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First published online August 17, 2006
Journal of Experimental Biology 209, 3269-3280 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02402
Body density affects stroke patterns in Baikal seals
1 Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai,
Nakano, Tokyo 164-8639, Japan
2 Limnological Institute, Siberian Division, Russian Academy of Sciences,
Ulan-Batorskaya Street 3, Irkutsk 664033, Russia
3 International Coastal Research Center, Ocean Research Institute, The
University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102,
Japan
4 National Institute of Polar Research, 1-9-10, Kaga, Itabashi, Tokyo
173-8515, Japan
* Author for correspondence (e-mail: yuuki{at}ori.u-tokyo.ac.jp)
Accepted 22 June 2006
Buoyancy is one of the primary external forces acting on air-breathing divers and it can affect their swimming energetics. Because the body composition of marine mammals (i.e. the relative amounts of lower-density lipid and higher-density lean tissue) varies individually and seasonally, their buoyancy also fluctuates widely, and individuals would be expected to adjust their stroke patterns during dives accordingly. To test this prediction, we attached acceleration data loggers to four free-ranging Baikal seals Phoca sibirica in Lake Baikal and monitored flipper stroking activity as well as swimming speed, depth and inclination of the body axis (pitch). In addition to the logger, one seal (Individual 4) was equipped with a lead weight that was jettisoned after a predetermined time period so that we had a set of observations on the same individual with different body densities. These four data sets revealed the general diving patterns of Baikal seals and also provided direct insights into the influence of buoyancy on these patterns. Seals repeatedly performed dives of a mean duration of 7.0 min (max. 15.4 min), interrupted by a mean surface duration of 1.2 min. Dive depths were 66 m on average, but varied substantially, with a maximum depth of 324 m. The seals showed different stroke patterns among individuals; some seals stroked at lower rates during descent than ascent, while the others had higher stroke rates during descent than ascent. When the lead weight was detached from Individual 4, the seal increased its stroke rate in descent by shifting swimming mode from prolonged glides to more stroke-and-glide swimming, and decreased its stroke rate in ascent by shifting from continuous stroking to stroke-and-glide swimming. We conclude that seals adopt different stroke patterns according to their individual buoyancies. We also demonstrate that the terminal speed reached by Individual 4 during prolonged glide in descent depended on its total buoyancy and pitch, with higher speeds reached in the weighted condition and at steeper pitch. A simple physical model allowed us to estimate the body density of the seal from the speed and pitch (1027-1046 kg m-3, roughly corresponding to 32-41% lipid content, for the weighted condition; 1014-1022 kg m-3, 43-47% lipid content, for the unweighted condition).
Key words: buoyancy, diving, swimming, body composition, body density, data logger, Baikal seal, Phoca sibirica
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