Breaststroke, butterfly, front crawl and back crawl: many of us learnt these swimming styles as kids while splashing around in our local pool. These strokes all involve the swimmer lying on their back or front, much like the way most fish swim. Of course, some people prefer to swim tilted to one side rather than face down even though the side-stroke evokes images of elderly swimmers in bathing caps rather than speed and athletic prowess. However, a new study from a team of international researchers suggests that your grandparents’ side-tilted swimming style may have something in common with a rather different type of sea creature: the great hammerhead shark.
Nicholas Payne from the University of Roehampton, UK, Gil Iosilevskii from the Israel Institute of Technology, and colleagues from various international institutions wanted to study the swimming behavior of these large predators in nature. They first fitted triaxial accelerometers, devices used to measure body pitch and roll angles, to the dorsal fins of two great hammerhead sharks, Sphyrna mokarran, in Australia and Belize. These devices recorded the body posture of each individual as it swam freely in the wild over the next 18–66 h. The team also attached body cameras to four individuals to get a shark's eye view of the fish's swimming position. Having recorded the fish's swimming styles, the group then tested the efficiency of various swimming postures using a miniature shark model positioned in a wind tunnel to mimic the drag forces acting on real sharks while swimming. By varying the tilt and rotation of the model, they calculated how much drag versus lift was produced in different positions for a given wind speed and estimated the most efficient angle for swimming.
Initially, the team was startled by the accelerometer data. The tagged sharks spent up to 90% of their time swimming at bank angles between 50 and 75 deg; in other words, rolled on their sides rather than upright. Video footage confirmed this side-stroke swimming preference. The researchers even observed videos of sharks swimming in public aquaria in the USA to verify that untagged, unhandled great hammerheads display the same tilted swimming behaviour, which they do. Interestingly, there is a real hydrodynamic advantage to this non-traditional stroke. As sharks are negatively buoyant, most species rely on long pectoral fins to generate lift and keep them from sinking. However, unlike most sharks, the great hammerhead's dorsal fin is significantly longer than its pectoral fins. This means that swimming with its dorsal fin tilted to one side allows the great hammerhead to generate more efficient lift than it would swimming ventrally. Simulations in the wind tunnel also showed that swimming side-stroke reduces drag, and therefore the cost of swimming, by about 10% compared with upright swimming.
Despite great hammerheads being such an iconic and easily recognizable shark, the team's study is the first to formally describe their peculiar tilted swimming behaviour. Whereas speed and agility might be best achieved by more traditional upright swimming, the advantages of the energy-savings provided by the unconventional side-stroke highlight the importance of efficient travel in the evolution of animal locomotion. Even though you may not see many side-strokers at the next Olympics, think about adding some sideways tilt to your swim the next time you're feeling tired in the water.
