Great hammerhead sharks swim on their side to reduce transport costs – that's the surprising title of a new scientific survey in Nature Communications.
Sharks’ dorsal fins usually help to propel them along and generate lateral hydrodynamic forces when they’re turning, while their pectoral fins generate vertical forces that offset negative buoyancy.
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In great hammerheads (Sphyrna mokarran), unusually, the dorsal fins are longer than the pectoral fins. The sharks have drastically reconfigured the normal function of their fins, say the researchers, and swim rolled onto one side to exaggerate the effect of the dorsal fin. By doing this, they generate lift and so reduce drag.
Tagging and videoing two great hammerheads in Australia and Belize revealed that they spent up to 90% of their time swimming at roll angles of between 50° and 75°. Hydrodynamic modelling then showed that this behaviour reduced drag by around 10%.
The Great Barrier Reef shark, monitored for 18 hours, would roll onto either its left or right side every 5-10 minutes, whether it was ascending, descending or swimming at constant depth. The Belize shark, monitored for almost three days, behaved similarly, swimming at 30-80° roll angles, although it tended to swim more upright during the day than at night. Captive specimens have been observed swimming in the same way.
As swimming sideways would seem to hinder use of the cephalofoil with which the sharks detect electrical signals from prey, the scientists conclude that the development offers hydrodynamic rather than foraging advantages, and that “efficient travel is a strong selective agent in driving the evolution of animals”.
The research was led by Nicholas Payne of London’s University of Roehampton in the UK and Gil Iosilevskii from Technion in Israel.
