Biuw Martin, McConnell Bernie, Bradshaw Corey J A, Burton Harry, Fedak Mike
Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife KY16 8LB, Scotland, UK.
J Exp Biol. 2003 Oct;206(Pt 19):3405-23. doi: 10.1242/jeb.00583.
Elephant seals regularly perform dives during which they spend a large proportion of time drifting passively through the water column. The rate of vertical change in depth during these "drift" dives is largely a result of the proportion of lipid tissue in the body, with fatter seals having higher (more positive or less negative) drift rates compared with leaner seals. We examined the temporal changes in drift rates of 24 newly weaned southern elephant seal (Mirounga leonina) pups during their first trip to sea to determine if this easily recorded dive characteristic can be used to continuously monitor changes in body composition of seals throughout their foraging trips. All seals demonstrated a similar trend over time: drift rates were initially positive but decreased steadily over the first 30-50 days after departure (Phase 1), corresponding to seals becoming gradually less buoyant. Over the following approximately 100 days (Phase 2), drift rates again increased gradually, while during the last approximately 20-45 days (Phase 3) drift rates either remained constant or decreased slightly. The daily rate of change in drift rate was negatively related to the daily rate of horizontal displacement (daily travel rate), and daily travel rates of more than approximately 80 km were almost exclusively associated with negative changes in drift rate. We developed a mechanistic model based on body compositions and morphometrics measured in the field, published values for the density of seawater and various body components, and values of drag coefficients for objects of different shapes. We used this model to examine the theoretical relationships between drift rate and body composition and carried out a sensitivity analysis to quantify errors and biases caused by varying model parameters. While variations in seawater density and uncertainties in estimated body surface area and volume are unlikely to result in errors in estimated lipid content of more than +/-2.5%, variations in drag coefficient can lead to errors of >or =10%. Finally, we compared the lipid contents predicted by our model with the lipid contents measured using isotopically labelled water and found a strong positive correlation. The best-fitting model suggests that the drag coefficient of seals while drifting passively is between approximately 0.49 (roughly corresponding to a sphere-shaped object) and 0.69 (a prolate spheroid), and we were able to estimate relative lipid content to within approximately +/-2% lipid. Our results suggest that this simple method can be used to estimate the changes in lipid content of free-ranging seals while at sea and may help improve our understanding of the foraging strategies of these important marine predators.
南象海豹经常进行潜水，在潜水中它们会花费很大一部分时间在水柱中被动漂流。在这些“漂流”潜水中深度的垂直变化率很大程度上是身体中脂质组织比例的结果，与较瘦的海豹相比，较胖的海豹具有更高（更正或更负）的漂流率。我们研究了24只刚断奶的南象海豹幼崽首次出海期间漂流率的时间变化，以确定这种易于记录的潜水特征是否可用于持续监测海豹在整个觅食行程中的身体组成变化。所有海豹随着时间推移都呈现出相似的趋势：漂流率最初为正，但在出发后的前30 - 50天（阶段1）稳步下降，这与海豹逐渐变得浮力减小相对应。在接下来大约100天（阶段2），漂流率再次逐渐增加，而在最后大约20 - 45天（阶段3），漂流率要么保持不变，要么略有下降。漂流率的每日变化率与水平位移的每日变化率（每日行进速率）呈负相关，并且每日行进速率超过约80公里几乎完全与漂流率的负变化相关。我们基于在野外测量的身体组成和形态测量数据、已发表的海水和各种身体成分的密度值以及不同形状物体的阻力系数值，开发了一个机理模型。我们使用这个模型来研究漂流率与身体组成之间的理论关系，并进行了敏感性分析以量化因模型参数变化引起的误差和偏差。虽然海水密度的变化以及估计的身体表面积和体积的不确定性不太可能导致估计的脂质含量误差超过±2.5%，但阻力系数的变化可能导致误差≥10%。最后，我们将模型预测的脂质含量与使用同位素标记水测量的脂质含量进行了比较，发现两者具有很强的正相关性。最佳拟合模型表明，海豹被动漂流时的阻力系数在大约0.49（大致对应于球形物体）和0.69（长椭球体）之间，并且我们能够将相对脂质含量估计在大约±2%脂质范围内。我们的结果表明，这种简单方法可用于估计自由放养海豹在海上时脂质含量的变化，并可能有助于增进我们对这些重要海洋捕食者觅食策略的理解。
