It takes a lot of energy for marine mammals, birds and reptiles to move. A humpback whale (Megaptera novaeangliae) population groups travel 5,000 kilometers annually. Magellanic penguins (Spheniscus magellanicus) migrate along the coasts of Argentina and Chile looking for food. To conserve crucial energy, migratory animals have adapted to swim at similar relative depths when traveling and not feeding, regardless of how large or small their bodies are. This “sweet spot” is described in a study published Dec. 16 in the journal Proceedings of the National Academy of Sciences (PNAS).
Some semi-aquatic animals, including mink–generally swim at or near the surface of the water. At this depth, wave generation is a primary source of wasted energy. However, for marine mammals, birds and turtles that travel large distances during their lives, minimizing the energetic costs of traveling through water is necessary, especially during long journeys.
The extra drag resulting from the creation of waves can be reduced once it is a traveling object moves to a depth greater than three times its diameter. Comparing these reduced drag depths to the actual travel depths of wildlife has been difficult because it is so difficult to track wildlife.
[ Related: Why do birds migrate? Scientists have a few major theories. ]
In the new studya team of six academic institutions from five countries compared the swimming depths of different sea turtles and species and cross-referenced this with data on some whales. Little penguins (Eudyptula small) and loggerhead sea turtles (Caretta Caretta) swam to within 1.5 centimeters (or about half an inch) of the surface. Movement data and video footage from cameras mounted on some animals were also used and compared with satellite tracking data for long-distance migrations in green turtles and data from other studies of whales and penguins.
They found that some of the penguins, turtles and whales traveled in the available data about three times their body diameter from the surface at a drag-reducing sweet spot. The animals also swam at this depth, whether they were on their way to a foraging area or when swimming a long distance when they were not feeding. This sweet spot minimizes wave formation on the surface and the vertical distance the animal travels.
“There are of course examples where animals’ swimming depth is determined by other factors, such as searching for prey, but it was exciting to find that all published examples of non-foraging, air-breathing marine animals followed the predicted pattern,” says Kimberley Stokes, co-author of a study and ecologist/sea turtle conservationist at Swansea University in the UK, said in a statement. “This has rarely been recorded due to the difficulty in retrieving depth data from animals migrating over large distances, so it was great to find plenty of examples showing a common link between swimming depth and body size of animals across the size spectrum from 30cm onwards. [11.8 inches] up to about 20 meters [65 feet] in terms of length.”
In this way, animals large and small can do everything they can to maintain their strength to find food and ultimately thrive.
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