Hammerhead sharks swim tilted on their sides because it saves energy. By rolling to one side, they recruit their tall dorsal fin as an extra lifting surface, reducing drag by more than 10% compared to swimming upright. It looks strange, but it’s an elegant solution to a problem unique to their body shape.
The Dorsal Fin as a Second Wing
Most sharks stay buoyant while swimming by using their two pectoral fins (the pair that stick out from their sides like airplane wings) to generate upward lift. Hammerhead sharks can do this too, but they have an unusual advantage: their dorsal fin, the iconic one on top, is exceptionally tall. In great hammerheads, the dorsal fin is actually longer than the pectoral fins. That’s the reverse of virtually every other shark species on record.
When a great hammerhead rolls onto its side, that oversized dorsal fin shifts from pointing straight up (where it does nothing for lift) to angling out to the side (where it works like an additional wing). The shark effectively goes from generating lift with two small pectoral fins to generating lift with the pectoral fins plus a large dorsal fin. This increases the total horizontal span of its lifting surfaces, which is the key to the whole trick.
Why a Wider “Wingspan” Matters
In aerodynamics and hydrodynamics, a wider span of lifting surfaces produces the same amount of lift with less drag. It’s the same reason gliders and albatrosses have long, narrow wings rather than short, stubby ones. By tilting sideways and bringing that dorsal fin into play, the hammerhead essentially stretches its wingspan. Hydrodynamic modeling published in Nature Communications found this reduces drag, and therefore the energy cost of traveling, by around 10%. That’s a significant saving for an animal that never stops swimming.
The reduction matters most during routine cruising. Sharks, unlike bony fish, lack swim bladders and are slightly denser than seawater, so they must constantly generate lift to avoid sinking. Any body configuration that produces lift more cheaply translates directly into fuel savings over a lifetime of nonstop movement.
The Head Helps Too
The hammerhead’s wide, flattened head (called the cephalofoil) has long been suspected of functioning like a wing itself. In cross-section, each lobe of the head resembles what engineers call a cambered wing: more curved on top than on the bottom. That shape forces water to move faster over the upper surface, lowering the pressure there and creating lift, even when the head is held perfectly level in the water. A symmetrical shape wouldn’t do this. So the head likely contributes some passive lift during normal swimming, complementing whatever the fins are doing.
Whether the head provides a large or small amount of lift is still being quantified, but its wing-like profile supports the broader picture: hammerhead anatomy is built around generating lift efficiently from multiple body surfaces simultaneously.
Not Just Great Hammerheads
The behavior was first documented and studied in great hammerheads, the largest species in the family, but it isn’t exclusive to them. Scalloped hammerheads, a closely related species, also spend the majority of their time swimming on their sides. Researchers tracking scalloped hammerheads with motion-sensing tags found rolling patterns similar to those in great hammerheads, with shifts in how much they tilted depending on the time of day.
This makes sense given that scalloped hammerheads are the species closest to great hammerheads in fin proportions, approaching the same unusual ratio of a dorsal fin nearly as long as the pectoral fins. Species with shorter dorsal fins would gain less from rolling sideways, which may explain why the behavior appears concentrated in these two species rather than across all sharks.
What It Looks Like in Practice
Hammerheads don’t flip fully onto their sides like a flatfish. They roll to varying degrees, sometimes a moderate tilt, sometimes a steep one. As the roll angle increases, more lift transfers from the pectoral fins to the dorsal fin, and the total lifting span grows. The shark adjusts its roll angle the way a pilot adjusts wing flaps: dynamically, based on conditions.
Both great and scalloped hammerheads spend a majority of their swimming time in some degree of roll rather than perfectly upright. This isn’t occasional or situational. It’s their default cruising posture. The rolled position is, for these animals, normal swimming. Upright swimming is the exception, likely reserved for moments when maneuverability or burst speed matters more than efficiency.