Watanabe Yuuki, Baranov Eugene A, Sato Katsufumi, Naito Yasuhiko, Miyazaki Nobuyuki
Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan.
J Exp Biol. 2006 Sep;209(Pt 17):3269-80. doi: 10.1242/jeb.02402.
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 (1,027-1,046 kg m(-3), roughly corresponding to 32-41% lipid content, for the weighted condition; 1,014-1,022 kg m(-3), 43-47% lipid content, for the unweighted condition).
浮力是作用于用肺呼吸的潜水者的主要外力之一，它会影响其游泳能量学。由于海洋哺乳动物的身体组成（即低密度脂质和高密度瘦肉组织的相对含量）随个体和季节而变化，它们的浮力也会大幅波动，预计个体在潜水时会相应地调整其划水模式。为了验证这一预测，我们将加速度数据记录器附着在贝加尔湖四只自由活动的贝加尔海豹（Phoca sibirica）身上，监测鳍状肢划水活动以及游泳速度、深度和身体轴线（俯仰）的倾斜度。除了记录器外，一只海豹（个体4）还配备了一个铅坠，在预定时间段后丢弃，以便我们对同一海豹在不同身体密度下进行一系列观察。这四组数据集揭示了贝加尔海豹的一般潜水模式，也为浮力对这些模式的影响提供了直接见解。海豹反复进行平均时长7.0分钟（最长15.4分钟）的潜水，平均水面停留时长为1.2分钟。潜水深度平均为66米，但差异很大，最大深度为324米。海豹个体之间表现出不同的划水模式；一些海豹在下降时的划水频率低于上升时，而另一些海豹在下降时的划水频率高于上升时。当个体4身上的铅坠被取下后，这只海豹通过将游泳模式从长时间滑行转变为更多的划水与滑行游泳，增加了下降时的划水频率，并通过从持续划水转变为划水与滑行游泳，降低了上升时的划水频率。我们得出结论，海豹根据其个体浮力采用不同的划水模式。我们还证明，个体4在下降时长时间滑行过程中达到的终端速度取决于其总浮力和俯仰，在有铅坠状态下以及俯仰更陡时速度更高。一个简单的物理模型使我们能够根据速度和俯仰估算出海豹的身体密度（有铅坠状态下为1027 - 1046千克/立方米，大致对应32 - 41%的脂质含量；无铅坠状态下为1014 - 1022千克/立方米，43 - 47%的脂质含量）。
