Great hammerhead sharks swim on their side to reduce transport costs

Abstract

Animals exhibit various physiological and behavioural strategies for minimizing travel costs. Fins of aquatic animals play key roles in efficient travel and, for sharks, the functions of dorsal and pectoral fins are considered well divided: the former assists propulsion and generates lateral hydrodynamic forces during turns and the latter generates vertical forces that offset sharks' negative buoyancy. Here we show that great hammerhead sharks drastically reconfigure the function of these structures, using an exaggerated dorsal fin to generate lift by swimming rolled on their side. Tagged wild sharks spend up to 90% of time swimming at roll angles between 50° and 75°, and hydrodynamic modelling shows that doing so reduces drag-and in turn, the cost of transport-by around 10% compared with traditional upright swimming. Employment of such a strongly selected feature for such a unique purpose raises interesting questions about evolutionary pathways to hydrodynamic adaptations, and our perception of form and function.

Figure 1. Rolled swimming in great hammerhead sharks Sphyrna mokarran.

For a to d, roll and pitch angles were measured by an electronic tag attached to a 295 cm shark's dorsal fin, and monitored as it swam freely at the Great Barrier Reef, northern Australia. (a,c) A typical hour-long time series for that animal. (b,d) Probability distributions of roll and pitch angles based on the last 15 h of the monitoring period for the Great Barrier Reef shark. Images in e,f were taken with a fin-mounted video camera attached to another wild S. mokarran (∼350 cm) as it swam rolled to the left and right (respectively) at South Bimini Island, the Bahamas, at absolute roll angles of ∼60° (see Supplementary Movie 2 for examples of this and other wild S. mokarran swimming rolled).

Figure 2. Reconfiguration of lifting surfaces in great hammerhead sharks S. mokarran.

By swimming rolled, a shark changes the surfaces that generate lift, L, from the pair of pectoral fins at zero roll angle (left) to the combination of the pectoral and dorsal fins at greater roll angles (right). For the great hammerhead, doing so increases the effective span of the lifting surfaces, b. The model to the right is rolled 65°.

Figure 3. Hydrodynamics of rolled swimming in great hammerhead sharks S. mokarran.

(a) Contours of constant lift C_L and (b) drag coefficients C_D for a range of pitch and roll angles, measured through wind tunnel experiments with a physical S. mokarran model. (c) Contours of constant COT for a 2.95 m shark for a range of roll angle and either pitch angles or (d) swimming speeds. COT was estimated from wind tunnel data summarized in b, and by assuming values for standard metabolic rate and both chemomechanical and propulsive efficiencies (see Supplementary Notes 4). In a, the difference between adjacent contours is 0.2, and in b–d, the difference is 0.02.