Sea Mammal Research Unit, School of Biology, University of St Andrews, Fife KY16 9QQ, UK.
J Exp Biol. 2012 Oct 15;215(Pt 20):3622-30. doi: 10.1242/jeb.070128.
Efficient locomotion between prey resources at depth and oxygen at the surface is crucial for breath-hold divers to maximize time spent in the foraging layer, and thereby net energy intake rates. The body density of divers, which changes with body condition, determines the apparent weight (buoyancy) of divers, which may affect round-trip cost-of-transport (COT) between the surface and depth. We evaluated alternative predictions from external-work and actuator-disc theory of how non-neutral buoyancy affects round-trip COT to depth, and the minimum COT speed for steady-state vertical transit. Not surprisingly, the models predict that one-way COT decreases (increases) when buoyancy aids (hinders) one-way transit. At extreme deviations from neutral buoyancy, gliding at terminal velocity is the minimum COT strategy in the direction aided by buoyancy. In the transit direction hindered by buoyancy, the external-work model predicted that minimum COT speeds would not change at greater deviations from neutral buoyancy, but minimum COT speeds were predicted to increase under the actuator disc model. As previously documented for grey seals, we found that vertical transit rates of 36 elephant seals increased in both directions as body density deviated from neutral buoyancy, indicating that actuator disc theory may more closely predict the power requirements of divers affected by gravity than an external work model. For both models, minor deviations from neutral buoyancy did not affect minimum COT speed or round-trip COT itself. However, at body-density extremes, both models predict that savings in the aided direction do not fully offset the increased COT imposed by the greater thrusting required in the hindered direction.
在深度处的猎物资源和在表面处的氧气之间高效移动，对于屏住呼吸的潜水者最大限度地延长觅食层中的停留时间并从而最大化净能量摄入率至关重要。潜水者的身体密度会随身体状况而变化，决定了潜水者的表观重量（浮力），这可能会影响往返于表面和深度的运输成本（COT）。我们评估了外部功和执行器盘理论对非中性浮力如何影响往返于深度的往返 COT 以及稳态垂直过渡的最小 COT 速度的替代预测。毫不奇怪，这些模型预测当浮力有助于（阻碍）单向运输时，单程 COT 会减少（增加）。在偏离中性浮力的极端情况下，在终端速度下滑翔是浮力所辅助的方向上的最小 COT 策略。在浮力阻碍的过渡方向上，外部功模型预测在更大的偏离中性浮力时最小 COT 速度不会改变，但根据执行器盘模型预测最小 COT 速度会增加。正如之前对灰海豹的记录，我们发现 36 只象海豹的垂直过渡速度在两个方向上都随着身体密度偏离中性浮力而增加，这表明与外部功模型相比，执行器盘理论可能更接近预测受重力影响的潜水者的功率需求。对于这两种模型，略微偏离中性浮力不会影响最小 COT 速度或往返 COT 本身。但是，在身体密度极端情况下，这两种模型都预测，在辅助方向上的节省并不能完全抵消在受阻方向上所需的更大推力所带来的增加的 COT。
