Researchers have established that marine animals travel at optimal depths roughly three times their body size to conserve energy, minimizing wave drag.
This pattern, confirmed through advanced tracking technologies, holds true across species from penguins to whales.
Marine Animal Swim Depths
Researchers from Swansea and Deakin Universities have discovered that marine animals, including mammals, birds, and reptiles, swim at similar relative depths when traveling without feeding. This behavior helps them conserve energy by reducing water resistance.
The research, led by Dr. Kimberley Stokes, Professor Graeme Hays, and Dr. Nicole Esteban, involved six institutions across five countries. They studied the swimming depths of various sea turtles, penguins, and whales. The team found that these animals typically travel at depths about three times their body length from the surface. This depth represents a ‘sweet spot’ that minimizes both wave formation at the surface and the vertical distance the animals must move through the water.
Energy Efficiency in Aquatic Travel
In contrast, some semi-aquatic animals like mink swim at the surface, where creating waves wastes significant energy. For marine birds, mammals, and reptiles that undertake long-distance migrations, evolving to swim at energy-efficient depths is crucial for reducing the cost of travel over their lifetimes.
It has long been known that additional drag from wave creation minimizes once a traveling object is at depths greater than three times its diameter, but it was hard to compare with travel depths of wild animals due to tracking limitations.
Technological Advancements in Animal Tracking
In this new study published today (December 16) in Proceedings of the National Academy of Sciences (PNAS) near surface swim depths were recorded to within 1.5 centimeters in little penguin and loggerhead turtles, along with motion data and video footage from animal-borne cameras. This was compared with satellite tracking data for long-distance migrations in green turtles and data from other studies on penguins and whales. It was found that these animal swim at optimal depths predicted from physics when either ‘commuting’ to a foraging patch in the wild or migrating over longer distances while not feeding.
Swansea University’s Dr. Kimberley Stokes, lead author of the study said:
“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. 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 to about 20 m in length.”
Reference: “Optimization of swim depth across diverse taxa during horizontal travel” by Kimberley L. Stokes, Nicole Esteban, Paolo Casale, André Chiaradia, Yakup Kaska, Akiko Kato, Paolo Luschi, Yan Ropert-Coudert, Holly J. Stokes and Graeme C. Hays, 16 December 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2413768121
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