Tireless travellers: sea turtles swim continuously during homing 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 multichannel 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 three-dimensional acceleration measurements. Turtles were translocated to open-sea locations and induced to complete oceanic journeys of 10 km to return to their nesting beach, performing active, oriented movements. Across a total of >600 h of high-resolution data, we observed (1) a constant flipper frequency indicating that turtles never ceased movement, (2) intense subsurface swimming for about half the time and (3) 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 lower on deep dives compared with 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 extended homing journeys.