Tireless travellers: Sea turtles swim continuously during long-distance movements

While rest and sleep are crucial to animals, our understanding of whether and how long-distance migrants rest has been thwarted by the inability to relay high-resolution data from multi-channel loggers via satellite. We overcame these obstacles for an iconic long-distance migrator by equipping five loggerhead sea turtles (Caretta caretta) with satellite tags and data-loggers providing depth and 3-D acceleration measurements. Turtles were translocated to open-sea locations and induced to complete oceanic migrations of tens of km to return to their nesting beach performing active, oriented movements. Across a total of >600 hours of high-resolution data, we observed (i) constant flipper frequency (ca. 0.5 Hz) indicating that turtles never ceased movement, (ii) intense subsurface swimming (mostly around 1 m) for about 50% of time and (iii) deeper, less active dives up to 80 m, which were made day and night and more frequently in offshore waters. Flipper beat amplitude was much smaller in deep dives; hence the estimated energy expenditure was 37% lower on deep dives compared to subsurface swimming. These findings suggest that turtles, which can complete migrations of >2000 km, alternate between phases of intense near-surface swimming and periods of lower activity at depth, without fully resting during migration.

尽管休息与睡眠对动物至关重要，但受限于无法通过卫星中继多通道记录仪的高分辨率监测数据，我们对长途迁徙动物是否休息、以及如何开展休息行为的相关认知始终受阻。针对这一标志性的长途迁徙物种——蠵龟（Caretta caretta），我们攻克了上述难题：为5只蠵龟佩戴了卫星标签与可采集深度及三维加速度数据的数据记录仪。研究人员将这些海龟转移至远海区域，诱导其以主动定向的运动方式完成数十公里的远洋迁徙并返回筑巢海滩。在总计超过600小时的高分辨率监测数据中，我们观测到三类核心现象：其一，海龟的鳍肢拍打频率始终保持恒定（约0.5 Hz），表明其从未停止运动；其二，约50%的监测时长内，海龟会在近表层水域开展高强度游泳（水深多集中于1米左右）；其三，昼夜均会进行深度可达80米的低活性下潜，且该行为在远海海域更为频发。深潜时海龟的鳍肢拍打幅度显著更小，因此其深潜阶段的估算能量消耗相较于近表层游泳降低了37%。上述研究结果表明，能够完成2000公里以上迁徙的蠵龟，会在高强度近表层游泳阶段与低活性深潜阶段之间交替进行，迁徙全程未出现完全休息的状态。