In order to reduce energy consumption during extended voyages, marine animals do not swim at the surface; rather, they swim at a depth that is approximately three times the diameter of their bodies. This strategy, common to birds, mammals, and reptiles was discovered by a study coordinated by the universities of Swansea in Great Britain 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 University of Pisa's Department of Biology took part. "Many semi-aquatic species, including humans, frequently swim in the air-water interface, but this generates surface waves that cause an expenditure of energy", explains Luschi. "Marine animals that travel long distances throughout their lives have developed a strategy to avoid this waste, which consists of swimming just below the surface, similar to what swimming athletes do immediately after the start, when they remain underwater as long as possible before making the first surfacing".