Webb P M, Crocker D E, Blackwell S B, Costa D P, Boeuf B J
Department of Biology, University of California, Santa Cruz, CA 95064, USA.
J Exp Biol. 1998 Aug;201(Pt 16):2349-58. doi: 10.1242/jeb.201.16.2349.
Marine mammals experience radical seasonal changes in body composition, which would be expected to affect their buoyancy in the water. The aim of this study was to examine the relationship between such changes in buoyancy and diving behavior in northern elephant seals Mirounga angustirostris. This was achieved by modifying the buoyancy of 13 juvenile elephant seals translocated from Año Nuevo State Reserve, CA, USA, and released at various sites in Monterey Bay, CA, USA. The buoyancy of each seal was calculated and was increased or decreased using syntactic foam or lead weights, and their diving behavior was recorded as they returned to Año Nuevo. The seals were divided into three groups: increased buoyancy (B+), reduced buoyancy (B-) and control seals (Bc). Mean descent rates were 0.77+/-0.3 ms-1 for the B+ seals, 0.82+/-0.2 ms-1 for the control seals and 0.87+/-0.3 ms-1 for the B- seals, and were significantly different. Mean ascent rates for the three treatments were 0.82+/-0.3 ms-1 for the B+ seals, 0.86+/-0.3 ms-1 for the control seals and 0.82+/-0.3 ms-1 for the B- seals. All the B+ seals ascended faster than they descended, while four of the five B- seals descended faster than they ascended. There was a significant negative correlation between buoyancy and descent rate, with less buoyant seals descending faster than more buoyant seals. There was, however, no correlation between ascent rate and buoyancy. This suggests that seals may use negative buoyancy to drift passively during descent, but that all seals may swim continuously during ascent. There was a significant correlation between buoyancy and the drift descent rate of C-type drift dives, including upwards drift in the most buoyant seal. Buoyancy was not correlated with diving depth, trip duration, dive duration or surface-interval duration. This study demonstrates that buoyancy plays a significant role in shaping diving behavior in northern elephant seals and that elephant seals may adjust their behavior to suit their buoyancy, rather than adjusting their buoyancy to suit a dive. This study also validated the truncated cones method of calculating body composition in this species by comparing it with body composition determined using tritium dilution.
海洋哺乳动物的身体组成会经历剧烈的季节性变化，预计这会影响它们在水中的浮力。本研究的目的是检验在北海象海豹（Mirounga angustirostris）中，这种浮力变化与潜水行为之间的关系。通过改变13只从美国加利福尼亚州阿诺新国家自然保护区转移并在美国加利福尼亚州蒙特雷湾不同地点放归的幼年象海豹的浮力来实现这一目的。计算每只海豹的浮力，并用复合泡沫或铅块增加或降低其浮力，在它们返回阿诺新时记录其潜水行为。这些海豹被分为三组：浮力增加组（B+）、浮力降低组（B-）和对照组（Bc）。B+组海豹的平均下潜速度为0.77±0.3米/秒，对照组为0.82±0.2米/秒，B-组为0.87±0.3米/秒，差异显著。三种处理方式下的平均上升速度，B+组海豹为0.82±0.3米/秒，对照组为0.86±0.3米/秒，B-组为0.82±0.3米/秒。所有B+组海豹上升速度比下潜速度快，而五只B-组海豹中有四只下潜速度比上升速度快。浮力与下潜速度之间存在显著的负相关，浮力较小的海豹比浮力较大的海豹下潜速度更快。然而，上升速度与浮力之间没有相关性。这表明海豹在下潜过程中可能利用负浮力被动漂移，但所有海豹在上升过程中可能持续游泳。在C型漂移潜水中，浮力与漂移下潜速度之间存在显著相关性，包括最具浮力的海豹向上漂移的情况。浮力与潜水深度、行程持续时间、潜水持续时间或水面间隔时间均无相关性。本研究表明，浮力在塑造北海象海豹的潜水行为中起重要作用，并且象海豹可能会调整其行为以适应自身浮力，而不是调整浮力以适应潜水。本研究还通过将截断圆锥体法计算该物种身体组成的结果与使用氚稀释法测定的身体组成结果进行比较，验证了该方法。
