Optimization of swim depth across diverse taxa during horizontal travel

Semi-aquatic taxa, including humans, often swim at the air-water interface where they waste energy generating surface waves. For marine animals, theory predicts the most cost-efficient depth-use pattern for migrating, air-breathing species that do not feed in transit is to travel at around 2–3 times the depth of their body diameter, to minimise vertical distance travelled whilst avoiding wave drag close to the surface. This has rarely been examined, however, due to depth measurement resolution issues at the surface. Using satellite-relayed data for green turtles (Chelonia mydas), high-resolution depth-accelerometry and video data for little penguins (Eudyptula minor) and loggerhead turtles (Caretta caretta), and literature data for further sea turtle, penguin and whale species, we show that near-surface swimming is likely used broadly across non-foraging diving animals to minimise the cost of transport.

Methods
See methods detailed in 'Optimization of swim depth across diverse taxa during horizontal travel' and Supporting Information.