It takes a lot of energy for marine mammals, birds, and reptiles to get around. Some humpback whale (Megaptera novaeangliae) populations travel 5,000 miles every year. Magellanic penguins (Spheniscus magellanicus) migrate along the coasts of Argentina and Chile in search of food. To conserve crucial energy, migratory animals have adapted to swim at similar relative depths when they are traveling and not feeding, no matter how big or small their bodies are. This “sweet spot” is described in a study published December 16 in the journal Proceedings of the National Academy of Sciences (PNAS).
Some semi-aquatic animals–including mink–generally swim at or near the water’s surface. At this depth, wave generation is a primary source of wasted energy. However, for marine mammals, birds, and turtles that travel great distances over their lives, minimizing the energetic cost of moving through the water is necessary for particularly on long journeys.
The additional drag from wave creation can be reduced once a traveling object moves to a depth greater than three times its diameter. Comparing these drag-reduced depths with the actual travel depths of wild animals has been difficult due to just how hard it is to track animals in the wild.
[ Related: Why do birds migrate? Scientists have a few major theories. ]
In the new study, a team from six academic institutions from five countries compared the swim depths of several sea turtle and speies and crossed referenced it with data on some whales. Little penguins (Eudyptula minor) and loggerhead turtles (Caretta caretta) swam to within 1.5 centimeters–or about half an inch–of the surface. Motion data and video footage from cameras affixed to some of the animals was also used and compared with satellite tracking data for long-distance migrations in green turtles and the data from other studies of whales and penguins.
They found that some of the penguins, turtles, and whales in the available data travelled at about three times their body diameter down from the surface in a drag-reducing sweet spot. The animals also swam at this depth whether they were moving towards a foraging ground or when swimming over a long distance when not feeding. This sweet spot minimizes the wave formation at the surface and the vertical distance that the animal is traveling.
“There are of course examples where animal swim depth is driven 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,” Kimberley Stokes, a study co-author and sea turtle ecologist/conservationist at Swansea University in the United Kingdom, said in a statement. “This has rarely been recorded because of the difficulty in retrieving depth data from animals that migrate over large distances, so it was great to find enough examples to show a common relationship between swim depth and body size from animals across the size spectrum from 30 cm [11.8 inches] to about 20 m [65 feet] in length.”
This way, animals big and small can do all that they can to keep up their strength to find food and ultimately thrive.
