To minimise energy consumption during long journeys, marine animals do not swim on the surface, but at a depth that corresponds to about three times the diameter of their bodies. This common stratagem among birds, mammals and reptiles was discovered in a study coordinated by the Universities of Swansea in the UK and Deakin in Australia, and published in the journal PNAS - Proceedings of the National Academy of Sciences USA, in which Professors Paolo Luschi and Paolo Casale of the Department of Biology at the University of Pisa participated.
“Many semi-aquatic species, including humans, often swim at the air-water interface, generating surface waves that cause a waste of energy,” explains Luschi. “Marine animals that travel long distances during their lives have instead developed a strategy to avoid this waste by swimming just below the surface, like swimming athletes do right after departure when they stay underwater as long as possible before emerging for the first time.”
Caretta caretta turtle with detection system. Credits: DEKAMER archive
The research analysed data obtained from depth gauges applied to sea turtles and penguins during phases of active movement, which were then supplemented with data from existing scientific literature on whales. The results showed that such different animals use the same strategy of swimming at a depth corresponding to three times their diameter, irrespective of their size, which can range from 50 centimetres to 20 metres.
“Obviously, there are cases where swimming depth is determined by other factors, such as the search for prey,” Luschi points out, “but the general data confirm the model, which has important conservation implications, helping to reduce collisions with vessels and accidental catches during fishing.”
In particular, the University of Pisa contributed to the research by providing data on the Caretta caretta turtles nesting in Turkey. “This type of data is extremely difficult to obtain,” adds Casale, “because it is too detailed to be transmitted by satellite and therefore requires special experimental procedures to recover the instruments used on free-roaming animals”.
The research group led by Professors Luschi and Casale has long used advanced animal telemetry techniques to study the behaviour of sea turtles during their extensive movements, which include migrations of up to hundreds of kilometres.