The hammerhead shark’s distinctive “hammer” is just one of the many bizarre evolutionary adaptations nature has sculpted. Like nearly everything in the natural world, even this peculiar formation serves a purpose, though scientists don’t entirely agree on what that is.
Just imagine a giant moose antler, capable of reaching 2.5 meters in diameter and weighing over 40 kilograms, or the ultra-long giraffe neck, or even the enchanting, yet incredibly impractical train of a peacock. At first glance, a whole host of disadvantages for the owners of such “outgrowths” immediately spring to mind. But upon closer inspection, it quickly becomes clear: each of these anatomical peculiarities has its purpose.
The moose needs its headgear for display and combat, and perhaps even for shovelling snow – here, bigger is still better. The giraffe, with its neck, can reach the leaves of trees that other hoofed animals can only dream of. And while the peacock might draw the short straw if a tiger is lurking nearby, it otherwise secures the prettiest female and the most offspring, all thanks to its train.
These supposed whims of nature are, in most cases, the result of a tough cost-benefit analysis. If it brings no advantage, it disappears.
The Enigma of the Hammerhead’s Cephalofoil
The hammerhead shark, too, stands out from its counterparts in the shark world due to an unusual structure: its head shape. This hammer-shaped head, called a cephalofoil—a blend of the Greek word for “head” (cephalos) and the Latin for “foil” (folium)—is the hallmark of the Sphyrnidae family, which comprises a total of nine species. Among them, the great hammerhead, at nearly six meters, is the largest, and the scalloped hammerhead is the most common.
Surprisingly, despite 50 years of research, the origin and biological function of the cephalofoil remain relatively unknown, unlike other curious or graceful structures in the animal kingdom.
Der »Hammer«, in that distinctive “hammer,” or cephalofoil as it’s known scientifically, has certainly proven its worth over time – over 55 million years of evolution if the oldest fossil records are anything to go by.
The Hydrodynamics Hypothesis
The “Hydrodynamic Lift Hypothesis”
According to the “hydrodynamic lift hypothesis,” the hammerhead’s distinctive “hammer” supposedly functions much like an aeroplane wing, specifically like a canard (or “duck plane”) wing, providing lift to the front of the body. Interestingly, their pectoral fins perform a similar function, but in hammerheads with a large cephalofoil, these fins are smaller than in other shark species.
One compelling fact supporting this theory is that the total surface area of the cephalofoil and pectoral fins is consistently the same across all hammerhead species. Essentially, they all get the same amount of “lift,” whether it’s from the size of their hammer or their fins. Conversely, shark species without a cephalofoil have correspondingly larger pectoral fins to compensate for the lack of that “hammer-wing” surface.
The second hydrodynamics hypothesis suggests that the hammer enhances the shark’s manoeuvrability. Experiments have shown that hammerhead sharks are, in fact, more agile and flexible than sandbar sharks, lending credence to this idea.
The Prey Theory
A third intriguing hypothesis suggests that hammerheads use their distinctive cephalofoil not just for detection, but to manipulate their prey.
On rare occasions, observers in the Bahamas have witnessed large hammerheads actually “knocking out,” pinning down, and subduing eagle rays and American stingrays with their hammers.
While the unfortunate ray was pressed against the seabed, the shark would then swivel its own body to achieve a prime biting position.
The wider the hammerhead’s head, the more sensory cells it can house.
Hypotheses Four and Five: Sensory Perception
The fourth and fifth hypotheses propose that the hammer significantly improves the shark’s sense of smell and vision.
The “olfactory gradient hypothesis” suggests, among other things, that the hammer allows for better orientation along a scent trail (known as “olfactory klinotaxis”). Essentially, smell gradients—where a scent gets stronger or weaker—can be perceived more effectively when the nostrils are further apart. Thanks to the wide placement of the nostrils on the hammer, and their increased size made possible by the hammer’s shape, the shark can also sample more seawater for scent molecules. Finally, larger nostrils could mean more olfactory epithelium is available to the shark, further enhancing its sense of smell.
While existing findings support the first two assumptions of this olfactory hypothesis (better scent trail alignment and more water sampling), it hasn’t been experimentally proven that hammerheads possess a larger olfactory epithelium – the tissue layer equipped with scent cells – compared to sharks without a cephalofoil.
The “improved binocular vision hypothesis,” on the other hand, posits that the lateral positioning of the eyes on the hammer enhances binocular vision and expands the overall field of view. Studies supporting this hypothesis already exist, with results backing both of these claims.
Sharks possess a remarkable ability: using their Ampullae of Lorenzini, they can locate prey hidden even beneath the sand.
The Primary Hypothesis: Electroreception
The most well-known and frequently cited hypothesis is the “enhanced electroreception hypothesis.” This theory proposes that the cephalofoil’s larger surface area must be covered with more electroreceptors to achieve a similar pore density as other sharks. This would provide the hammerhead with a greater overall number of pores, and consequently, a significantly improved electrical sense. Sharks use this sensory system to detect bioelectric impulses involuntarily emitted by prey. These impulses allow them to easily pinpoint animals hidden, for example, even beneath the sand.
Beyond just more receptors, the electrical sense can also achieve higher sensitivity through lengthening the channels within the electrical pores. In hammerheads, the cephalofoil makes precisely this extension possible. There’s also a considerable body of research supporting this hypothesis, at least in part.
Just like with the moose, which uses its antlers for fighting, displaying dominance, and even shovelling, it’s highly probable that the hammerhead shark’s remarkable head shape evolved for multiple reasons. This unique feature, after all, hasn’t significantly changed over the past 55 million years.
Hammerhead Shark
Hammerhead SharkHere’s a breakdown of the classification for hammerhead sharks, from broader categories down to individual species:
Classification of Hammerhead Sharks
- Subclass: Euselachii (Sharks and Rays)
- Neoselachii (Modern Sharks and Rays)
- Selachii (Sharks)
- Neoselachii (Modern Sharks and Rays)
- Superorder: Galeomorphii (Encompasses four orders of modern sharks)
- Order: Carcharhiniformes (Ground Sharks)
- Family: Sphyrnidae (Hammerhead Sharks)
- Genus: Eusphyrna
- Species: Eusphyra blochii (Winghead Shark)
Genus: Sphyrna
- Species: Sphyrna corona (Scalloped Bonnethead)
- Sphyrna lewini (Scalloped Hammerhead)
- Sphyrna media (Scoophead)
- Sphyrna mokarran (Great Hammerhead)
- Sphyrna tiburo (Bonnethead Shark)
- Sphyrna tudes (Small-eye Hammerhead)
- Sphyrna zygaena (Smooth Hammerhead)
