Optimization of swim depth across diverse taxa during horizontal travel

Semiaquatic taxa, including humans, often swim at the air-water interface where they waste energy generating surface waves. For fully marine animals however, 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 to 3 times the depth of their body diameter, to minimize the vertical distance traveled while avoiding wave drag close to the surface. This has rarely been examined, however, due to depth measurement resolution issues at the surface. Here, we present evidence for the use of this strategy in the wild to the nearest centimeter and document the switch to shallow swimming during naturally occurring long-distance migrations. Using high-resolution depth-accelerometry and video data for little penguins (Eudyptula minor) and loggerhead turtles (Caretta caretta), satellite-relayed data for green turtles (Chelonia mydas), and literature data for further sea turtle, penguin, and whale species, we show that near-surface swimming is likely used broadly across nonforaging diving animals to minimize the cost of transport.