Marine vertebrates, such as sharks and seals, swim in undulating patterns to conserve energy, similar to birds changing altitude, reports a study in Nature Communications this week. This finding implies convergent evolution in the locomotion patterns of flying and swimming.Adrian Gleiss and colleagues equipped animals with accelerometers which recorded the swim patterns of four marine vertebrates including white and whale sharks, and northern fur and elephant seals. The recordings showed the animal performing undulating flight, where passive gliding is interspersed with upward movement in the water. This ascent acts as propulsion, which the authors now show to be used by animals to extend energy reserves whilst moving through water as well as air.