Adachi Taiki, Maresh Jennifer L, Robinson Patrick W, Peterson Sarah H, Costa Daniel P, Naito Yasuhiko, Watanabe Yuuki Y, Takahashi Akinori
Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Tokyo, Japan
Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA, USA.
Proc Biol Sci. 2014 Dec 22;281(1797). doi: 10.1098/rspb.2014.2120.
Foraging theory predicts that breath-hold divers adjust the time spent foraging at depth relative to the energetic cost of swimming, which varies with buoyancy (body density). However, the buoyancy of diving animals varies as a function of their body condition, and the effects of these changes on swimming costs and foraging behaviour have been poorly examined. A novel animal-borne accelerometer was developed that recorded the number of flipper strokes, which allowed us to monitor the number of strokes per metre swam (hereafter, referred to as strokes-per-metre) by female northern elephant seals over their months-long, oceanic foraging migrations. As negatively buoyant seals increased their fat stores and buoyancy, the strokes-per-metre increased slightly in the buoyancy-aided direction (descending), but decreased significantly in the buoyancy-hindered direction (ascending), with associated changes in swim speed and gliding duration. Overall, the round-trip strokes-per-metre decreased and reached a minimum value when seals achieved neutral buoyancy. Consistent with foraging theory, seals stayed longer at foraging depths when their round-trip strokes-per-metre was less. Therefore, neutrally buoyant divers gained an energetic advantage via reduced swimming costs, which resulted in an increase in time spent foraging at depth, suggesting a foraging benefit of being fat.
觅食理论预测，屏气潜水者会根据游泳的能量消耗来调整在深度觅食的时间，而游泳能量消耗会随浮力（身体密度）变化。然而，潜水动物的浮力会随其身体状况而变化，而这些变化对游泳成本和觅食行为的影响却很少被研究。我们开发了一种新型的动物携带式加速度计，它可以记录鳍状肢划水的次数，这使我们能够监测雌性北象海豹在长达数月的海洋觅食洄游过程中每游一米的划水次数（以下简称每米划水次数）。随着负浮力的海豹增加其脂肪储备和浮力，在浮力辅助方向（下降）每米划水次数略有增加，但在浮力阻碍方向（上升）则显著减少，同时游泳速度和滑行持续时间也发生了相应变化。总体而言，当海豹达到中性浮力时，往返每米划水次数减少并达到最小值。与觅食理论一致，当海豹往返每米划水次数较少时，它们在觅食深度停留的时间更长。因此，中性浮力的潜水者通过降低游泳成本获得了能量优势，这导致在深度觅食的时间增加，表明肥胖具有觅食优势。
