Factors affecting stroking patterns and body angle in diving Weddell seals under natural conditions
1 National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo
173-8515, Japan
2 Graduate University for Advanced Studies, 1-9-10 Kaga, Itabashi, Tokyo
173-8515, Japan
3 Department of Ecology, Evolution and Behavior, University of Minnesota,
100 Ecology, St Paul, Minnesota 55455 USA
Present address: National Marine Mammal Laboratory/Alaska Fisheries Science
Center, NOAA, 7600 Sand Point Way NE, Seattle, Washington 98115, USA
* Author for correspondence (e-mail: ksato{at}nipr.ac.jp)
Accepted 27 January 2003
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Summary |
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Key words: acceleration data logger, body angle, stroke frequency, prolonged glide, stroke-and-glide, Weddell seal, Leptonychotes weddellii.
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Introduction |
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Oxygen stores are depleted through metabolic processes in the locomotory
muscles. Therefore, efficient locomotion is important for air-breathing divers
(Skrovan et al., 1999). When
submerged, they must balance the energetic demands of movement with the
conservation of a limited oxygen store
(Castellini et al., 1985
;
Skrovan et al., 1999
). High
levels of exercise will presumably lead to the termination of a dive as oxygen
reserves are quickly depleted (Skrovan et
al., 1999
). Aquatic animals use a variety of strategies to reduce
the cost of locomotion. For example, burst-and-glide swimming in fishes
promotes energy conservation (Weihs,
1974
; Fish et al.,
1991
). Williams et al.
(2000
) demonstrated that
prolonged gliding in Weddell seals affected their recovery oxygen consumption
at the sea surface between dives. They monitored a seal's flipper movements
using a backward-looking video system, mounted on the seal's back, and a
tail-mounted accelerometer. However, these observations were of seals
translocated to an isolated hole drilled through sea ice over deep water and
away from any existing breeding colonies
(Davis et al., 1999
).
Using acceleration data loggers, Yoda et al.
(2001) developed a new
technique for monitoring the behavior of free-ranging penguins. They showed
that an acceleration profile could be used for detecting fine-scale movements
(lying, standing, walking, tobogganing, diving, resting at the water surface
and porpoising). Similarly, Tanaka et al.
(2001
) described the tail
movements and body angle of chum salmon Oncorhynchus keta. We
monitored the flipper movements and body angle of free-ranging Weddell seals
Leptonychotes weddellii at their own breeding sites using small data
loggers that can record swimming speed, depth, two-dimensional (2-D)
accelerations (flipper movements and body angle) and temperature. Acceleration
data were used to define different swimming strategies and their pattern of
use. Changes in swimming behaviors are discussed with respect to optimal
stroking patterns according to individual and geographic conditions.
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Materials and methods |
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Index of fatness
The proportion of positively buoyant tissues, such as blubber, to other
tissues, such as bone and muscle, influences a seal's body composition and
overall buoyancy. Crocker et al.
(1997) and Beck et al.
(2000
) suspected that these
factors could affect the diving behaviors of northern elephant seals
Mirounga angustirostris and grey seals Halichoerus grypus.
Suspecting that buoyancy may similarly affect diving Weddell seals, we
employed Stirling's `index of fatness' (calculated as axillary girth/standard
length) (Stirling, 1971
) to
quantify the body condition of each instrumented female. Using data reported
by Fujise et al. (1985
) on the
external measurements and organ masses of five Weddell seals collected at
Syowa Station, we calculated the linear regression:
![]() | (1) |
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Multi-sensor data logger
A multi-sensor data logger (UWE1000-PD2GT: 22 mm diameter, 124 mm length;
80 g in air; Little Leonardo Corp., Tokyo, Japan) was used to record swimming
speed and depth at 1 s intervals, 2-D accelerations (for determining flipper
movement and body angle) at 1/16 s intervals, and temperature at 30 s
intervals. The cross-sectional area of the instrument was less than 0.2% of
the maximal cross-sectional area of an adult Weddell seal and, therefore, we
felt that any influence on swimming behavior would be minimal
(Wilson et al., 1986). The
logger uses a two-axis acceleration sensor (Model ADXL210, Analog Devices,
Inc., Norwood, USA). The sensor can measure both dynamic acceleration (such as
propulsive activities) and static acceleration (such as gravity), allowing it
to be used as a tilt sensor. The measuring range of the accelerometer is
±49 m s2 with a resolution of 0.02 m s-2.
However, the sensitivity of the sensor can be affected by thermal changes. The
temperature was therefore simultaneously recorded to allow for
post-hoc data corrections. This was unnecessary, however, as the
ambient saltwater temperature remained constant at 1.8°C for the
duration of the experiment.
Swimming speed was calculated using the rotation (revs s1) of an external propeller. However, the flow of water across the propeller may also vary with the location of the instrument or the girth of the seal. Therefore, the rotation value was converted to actual swimming speed (m s1) using a calibration line that was estimated for each animal. The calibration line was created from a linear regression of revs s1 against a second independent method of calculating swimming speed. Information on the body angle (from an acceleration sensor along the longitudinal axis of the seal) and vertical speed (as determined from the depth recorder) provided us with an independent estimate of the actual swimming speed. For example, an animal with a vertical speed of 1.3 m s1 and a body angle of 60° would have a true swimming speed of 1.5 m s1 (i.e. 1.3 m s1/sin60). This second method is only reliable for steeper body angles, however, and as such it was not used as the primary method of determining swimming speed. Calibration lines were obtained from each animal with correlation coefficients higher than 0.944 (Table 2). T he propeller of one seal did not rotate well and was not used (Table 2). Rotation values of revs s1 were not converted to swimming speed when they were lower than the stall revs s1 of the logger, determined experimentally to be 0.3 m s1.
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The maximum range of the depth sensor was 1000 m, with a resolution of 0.24
m. The beginning or end of a dive was considered to be the moment when depth
was greater than, or less than, 2 m, respectively. Dive duration was defined
as the time elapsed between the start and end of the dive. Dive depth was
defined as the maximum depth of the dive. Bottom time, which was assumed to be
the time spent in the foraging area, was defined as the duration from the
first ascent to last descent in each dive. According to Kooyman
(1968), dive duration is
positively related to the number of breaths after the dive rather than the
number of breaths before the dive. Therefore, post-surface time was used in
analyses of dive recovery time. In these analyses, we only used dives with a
maximum depth greater than 50 m. The measuring range of the temperature sensor
was 22°C to 50°C, with a resolution of 0.018°C and an
accuracy of 0.1°C.
Acceleration data analysis
A preliminary experiment was conducted on a captive male (body mass 76 k g)
Larga seal Phoca largha in the Minamichita Beachland Aquarium on June
19, 1998. Equipped with a PD2GT logger, the seal was released in an outdoor
pool and its behavior was monitored using a video camera (DCR-TRV9, Sony). The
resulting acceleration profiles were compared to the visual record, confirming
that side-to-side flipper movements could be detected as fluctuations in
acceleration along the transverse axis (hereafter referred to as swaying
acceleration). Additionally, fluctuations in acceleration along the
longitudinal axis (surging acceleration) and the estimated body angle of the
seal were consistent (Sato and Naito,
2002).
We attached PD2GT loggers to free-ranging adult female Weddell seals to record their flipper stroke frequencies and body angle. Swaying acceleration often contained low frequency variations that were assumed to be the result of various turning and rolling movements. These variations were separated using a 0.1 Hz highpass filter (IFDL Version 3.1; WaveMatrics, Inc., USA). The remaining peaks and troughs with absolute amplitudes greater than 0.5 m s2 were considered to be strokes and used in analyses. Peaktrough duration or troughpeak duration corresponds to a single flipper stroke (i.e. left-to-right or right-to-left). Flipper stroke frequencies (Hz) during descending and ascending were calculated from the total number of strokes divided by the duration of each phase (ascending and descending) for each dive deeper than 50 m.
The acceleration sensor along the longitudinal body axis of the body
measured the surging accelerations, which are affected by both the forward
movements of the animal and gravity (Yoda
et al., 2001; Tanaka et al.,
2001
). High frequency variations in the surging acceleration
record are believed to be caused by flipper movements. These were filtered out
using a 0.1 Hz lowpass filter (IFDL Version 3.1; WaveMatrics, Inc., USA). As
described by Tanaka et al.
(2001
), when the animal is
still or moving at a constant speed, the gravity vector will change in
response to the body angle. Together, these vectors were used to calculate the
body angle. Fig. 2A shows the
direction of the surging acceleration recorded by a PD2GT logger attached on
the back of a seal. A descending seal would have surging accelerations and
body angles represented as negative values. Body angles and surging
accelerations of ascending seals are positive.
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The data logger was attached to the seal's back at the point of maximum
girth. But length from the nose to the logger varied from 43% to 63% of the
standard length (from the nose to the end of the tail). This distance was
decided according to the degree of fatness of each seal, with `fatter' seals
having a relatively longer distance. It was impossible to align the PD2GT
logger exactly parallel to the longitudinal axis of a seal. Therefore, it was
necessary to calculate this difference as an adjustment angle
(Fig. 2A). The relationships
between surging acceleration A(i) (m s2), the
acceleration of gravity g (=9.8 m s2), body
angle
(i) (degrees) and the adjustment angle
(degrees)
can be expressed by:
![]() | (2) |
![]() | (3) |
![]() | (4) |
![]() | (5) |
![]() | (6) |
The adjustment angle for a specific dive can therefore be
determined through iterative procedures using Equation 6.
Fig. 3 shows the results of one
dive calculated using several values for Dg. This procedure was repeated on
five dives for each animal, the mean of which was used as the adjustment angle
for the given seal. The mean adjustment angle varied between seals; however,
reliable values could be obtained for each seal with small standard deviations
(S.D.) (Table 2).
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The results are presented as means ± S.D. Statistical analysis
followed Zar (1984). We used
StatView (version5.0) for statistical tests and considered results
statistically significant if P<0.01.
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Results |
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Comparisons of diving behaviors at both sites revealed that seals at Big Razorback Island (hindered from diving vertically to reach deeper water by the island's more gradual slope) conducted dives with a longer dive duration, a longer travel time, a longer post-surface time (except for Jumbo), and shallower body angles than seals instrumented at Turks Head (where there was a steeper bathymetric slope) (Fig. 1, Table 1). The data presented in Fig. 4 describe a typical dive at each site. The bottom times of both seals were similar (8.9 min and 8.3 min), but Sarah from Big Razorback Island had a longer travel time (descent time plus ascent time = 16.9 min) than Mina (6.4 min) from Turks Head. Additionally, Sarah's mean descent and ascent body angles were ±30°, compared to Mina's steeper angles of ±50° (Fig. 4).
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From the swaying acceleration data, `Sarah' adopted a stroke-and-glide method throughout her dive (i.e. both descending and ascending) (Figs 4A, 5A). In contrast, Mina's flipper movements were only substantial during the beginning of her descent, gliding from about halfway through her descent until reaching the bottom (Fig. 4B). After continuous stroking at the beginning of her ascent the remainder was characterized by a stroke-and-glide pattern similar to Sarah's (Fig. 4B). This stroke-and-glide behavior is defined by intermittent strokes and corresponding fluctuations in swimming speed (Fig. 5A,B). Swimming speed either increased or remained constant while stroking continuously (Fig. 5B,D); interestingly, gliding seals also managed to keep their swimming speed constant (Fig. 5C).
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There was a significant negative relationship between the mean stroke frequencies during descent and ascent (Spearman rank correlation = 0.427, N=179 dives of eight seals, P<0.0001; Fig. 6), indicating that seals who descended using lower frequency strokes, ascended with higher frequency strokes, and vice versa. Seals with a mean stroke frequency ratio (i.e. ascent/descent) <1 were characterized as stroke-and-glide swimmers, while those with a mean stroke frequency ratio >1 were characterized as prolonged gliders. Interestingly, the five thinner seals were categorized as prolonged gliders, while the three fatter seals were categorized as stroke-and-glide swimmers (Table 1, Fig. 6). Comparing swimming methods among four seals at Big Razorback Island, where the gradual slope prevents seals from vertical dives, only Jumbo, a thinner seal, was characterized as a prolonged glider. All seals at Turks Head, where seals can dive vertically, adopted a prolonged glide method independent of their body angles.
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Table 4 compares the results between stroke-and-glide swimmers and prolonged gliders. During a descent, the stroke frequency of stroke-and-glide swimmers (0.68±0.16 Hz) was greater than that of prolonged gliders (0.30±0.21 Hz), yet prolonged gliders had a faster swimming speed (1.7±0.3 m s1 versus 1.3±0.2 m s1). In comparison, the stroke frequency of prolonged gliders during the ascent phase (0.99±0.28 Hz) was greater than that of stroke-and-glide swimmers (0.40±0.20 Hz), and prolonged gliders had a slightly faster swimming speed (1.6±0.3 m s1 versus 1.5±0.3 m s1). There was no significant difference in total stroke frequency (descent and ascent) between stroke-and-glide swimmers (0.54±0.23 Hz) and prolonged gliders (0.65±0.43 Hz). The ratio of the post-surface time to dive duration in stroke-and-glide swimmers (0.44±0.25) was significantly larger than that for prolonged gliders (0.29±0.42).
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Discussion |
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For thinner, negatively buoyant seals, body angle should also be important.
While descending, steeper body angles would bring the vector of forward motion
closer to that of the force of gravity, making it easier to descend. Among the
prolonged gliders at Turks Head, the more stout seals, Windy and Mina, had
much steeper body angles (Table
1), presumably enabling them to glide, and this would also
decrease the time necessary to reach the bottom. Andrea was the thinnest and
also had the most shallow body angle (Table
1), which enabled her to reach further for exploration. Jumbo,
despite being a prolonged glider, had a relatively shallow body angle
(Table 1); however, she was
diving at Big Razorback Island and was restricted from steeper angles by the
shape of the slope at that colony (Fig.
1B). Similarly, as each of the three fatter seals were at Big
Razorback Island, it is perhaps not surprising that they were all
stroke-and-glide swimmers, considering their inability to descend at steep
angles. Unfortunately, the two fatter seals instrumented at Turks Head (with
IF = 0.83 and 0.81) did not dive deeper than 50 m
(Sato et al., 2002a),
therefore we could not fully test this hypothesis. Overall, a wide variety of
factors including topography, seal morphology and perhaps prey distribution
could affect their body angles.
Davis et al. (2001)
identified three modes of swimming (prolonged gliding, stroke-and-glide and
continuous stroking), based on the interval between strokes. We chose to
categorize diving behavior based also on relative swimming speeds because we
found transitions between these categories to be gradual. While continuously
stroking, swimming speed was constant (Fig.
5D) or accelerated (Fig.
5B), indicating that thrust was equal to or exceeded the drag and
any external forces such as gravity or buoyancy. This can be expressed more
formally as:
![]() | (7) |
![]() | (8) |
Gravity gliding among diving animals has been reported previously for the
Weddell seal, northern elephant seal Mirounga angustirostris, bottle
nose dolphin Tursiops truncatus and the blue whale Balaenoptera
musculus (Skrovan et al.,
1999; Williams et al.,
2000
), while the corollary, buoyant gliding, has been reported in
right whales, a northern elephant seal, penguins and some flying birds
(Nowacek et al., 2001
;
Davis et al., 2001
;
Sato et al., 2002b
;
Ross, 1976
;
Tome and Wrubleski, 1988
;
Stephenson et al., 1989
;
Lovvorn, 1994
;
Watanuki et al., 2003
).
Buoyant gliding in penguins is accomplished by changing the volume of
gas-filled cavities using water pressure. This creates a dynamic buoyancy
component that varies with depth and may actually accelerate the swimming
speed of some ascending penguins (Sato et
al., 2002b
). The swimming speed of gravity-gliding Weddell seals
did not change with respect to depth (Fig.
4B) because body tissues such as blubber, bone and muscle are less
compressible than air. However, in both cases, external net forces such as
gravity and buoyancy, are equal to or larger than the drag of gliding animals,
enabling them to glide for long periods with a constant or increasing
speed.
Finally, a surprising result is that despite the extra effort necessary for
seals to reach deep depths at Big Razorback Island (due to the shallow body
angles required by bathymetry) versus Turks Head, the maximum depth
and bottom time of seals at the two colonies were not statistically different
(Table 1). Theoretical studies
by Thompson and Fedak (2001)
and Mori et al. (2002
)
predicted a positive correlation between patch quality and patch residence
time. If true, differences in prey density (higher at Big Razorback Island)
may explain this result. Using digital still cameras mounted on the back of
diving adult Weddell seals, a concurrent study calculated a higher index for
prey distribution at Big Razorback Island than at Turks Head
(Watanabe et al., in press
).
Further quantitative investigation is needed to verify this as a proximate
cause.
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Acknowledgments |
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