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Factors affecting stroking patterns and body angle in diving Weddell seals under natural conditions

Katsufumi Sato^1,2,*, Yoko Mitani², Michael F. Cameron^3,, Donald B. Siniff³ and Yasuhiko Naito^1,2

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

View larger version (17K): [in a new window]   Fig. 1. (A) Study sites (closed circles) near McMurdo Station, Antarctica. Land and the ice tongue are represented by gray color. The sea (white) was covered by fast ice during the study period. (B) The bathymetry of Big Razorback Island and (C) Turks Head. The location of ice holes and tidal cracks are shown by black horizontal bars (B,C).

View larger version (19K): [in a new window]   Fig. 2. (A) Schematic diagram showing the direction of surging acceleration A(i), recorded by a PD2GT logger placed on a seal, and gravity g (=9.8 m s–2). The angle of the logger is composed of the body angle of a seal (i) and the adjustment angle . Descending body angles are represented as negative values. (B) A schematic of a dive profile calculated from swimming speed S(i) and body angle (i).

View larger version (21K): [in a new window]   Fig. 3. The comparison between a measured dive profile (black line) and calculated dive profiles (red line) using several adjustment angles . An adjustment angle of 13.3° is appropriate for this dive.

View larger version (33K): [in a new window]   Fig. 4. Typical dives at (A) Big Razorback Island, with a gradual slope, and (B) Turks Head, with a steep slope. The x-axis is time, swimming speed is in red, and dive depth is in black; the black dots over the green lines represent strokes recorded as swaying acceleration, and the vertical dashed lines delineate the separation of the dive into three phases: descent, bottom and ascent. Swimming behavior is categorized as continuous stroking (black horizontal bar), stroke-and-glide swimming (grey horizontal bar) and prolonged gliding (open horizontal bar). Blue lines represent the body angle calculated from surging acceleration.

View larger version (32K): [in a new window]   Fig. 5. Typical profiles of swimming speed (red lines), swaying acceleration (green lines), and flipper strokes (black dots) that correspond with stroke-and-glide swimming (grey horizontal bar), prolonged gliding (open horizontal bar) and continuous stroking (black horizontal bar).

View larger version (21K): [in a new window]   Fig. 6. The relationships between the mean stroke frequencies during descent and ascent. The thick diagonal line, representing identical stroke frequencies in descent and ascent, divides seals into prolonged gliders or stroke-and-glide swimmers. The numbers in parentheses indicate the given seals' fatness index. Note that stroke-and-glide swimmers are the `fatter' seals. r, Spearman rank correlation.

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