Data from: Body condition changes at sea: Onboard calculation and telemetry of body density in diving animals

The ability of marine mammals to accumulate sufficient lipid energy reserves is vital for mammals' survival and successful reproduction. However, long-term monitoring of at-sea changes in body condition, specifically lipid stores, has only been possible in elephant seals performing prolonged drift dives (low-density lipids alter the rates of depth change while drifting). This approach has limited applicability to other species. Using hydrodynamic performance analysis during transit glides, we developed and validated a novel satellite-linked data logger that calculates real-time changes in body density (∝lipid stores). As gliding is ubiquitous amongst divers, the system can assess body condition in a broad array of diving animals. The tag processes high sampling-rate depth and 3-axis acceleration data to identify 5 s high pitch angle glide segments at depths >100 m. Body density is estimated for each glide using gliding speed and pitch to quantify drag versus buoyancy forces acting on the gliding animal. We used tag data from 24 elephant seals (Mirounga spp.) to validate the onboard calculation of body density relative to drift rate. The new tags relayed body density estimates over 200 days and documented lipid store accumulation during migration with good correspondence between changes in body density and drift rate. Our study provided updated drag coefficient values for gliding (Cd,f = 0.03) and drifting (Cd,s = 0.12) elephant seals, both substantially lower than previous estimates. We also demonstrated post-hoc estimation of the gliding drag coefficient and body density using transmitted data, which is especially useful when drag parameters cannot be estimated with sufficient accuracy before tag deployment. Our method has the potential to advance the field of marine biology by switching the research paradigm from indirectly inferring animal body condition from foraging effort to directly measuring changes in body condition relative to foraging effort, habitat, ecological factors, and anthropogenic stressors in the changing oceans. Expanding the method to account for diving air volumes will expand the system's applicability to shallower-diving (<100 m) species, facilitating real-time monitoring of body condition in a broad range of breath-hold divers.

海洋哺乳动物积累充足脂质能量储备的能力，对其生存与成功繁衍至关重要。然而，长期监测其海上身体状况——尤其是脂质储备——的变化，此前仅在开展长时程漂移潜水的象海豹中得以实现：低密度脂质会改变漂移过程中的深度变化速率。该方法对其他物种的适用性较为有限。我们通过行进滑翔过程中的流体动力学性能分析，研发并验证了一款新型卫星遥测数据记录仪（satellite-linked data logger），该设备可实时计算身体密度变化（与脂质储备呈正比关系）。由于滑翔行为在潜水动物中普遍存在，该系统可对广泛类群的潜水动物身体状况进行评估。该记录仪对高采样率深度数据与三轴加速度（3-axis acceleration）数据进行处理，以识别深度大于100米处、时长5秒的高俯仰角滑翔段。通过滑翔速度与俯仰角，可估算每个滑翔段的身体密度，以此量化作用于滑翔动物的阻力与浮力。我们利用24头象海豹（*Mirounga* 属）的记录仪数据，验证了基于漂移速率的板载身体密度计算方法。新型记录仪可中继传输长达200余天的身体密度估算结果，并记录了迁徙过程中的脂质储备积累情况，身体密度变化与漂移速率呈现良好的对应关系。本研究更新了象海豹滑翔（Cd,f = 0.03）与漂移（Cd,s = 0.12）过程中的阻力系数值，二者均远低于此前的估算结果。我们还证实，可利用传输数据事后估算滑翔阻力系数与身体密度，这在记录仪部署前无法准确估算阻力参数时尤为实用。我们的方法有望推动海洋生物学领域的研究范式转变——从通过觅食努力间接推断动物身体状况，转变为直接测量变化海洋中与觅食努力、栖息地、生态因子及人为压力因子相关的身体状况变化。若将该方法拓展至考量潜水气量的范畴，可将系统的适用范围拓展至浅潜（<100米）物种，从而实现对广泛类群屏气潜水动物身体状况的实时监测。