dolphins. The mean distance
per stroke was approximately 0.5 m (27 %) less for the
instrumented dolphin than for uninstrumented dolphins
swimming at comparable speeds.
Variations in gait
In addition to the three distinct gaits described above,
dolphins utilized several variations of these patterns. Smallamplitude strokes (<20 % of body length) occurred
intermittently as animals made the transition between active
swimming and gliding. These smaller strokes also occurred
between periods of medium-amplitude stroking. A variety of
braking motions that included holding the fluke up, down or to
either side were used by dolphins to decelerate.
Locomotor mode during swimming and diving
Horizontal swimming
Horizontal swimming by dolphins near the water surface or
submerged at 5–16 m involved similar locomotor modes. For
horizontal distances less than 15 m, the dolphins initially
accelerated using large-amplitude strokes, followed by a period
of decreasing stroke amplitude and finally passive gliding to
the end point. The initial acceleration enabled the dolphins to
reach speeds of 2.0–3.5 m s−1 in less than 2 s. Longer periods
of steady-state swimming on the water surface at 1.5–3.7 m s−1
were accomplished by medium-amplitude stroking. Glide
periods during steady-state swimming rarely exceeded 2 s.
Shallow dives
All shallow dives matched one of the following patterns with
minor variation. Dives to 12 m by uninstrumented dolphins
began with one or two large-amplitude strokes, resulting in a
travel speed of 2.0 m s−1. Starting at a depth of 4–6 m, the
animals glided for approximately 5 m before braking or veering
into a horizontal glide. Because of the short distance involved,
the dolphins were able to glide to the surface after one or two
medium-amplitude strokes at the start of the ascent. Dives to
16 m by the instrumented dolphin also began with a short
period of active stroking followed by a short glide. The dolphin
actively swam downwards for 9.0±1.9 m (N=10) before gliding
the remaining 7.1±1.9 m (N=10). After braking, the dolphin
used large-amplitude strokes to begin the ascent. Mediumamplitude strokes were used throughout the mid portion of the
Biomechanics of diving dolphins 2755
0
60
A
Distance of glide (m)
Depth (m)
-20
-40
-60
-80
-100
-120
Speed (m s-1)
Ascent
40
30
20
10
B
(7)*
Descent
50
(10)
(10)
(10)
(10)
(7)
0
2
16
50
100
Dive depth (m)
1
0
20
40
60
80 100
Elapsed time (s)
120
140
160
Fig. 3. Representative changes in depth (A) and speed and stroke
pattern (B) in relation to dive time for an instrumented dolphin.
Maximum dive depth was 112 m. Grey bars denote periods of
stroking in which glide periods were less than 1 s in duration. Open
areas show periods of continuous gliding or stationing. Black bars
indicate braking at the end of the descent and ascent. Note the
change in deceleration at 60 s midway through the gliding decent.
ascent. As the dolphins approached the surface, stroke
amplitude decreased to zero, with the dolphin gliding the
remaining 9.1±2.6 m (N=10).
Deep dives
The instrumented dolphin performed ten dives to depths of
50 m and eight dives to depths of 100–112 m. As observed for
shallow dives, the dolphin used large-amplitude strokes to
begin the descent, followed by medium-amplitude stroking.
Intermittent stroking patterns incorporating short periods of
gliding between active stroking often occurred during deep
dives (Fig. 3). These periods of intermittent propulsion were
characteristic for descents and ascents of deep dives but were
not observed for shallow dives.
The percentage of time spent gliding during descent changed
with depth for the diving dolphins. During 50 m dives, the
dolphin glided for 30.3±2.8 % (N=10) of the descent. This
increased significantly (at P<0.001) to 51.2±3.3 % (N=8)
during the 100–112 m dives.
Glide distance for the 50 m dives, 12.3±3.6 m (N=10), was
not significantly different (at P=0.21) from the average for
16 m dives. However, glide distance during the descent
increased significantly (P=0.01) with dive depths greater than
50 m. The total length of the glide was 43.6±7.0 m (N=8)
during 100–112 m dives (Fig. 4). Glides occurring during the
ascent showed no significant changes with depth (P=0.27).
The speed of the dolphins during diving was correlated with
Fig. 4. Glide distance during descent (open columns) and ascent
(filled columns) segments of dives in relation to depth for bottlenose
dolphins. The height of the columns and lines shows the mean value
+1 S.E.M. Numbers in parentheses indicate the total number of dives
examined. An asterisk indicates a significant difference between the
descent and ascent values for 100 m dives. Glide distances were not
significantly different (at P<0.05) between ascent and descents for
dives ranging from 16 to 50 m. In contrast, significant differences (at
P<0.001) were found between glide distances for the descent and
ascent segments of the 100 m dives.
gliding or stroking periods. An example is shown in Fig. 3.
During stroking on the descent of a 112 m dive, the speed of
the dolphin was approximately 1.9 m s−1. Cessation of stroking
resulted in a period of deceleration that was followed by a
constant speed of 1.2 m s−1 during the remainder of the
descending glide. Speed during the ascent was more variable
and corresponded with burst-and-glide activity (Fig. 3).
As with shallower dives, a braking motion occurred at the
end of descent, followed by large-amplitude strokes at the
beginning of ascent. Average glide distance to the surface was
10.4±2.2 m (N=10) on 50 m dives and 8.5±2.9 m (N=8) on
100–112 m dives.
Drag and buoyant forces
The drag of the dolphins increased significantly with speed
and was comparatively higher for the instrumented dolphin
(Fig. 5). The least-squares curvilinear regression for the
instrumented dolphin was:
D = 1.78 + 8.93v2.99
(5)
(r2=0.64, P=0.220, N=6). The regression for uninstrumented
dolphins was:
D = 4.15v2.00
(6)
(r2=0.65, P=0.097, N=5), where drag (D) is in newtons and
speed (v) is in m s−1 for both equations.
For deep-diving dolphins, measured deceleration during
gliding changed with depth because of changes in buoyant
force. For example, the mean depth during prolonged (>2 s)
descending glides was 67.5±23.0 m, with a glide speed of
1.5±0.3 m s−1 and a mean deceleration of 0.03±0.06 m s−2
2756 R. C. SKROVAN AND OTHERS
100
Drag force (N)
80
Instrumented
60
40
Uninstrumented
20
0
0
1
2
Speed (m s-1)
3
4
Fig. 5. Body drag in relation to horizontal glide speed for
instrumented (filled circles) and uninstrumented (open circles)
bottlenose dolphins. Solid lines denote the least-squares curvilinear
relationships through the data points. All glide sequences took place
at depths greater than three body diameters below the water surface
to avoid surface wave effects. Speed represents the mean speed
during each glide sequence. Equations for the relationships are given
in the text.
(N=27 glide sequences). The decelerating force acting on
vertically diving dolphins, calculated from the product of
deceleration (0.03 m s−2) and the mass of the instrumented
dolphin including entrained water (202 kg), was 6.1 N. This
compares with a drag of 31.8 N for gliding dolphins moving
horizontally at the same speed (Fig. 5; equation 5).
Presumably, the drag of the vertically diving dolphin was
countered by a downward force of 25.7 N (31.8−25.7=6.1)
(Table 3). Similar calculations for the ascent phase
demonstrate the positive effect of buoyancy as a dolphin nears
the water surface. The mean depth of gliding for ascent from
instrumented dives was 5.5±2.2 m, with a mean speed of
1.6±0.2 m s−1 and deceleration of 0.07±0.12 m s−2 (N=10). The
product of deceleration and mass is 14.1 N for the dolphins on
a vertical ascent. In comparison, the calculated drag for
horizontally gliding dolphins moving at 1.6 m s−1 is 38.4 N
(equation 5). Thus, the final ascent drag was countered by an
upward buoyant force of 24.3 N (38.4−24.3=14.1).
From these calculations, we find that the buoyant force
acting on the diving dolphins in this study changed from
+24.3 N near the water surface (5.5 m depth) to −25.7 N at a
depth of 67.5 m, a difference of 50.0 N. This is equivalent to a
change in water displacement of 5.1 l (50.0 N/9.8 m s−2=5.1 kg
or approximately 5.1 l of water). Such a change in
displacement is reasonable since dolphins dive following
inspiration and air compresses with depth. From equation 1, an
initial lung volume of 8–10 l would be needed to achieve this
magnitude of volume change in the diving dolphin, which is
within the reported range for a 177 kg dolphin (Ridgway et al.,
1969; Stahl, 1967).
Fig. 6 illustrates the changes in buoyant force of 8.5 l of air
with depth for diving dolphins. This curve and the calculated
buoyancy of the dolphin differed consistently by 33.2 N, and
we assume that this was due to the weight of the dolphin’s
body. Thus, for the instrumented dolphin, diving with a lung
volume of 8.5 l, equation 2 becomes:
BD = 83/(1 + 0.1h) − 33.2 ,
(7)
where BD is the buoyant force in newtons at depth h in m, 83 N
is the buoyant force of 8.5 l of air and −33.2 N is the buoyancy
of the dolphin’s body without air. The net force acting on the
gliding dolphin can then be calculated from the difference
between this buoyant force and total body drag (Fig. 5).
The above calculations are appropriate for gliding dolphins
in which swimming motions are absent. To calculate the drag
on swimming dolphins, we need to account for the additional
drag due to locomotor movements. A conservative estimate of
this active drag is three times that of the gliding animal
(Lighthill, 1975, 1971; Webb, 1975, 1984; Williams and
Kooyman, 1985; Fish, 1993).
Table 3. Locomotor variables, total body drag and buoyant forces for a bottlenose dolphin during a 100 m dive
Deceleration
(m s−2)
Speed
(m s−1)
Net force
(N)
Descent glide
0.03±0.06
(27)
1.5±0.3
(27)
6.1
Ascent glide
0.07±0.12
(10)
1.6±0.2
(10)
Descent swimming
Ascent swimming
Drag
(N)
Buoyancy
(N)
Depth
(m)
31.8
−25.7
67.5
−14.1
−38.4
24.3
5.5
1.7±0.0
160.6
136.3
24.3
5.5
1.9±0.0
−213.6
−187.9
−25.7
67.5
Gliding and swimming during ascent and descent are compared.
Deceleration, speed and depth were measured for a dolphin wearing an instrument pack.
Net force, drag and buoyancy were calculated as described in the text. Drag calculations for swimming on ascent and descent include an
active drag factor of 3 (Fish, 1993; Lighthill, 1971, 1975; Webb, 1975, 1984) to account for the additional drag associated with swimming
movements.
All downward forces relative to the water surface are indicated by a negative sign.
Numbers in parentheses indicate N for the measured variables.
Biomechanics of diving dolphins 2757
120
A
100
Buoyant force (N)
performance of uninstrumented dolphins performing deep
dives.
11 l
8.5 l
4l
80
60
40
20
0
0
20
40
60
Depth (m)
80
100
B
0.5
Deceleration (m s-2)
0.4
0.3
0.2
0.1
0
-0.1
-0.2
0
20
40
60
80
Depth (m)
100
120
140
Fig. 6. Changes in the buoyant force of lung air (A) and deceleration
(B) in relation to dive depth in bottlenose dolphins. Calculations for
buoyant force are based on equations presented in the text and are
compared for three different initial lung volumes. Note the rapid
decline in buoyant force with depth as the water pressure
progressively collapses the lungs. The decline in deceleration of
gliding dolphins (B) determined from video analyses paralleled that
calculated for buoyant force. Each point represents an individual
glide sequence. The solid line in B is not a regression for the data,
but rather the calculated deceleration based on buoyancy changes
with depth (see text).
Effects of instrumentation
Average speeds during diving and horizontal swimming
were 9–10 % lower for the instrumented dolphin than for
uninstrumented dolphins. Drag was 3.3 times higher for the
instrumented dolphin at the mean gliding speed of 1.47 m s−1
(Fig. 5). The elevated drag resulted in a 27 % reduction in
distance achieved per stroke and a concomitant increase in
stroke frequency (Fig. 2). Although stroke amplitude appeared
to be higher for the instrumented dolphin, differences in
measurement techniques for instrumented and uninstrumented
animals prevented accurate comparisons. Because of the
demonstrated effects of the instrumentation on drag and
swimming mechanics, the glide distances reported here
should be considered as conservative estimates of the true
Discussion
The importance of gait transitions during swimming and
diving
Foraging aquatic mammals must divide their time between
two important resources, oxygen located at the water surface
and prey items located at depth (Dunstone and O’Connor,
1979). The swimming modes selected by mammals moving
between these resources will affect their locomotor efficiency
and, ultimately, the cost/benefit relationships for foraging.
Previous studies with dolphins have shown that elevating
swimming speeds during ascent and descent to decrease the
duration of a dive leads to an extraordinarily rapid depletion
of limited oxygen reserves (Williams et al., 1993). Travel too
slowly, however, and time becomes limiting as basal metabolic
demands exhaust the available oxygen (Williams et al., 1999).
Data from the present study demonstrate that bottlenose
dolphins tailor their swimming patterns to diving depth, a
strategy that leads to energetic efficiency (Figs 3, 4). As found
for running animals (Heglund et al., 1974; Taylor, 1978; Hoyt
and Taylor, 1981), changes in gait by swimming and diving
dolphins were associated with specific tasks and speeds
(Table 2; Fig. 3). Dolphins switched gaits primarily in
conjunction with acceleration needs. During initial
acceleration from rest, stroke frequencies and fluke amplitudes
often exceeded those used during steady swimming. Largeamplitude movements of the head and back accompanied these
large fluke motions. The tip of the rostrum showed an
excursion of nearly 20 % of body length, while fluke amplitude
exceeded 40 % of body length (Table 2). As the dolphin’s
speed increased, stroke amplitude gradually decreased to the
values observed during steady-state swimming. These results
are consistent with models that predict increased mechanical
efficiency during low-speed swimming when thrust is
produced by accelerating a large mass of fluid (per time) to a
low velocity instead of accelerating a small mass to a high
velocity (Alexander, 1977).
During diving, dolphins minimized the use of largeamplitude strokes and incorporated prolonged glide periods as
speed and coincident drag increased. Large-amplitude strokes
only occurred for brief (<5 s) periods during the initial descent
and ascent. Except for these initial periods, diving dolphins
relied on medium-amplitude strokes and, when possible, even
smaller stroking movements. The smaller-amplitude strokes
occurred during transitions between steady-state stroking and
gliding, with stroke frequency remaining unchanged. These
results are not surprising when the hydrodynamics are
considered. High-amplitude movements are a departure from
the streamlined shape of the dolphin and theoretically result in
elevated levels of drag, especially as stroke amplitude is
increased (Fish et al., 1988; Lighthill, 1971; Webb, 1975). An
actively swimming animal may encounter a three- to fivefold
increase in total body drag over gliding values as a result of
2758 R. C. SKROVAN AND OTHERS
elevated pressure drag (Fish et al., 1988), separation or
thinning of the boundary layer (Lighthill, 1971) and increased
drag from thrust production (Webb, 1975). The marked effect
of even small adjustments in posture on drag and forward
movement of the dolphin was observed when an animal used
braking movements to reduce speed. Raising the fluke a
distance equivalent to 10 % of body length resulted in an 11.5fold increase in total body drag.
Although prolonged gliding allows diving dolphins to avoid
active drag, it places a limit on maintaining propulsion. To
circumvent this, dolphins and other swimmers often rely on a
burst-and-glide style of swimming that incorporates short
periods of stroking during prolonged glide sequences to
maintain forward speed (Videler, 1981; Videler and Weihs,
1982; Weihs, 1974). Despite elevated drag associated with reacceleration between glides, the calculated energetic cost for
this interrupted mode of swimming is significantly lower than
for continuous swimming (Blake, 1983).
Because dolphins produce power by oscillating their flukes
(Lang and Daybell, 1963; Slijper, 1961; Videler and
Kamermans, 1985), the mass of the fluke plus entrained water
must be decelerated to a stop then re-accelerated in the
opposite direction both at the top and bottom of each stroke.
The alternate storage and release of elastic energy in
conjunction with fluke movements could serve as a potential
energy-conserving mechanism. Changes in the axial body of
the swimming dolphin are qualitatively similar to those of
galloping terrestrial mammals in which the trunk is used as a
spring to store elastic energy (Taylor, 1978). Several springlike tissues have been implicated as energy-saving mechanisms
for swimming dolphins. Pabst (1990) described a crossed,
helically wound, fiber array encasing the dolphin body. The
fiber array, derived from ligaments, muscle tendons and
blubber tissue, gains rigidity because of the tension it is under.
This array may act as a spring, storing energy during part of
the stroke cycle and recovering it during the remainder (Pabst,
1990). Although intriguing, such elastic storage mechanisms
have yet to be tested in a freely swimming dolphin and warrant
further investigation.
The swimming mechanics of dolphins share other features
common to terrestrial animals and swimming humans. In
terrestrial mammals, stride frequency increases linearly with
speed during walking and trotting. As speed increases, many
runners switch to a gallop in which speed is achieved by
lengthening the stride rather than by increasing stride
frequency (Heglund et al., 1974). Conversely, human
swimmers decrease the distance per stroke (the aquatic
equivalent of stride length) and increase the stroke frequency
to achieve greater speeds (Costill et al., 1991). Horizontally
swimming dolphins combine both patterns and increase stroke
frequency linearly with speed while the distance per stroke
remains relatively constant (Fig. 2). Mean distance per stroke
was 2.4 m irrespective of speed. Stroke amplitude in dolphins
also remained constant during horizontal swimming, with
amplitude remaining at 20 % of body length for steady speeds
ranging from 1.2 to 6.0 m s−1 (Table 2; Fish, 1993).
Buoyancy, gliding and energy expenditure during diving
Locomotor performance by horizontally swimming and
vertically diving dolphins is influenced by very different
physical factors. During horizontal swimming near the water
surface, dolphins encounter high levels of drag associated with
wave generation (Hertel, 1969). The effects of wave drag are
negligible for diving dolphins once the animal is three body
diameters below the water surface. Diving dolphins, however,
face unique changes in buoyant forces with depth that become
a major influence on performance and behavior.
An interesting finding in this study was the use of prolonged
periods of gliding by dolphins for dives exceeding a depth of
50 m. Approximately 50 % of the descent phase was spent
gliding rather than actively swimming on dives to a depth of
100 m. Deceleration rate decreased progressively during
prolonged glides, finally reaching a point of zero deceleration
at a depth of 90 m (Fig. 6B). These extended glides occurred
only during the descent phase of deep dives (Fig. 4),
suggesting that physical factors rather than distance per se
dictated glide performance. Changes in buoyancy with depth
due to lung compression from increased pressure probably
contributed to these results. In general, dolphins dive after
inspiration and exhale upon surfacing, indicating that they dive
with inflated lungs (Ridgway et al., 1969; present study).
Goforth (1986) reported that the diving lung volume of
dolphins was approximately 75 % of maximum lung volume.
The bronchi and trachea as well as the alveoli of the cetacean
lung are collapsible, as determined in pressure chamber tests.
Only the bony nares, with a volume of 50 ml, are rigid
(Ridgway et al., 1969). Such a morphological structure permits
a progressive collapse of the thorax with increased pressure at
depth.
Compression of the air spaces in dolphins decreases volume
without an accompanying reduction in mass. As a result, the
dolphin becomes less buoyant with depth. Although it was not
possible to measure directly the volume of air in an actively
diving dolphin, the range of lung volumes and their effect on
buoyancy have been determined for excised lungs from a
200 kg bottlenose dolphin (Ridgway et al., 1969). Ridgway and
Howard (1979) calculated that alveolar collapse is complete
once bottlenose dolphins experience pressures equivalent to
65–70 m in depth. The theoretical changes in buoyant force
associated with this collapse are shown in Fig. 6. The
maximum respiratory volume of the dolphin (11 l) was
associated with neutral to slightly buoyant forces at full
inflation and with a negative buoyancy of 10 kg when the lungs
were deflated. In the present study, we found that changes in
the deceleration rate of gliding dolphins were similar in pattern
to the calculated changes in buoyant forces with lung
compression (Fig. 6), suggesting a correlation between
pressure and locomotor movements at depth.
Using this basic information, we can examine the
relationship between physical factors and the swimming
behavior of dolphins during diving. Major physical forces
include buoyancy, acting in an upward or downward direction
depending on diving depth, and drag opposing the forward
Biomechanics of diving dolphins 2759
Table 4. Calculated energetic costs for overcoming drag and
buoyancy during a 100 m dive by an adult bottlenose dolphin
Active swim
distance (m)
Descent Ascent
Locomotor
cost (J)
Descent Ascent
Total
Speed (m s−1)
Measured
53
92
1.73
1.90
26 064
Neutral buoyancy
Total time-fixed
Speed-fixed
95
95
95
95
1.76
1.73
1.76
1.90
28 608
31 465
Locomotor costs were determined from the product of net forces
(Table 3) and distance traveled.
Measured values for a dolphin wearing an instrument package are
compared with those for two models assuming neutral buoyancy.
The time-fixed model maintains the total dive time to that measured
for the diving dolphin. Swimming speed is adjusted to accommodate
the time requirement. The speed-fixed model maintains the
swimming speeds for ascent and descent to those measured for the
diving dolphin. However, the duration of the dive is adjusted to
accommodate the neutral buoyancy and speed requirements.
movement of the dolphin. For the straight-line trained dives in
the present study, total body drag acts upwards as the animal
descends and downwards relative to the motion of the dolphin
during ascent. The combined effects of these forces during
various segments of a 100 m dive by a bottlenose dolphin are
presented in Table 3. For the instrumented dolphin, passive
gliding predominated when the calculated net force opposing
the animal was less than 21 N. If the opposing force was higher,
prolonged gliding was untenable and the dolphin switched to
either stroking or short periods of burst-and-glide swimming.
This may explain in part the high proportion of gliding during
vertical diving in comparison with horizontal swimming by the
same animal. With no buoyancy advantage during horizontal
swims, the calculated drag for the speed range examined
exceeded 21 N, and little gliding occurred. As mentioned
above, descending glides during long descents reached zero
deceleration at depth of approximately 90 m depth. At this
depth, the downward force imparted by negative buoyancy
fully counteracted the calculated drag and provided the dolphin
with a theoretical ‘free ride’.
Although the progressive negative buoyancy with depth
provides a locomotor advantage during descent, the reverse
occurs during ascent. The same force that pulled the dolphin
down must be overcome for the animal to return to the surface,
seemingly negating any benefit. If energetic and hydrodynamic
factors are considered together, we find that gliding provides
an overall advantage for the diver. This is due to a significant
reduction in active drag, which contributes to the energetic
efficiency of burst-and-glide swimming (Blake, 1983). The
locomotor behavior of diving dolphins is analogous to a type
of burst-and-glide swimming, with exceptionally long glides
facilitated by changes in buoyancy with depth.
The energetic advantage of gliding may be determined
theoretically by calculating the total energy expended to
overcome drag and buoyancy (locomotor cost) in diving
dolphins. We estimated locomotor costs from the product of the
net forces and mean distance covered during gliding and
swimming (Tables 3, 4). On the basis of the straight-line dives
examined in this study, the locomotor cost for an instrumented
dolphin performing a 100 m dive was 26 064 J (0.74 J kg−1 m−1).
This compares with a minimum locomotor cost of
0.73 J kg−1 m−1 calculated from the difference between
maintenance costs and total minimum cost of transport for
bottlenose dolphins swimming near the water surface (Williams
et al., 1992; Williams, 1999). In view of the similarity in
locomotor costs between these swimmers and divers, it is
apparent that deep-diving dolphins select energetically efficient
modes of locomotion. Despite the effect of the instrumentation
(Figs 2, 5), the diving dolphins in the present study were able
to match the predicted minimum locomotor costs of swimming
dolphins by taking advantage of changes in buoyancy. The
theoretical costs are considerably higher if we assume that
dolphins are neutrally buoyant throughout the dive. If a dolphin
were neutrally buoyant and tried to maintain the same dive
duration for a 100 m dive, as measured in this study, then
locomotor costs would increase by 10 %. A neutrally buoyant
dolphin maintaining the same swimming speeds (with a
consequent shorter dive duration) for a 100 m dive experiences
a 21 % increase in energetic requirements (Table 4).
The reduction in active drag during gliding was the primary
factor leading to the energetic savings during diving rather than
changes in buoyancy per se. Because of the marked influence
of the instrument package on body drag (Fig. 5), these
calculations admittedly represent a conservative estimate of the
effects of buoyancy on gliding performance. It is likely that
uninstrumented dolphins will exhibit even greater gliding
performance with potentially greater energetic savings than
indicated in these calculations. The reduction in power
requirements and hence energetic costs associated with gliding
initially appears modest. However, the savings may provide a
significant advantage to free-ranging dolphins by allowing
extended foraging time through the conservation of limited
energy stores.
In conclusion, the present study illustrates how the
interrelationships between swimming mechanics, buoyancy
and underwater behavior support energetically efficient
locomotion in diving dolphins. Similar conclusions regarding
buoyancy and performance have been reached for other marine
animals, including free-ranging elephant seals (Mirounga
augustirostris) (Webb et al., 1998), diving ducks (Lovvorn et
al., 1991) and a variety of fish species (Alexander, 1990).
Variation in glide performance facilitated by changes in
buoyancy appears to be an important mechanism that enables
marine mammals to conserve limited oxygen stores during
submergence. A corollary to this study suggests that speed
alone is a relatively poor indicator of aquatic effort and may
be inadequate for assessing energetic costs in diving marine
mammals. Both gliding and active swimming often occur at
similar speeds. However, the energetic consequences of each
may be very different.
2760 R. C. SKROVAN AND OTHERS
This series of papers on the diving physiology of dolphins
was inspired by the work of Gerald L. Kooyman; they are
dedicated to him in celebration of his remarkable research
career and influence on all comparative physiologists. This
study was supported by an Office of Naval Research grant
and ASSEE fellowship (N00014-95-1-1023) to T.M.W. The
authors thank the many people that assisted in this study
including the trainers and dolphins at the SPAWAR facility in
San Diego and the Dolphin Experience in the Bahamas.
Computer assistance by S. Collier at Texas A&M University
is gratefully acknowledged. In addition, the authors appreciate
the critical evaluation of this manuscript by S. Noren, D.
Noren and S. Kohin and lively discussions with F. Fish. All
experimental procedures were evaluated and approved
according to animal welfare regulations specified by NIH
guidelines. UCSC and SPAWAR Systems Center (San Diego)
conducted institutional animal use reviews.
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