Why Do Hammerhead Sharks Have a Hammer? What the Science Says About the Cephalofoil

Hammerhead sharks appear to use their hammer for several jobs at once, and no single function has been shown to explain the head across the whole family. Tests show that the hammer-shaped head, or cephalofoil, gives young scalloped hammerheads a wider electrical search area than sandbar sharks. It also gives winghead and scalloped hammerheads more overlap between the fields of view of the two eyes than conventionally shaped sharks. The head appears to improve maneuverability as well, and great hammerheads use it to pin stingrays to the seafloor. The popular idea that the head lifts the shark like an aircraft wing has fared worse: flow models found that a head held level produces no significant lift.

This guide is written by the scientists of the Marine Megafauna Foundation, whose work in the Galápagos and Florida brings them into regular contact with hammerheads, and draws on the published research, including Dr. Alex Hearn’s long-term tracking of scalloped hammerheads in the Eastern Tropical Pacific.

What is the hammer on a hammerhead shark?

The hammer is a flattened, sideways extension of the head, called the cephalofoil, with an eye at each tip and the nostrils and electrosensory pores spread across it. Hammerheads (family Sphyrnidae) are recognized by this lateral expansion of the head and the placement of the eyes and nasal capsules toward its outer edges, and their heads are the most expanded of any shark or ray. All of the roughly ten hammerhead species have one, but its shape varies enormously from species to species.

At one extreme, the bonnethead (Sphyrna tiburo) has a broadly rounded, spade-like head and the least lateral head expansion of any hammerhead. At the other, the winghead (Eusphyra blochii) has a head that is extremely wide and narrow, drawn out into long, slender blades. The three large species sit between them. In these, the front edge of the head is the standard field mark: straight in the great hammerhead, two rounded lobes divided at the midline in the scalloped, and a single rounded lobe in the smooth. How to tell the species apart is covered in the types chapter of this guide.

Why do hammerhead sharks have a hammer-shaped head?

The short answer is that the hammer appears to do more than one job, and which job matters most may differ from species to species. Researchers have proposed that the head improves the senses, prey capture, maneuverability, and lift, yet the primary function of the cephalofoil remains unclear, and few of these hypotheses have been tested empirically. The tests that have been done support several proposed functions, cast doubt on one, and leave others open; how much each function shaped the head’s evolution remains uncertain.

Does the hammer help hammerheads find prey?

In tests of juvenile scalloped hammerheads against sandbar sharks, the hammer gave a wider search sweep rather than a sharper sense. Like all sharks, hammerheads carry ampullae of Lorenzini, jelly-filled electroreceptor organs that open through pores in the skin and detect very subtle variations in electric fields during hunting and navigation, including the weak fields around hidden prey. Spreading those pores across a wide head has been proposed to increase the sensory area and the precision of electrical prey detection, so a hammerhead might be expected to detect prey from farther away than other sharks.

That prediction failed its most direct test. Stephen Kajiura and Kim Holland compared similarly sized juvenile scalloped hammerheads (Sphyrna lewini) and sandbar sharks (Carcharhinus plumbeus) responding to small electric fields that imitate prey. Both species oriented to the fields from the same maximum distance, about 30 centimeters (12 inches), and both responded to fields weaker than one nanovolt (a billionth of a volt) per centimeter. The hammer did not make the scalloped hammerheads any more sensitive.

What differed was how the sharks approached. Scalloped hammerheads typically pivoted, holding the edge of the head nearest the field in place while bending the trunk to swing onto the target, whereas sandbar sharks swam in a broader arc. The authors concluded that the hammerhead’s head does not appear to confer greater sensitivity, but does provide a greater lateral search area, “which may increase the probability of prey encounter”. It also provides enhanced maneuverability, they added, “which may aid in prey capture”. The hammer works much as a wider metal detector does: it covers more ground with each sweep without being any more sensitive.

The test used juveniles, and a later review stated, citing other work, that the hammerhead electrosense detects electric fields from greater distances than those of other shark species. That statement and the juvenile trials point in different directions, and the difference remains unresolved. One recent analysis suggests that the wider search area may convey a selective advantage upon newborn and young juvenile hammerheads, which hunt small, agile prey on the seafloor. Older scalloped hammerheads hunting in open water, by contrast, are unlikely to benefit as much.

Can hammerhead sharks see better because of their hammer?

In the species with the widest heads, yes: the wider the head, the larger the zone that both eyes see at the same time, known as binocular overlap. Placing the eyes at the ends of a wide blade might be expected to leave a hammerhead unable to see straight ahead. To test this, D. Michelle McComb, Timothy Tricas, and Stephen Kajiura measured the visual fields of three hammerheads and two requiem sharks, the family of conventionally shaped sharks that includes the lemon and blacknose sharks. The winghead had the largest binocular overlap (48°), more than four times that of the lemon shark (10°) and blacknose shark (11°).

The scalloped hammerhead’s overlap (34°) was also larger than that of the bonnethead (13°) and the two requiem sharks, but the narrow-headed bonnethead did not differ from the requiem sharks. The authors concluded that binocular overlap has increased with lateral head expansion in hammerhead sharks, which is consistent with what they call the “enhanced binocular field” hypothesis.

The arrangement has a cost. Greater overlap farther ahead does not remove the large blind area directly in front of the head, and the hammerheads in the study did not rotate their eyes any more than the other sharks did. Instead, scalloped hammerheads and bonnetheads swung their heads from side to side more while swimming (16.9° and 15.6°) than lemon and blacknose sharks did (15.1° and 15.0°). The authors interpret this as a behavioral compensation for the blind area.

Does a hammerhead’s head work like a wing?

Probably not when held level: flow models found that the head produces no significant lift at that angle, and that it adds drag. In 1977, Keith Thomson and Dan Simanek argued that a swimming shark’s net tendency to sink is countered by the pectoral fins and the flattened underside of the head. These act as planing surfaces, which push the body upward as it moves forward. In the bonnethead and smooth hammerhead (Sphyrna zygaena), they found, the combined area of head and pectoral fins was virtually the same. That, they wrote, “may confirm the importance of the head and pectoral fins as planing surfaces”, but it was an inference from body measurements, not a measurement of force.

Forty-three years later, Matthew Gaylord, Eric Blades, and Glenn Parsons tested the idea directly. They cast models of the heads of all eight hammerhead species they recognized, plus three sharks with typical heads. Computational fluid dynamics (computer simulation of how water flows over a shape) then gave the lift and drag of each head at a range of angles. The hammer-shaped heads did not provide significant dynamic lift when held parallel to the flow, and created more drag than typical shark heads at every angle tested. The models were static heads rather than swimming sharks, and they did not rule out lift from a tilted head, so the wing analogy is at best a partial explanation.

If hammerheads gain help with efficient swimming, the evidence points elsewhere on the body. Hammerheads have unusually tall dorsal fins, and one study of scalloped hammerheads found that dorsal fin height grows faster than body length. Subadult and adult scalloped hammerheads sometimes swim rolled onto their side, a posture that models of the similarly shaped great hammerhead (Sphyrna mokarran) suggest may reduce drag or the cost of transport. In that posture, the large dorsal fin is hypothesized to generate thrust.

Does the hammer help hammerheads turn?

Probably: several independent lines of evidence point that way, although they come from anatomy, models, and behavior rather than a direct measurement of turning. The pivoting turns that scalloped hammerheads made in the electric-field trials were attributed to the hydrodynamic properties of the cephalofoil, which enables hammerheads to execute sharp turns at high speed. The sandbar sharks’ wider trunks, by contrast, prevented them from bending as tightly.

Anatomy points the same way. Kazuhiro Nakaya found that a hammerhead’s backbone is completely surrounded by muscle, unlike that of the other ground sharks, the larger group to which hammerheads belong. Hammerheads also could both raise and lower the head to a considerable degree, whereas other ground sharks could only raise theirs. Nakaya argued that a movable plate at the very front of the body would create large turning forces in the water, and that hammerheads “utilize their head for hydrodynamic purposes in various ways”. The same flow-modeling study that found no significant lift from a level head found that the cephalofoil provides greater maneuverability that may be important in prey capture.

The most direct field evidence comes from the species with the most extreme head. In 2026, Nicolas Lubitz and colleagues described drone footage of a winghead shark chasing a fish at Magnetic Island, Queensland. The shark alternated rapid turns with directed pursuit, repeatedly tilted its head up and down as the fish moved vertically, and made sharp turns before catching and eating it. It is a single observation, which the authors describe only as consistent with the idea that the expanded head enhances maneuverability. Such records matter because direct records of hammerheads hunting are limited, particularly for this Critically Endangered species.

How do great hammerheads use their head to catch stingrays?

Great hammerheads have been seen thrusting the head down onto a southern stingray and pinning it to the seafloor while pivoting to bite its fins, the clearest case of the head used as a tool. The behavior, later termed “pin and pivot”, has been documented in great hammerheads, and the feeding chapter describes these hunts in detail.

Could the hammer help hammerheads navigate?

Possibly, but this idea remains a proposal rather than a tested result. It grew out of tracking work led by Dr. Alex Hearn, now a professor at the Universidad San Francisco de Quito. His team showed that scalloped hammerheads at Wolf Island in the Galápagos gather by day at a few sites on the island’s southeastern corner and move offshore at night. The schooling chapter describes these daily movements.

Commuting precisely between an island hotspot and open-ocean feeding grounds, Hearn’s team wrote, would appear to require the ability to detect and interpret some environmental property. Their paper discusses an earlier proposal by A. Peter Klimley that the hammerheads’ nightly paths followed gradients in Earth’s magnetic field. Klimley suggested that the voltage difference between the electroreceptors on either side of the head, “thus the ‘hammer’ shape”, could be used to navigate. A magnetic sense is real in at least one hammerhead: experiments with wild-caught bonnetheads showed that magnetic map cues can elicit homeward orientation. Whether a wide head improves that sense, however, has not been measured.

Why can’t one explanation fit every hammerhead?

Probably because the family’s members live very different lives, so a head that helps one species hunt may matter less to another. Off eastern Australia, large great hammerheads rely mainly on other sharks and rays, with a preference for bottom-living prey such as cownose rays. The scalloped hammerhead, in contrast, feeds on a mixture of fish, crustaceans, and cephalopods (squid and their relatives), with squid as a primary prey item. Adults appear to do much of their hunting at night, away from the reefs where divers meet them. Scalloped hammerheads tracked at Wolf Island occasionally dived below the thermocline (the boundary between warm surface water and colder water below) while moving offshore, most likely for foraging. The authors describe that evidence as limited and inconclusive, however, and the feeding chapter sets out the night trips and deep dives in detail.

The bonnethead breaks the pattern further still. It eats mostly crustaceans and mollusks but also swallows large amounts of seagrass, up to 62.1% of its gut contents by mass. A feeding trial showed it to be the first shark demonstrated to have an omnivorous digestive strategy, as the feeding chapter describes. It also has the least lateral head expansion in the family, and its binocular overlap is no greater than that of ordinary requiem sharks. At the other end of the range, the long-bladed winghead has been filmed chasing an agile fish.

A ray-pinning predator, a squid-eating deep diver, a seagrass-digesting omnivore, and a fish-chasing winghead may each use the hammer differently. One recent analysis concluded that the evolution of the cephalofoil may be driven by a complex selective regime, a combination of evolutionary pressures with no obvious link to any single function. The same analysis notes evidence for several distinct head “designs” in the family, so the relative importance of each function may vary between species.

Does a hammerhead’s hammer change as it grows?

Yes: in scalloped hammerheads the head appears to become proportionally smaller as the shark grows, and in bonnetheads the males’ heads change shape when they mature. Young scalloped hammerheads, roughly 30 to 90 centimeters (1 to 3 feet) long, generally live in shallow, near-shore water and feed on bottom-living prey, whereas larger individuals move into deep, open-ocean habitats. Phillip Sternes and Timothy Higham measured museum specimens from 32 to 130 centimeters (13 to 51 inches) long. They found that head area increased less than it would if the head simply kept its proportions, so larger sharks had relatively smaller heads. In their sample, pectoral fin length, dorsal fin height, and some tail-fin proportions grew faster than body length, a shift they link to sustained, efficient swimming in the open ocean.

A second study measured 129 freshly landed scalloped hammerheads at a fishing camp in Baja California Sur, Mexico. It found that the width of the hammer between the eyes grew more slowly than body length across the size range sampled. Sternes and Higham’s smaller museum sample, by contrast, found eye-to-eye width keeping pace with body length, and the two studies also disagree on how pectoral fin length scales. The pattern of electrosensory pores on the underside of the head stays the same from juvenile to adult in the species studied. Noting that pore number is thought to be fixed, one anatomist has asked why the cephalofoil keeps growing while the sensory pores remain constant in number, and suggested that very old adults may develop extra pores. How the head scales with body size is covered further in the size chapter.

Bonnetheads add a twist linked to sexual maturity. Male bonnetheads develop a prominent bulge along the front of the head at the onset of sexual maturity, caused by the lengthening of three rod-like cartilages in the snout. The bulge appears as their claspers (the paired reproductive organs of male sharks) lengthen, while females, juveniles, and embryos of both sexes keep a smoothly rounded head. The 2005 study was the first report of a difference in head shape between the sexes in any shark. Its authors consider the bulge unlikely to have a function: it is more probably a byproduct of the hormonal changes of maturity, or possibly a signal of a male’s condition to females. They note, however, that the change may also affect the feeding apparatus, which they did not measure.

When did hammerhead sharks evolve their hammer?

Hammerheads are young by shark standards: a 2014 review puts their divergence from the rest of the ground sharks at around 10–20 million years ago, which places them among the most recently evolved of all living sharks. The ground sharks (order Carcharhiniformes) include the lemon, blacknose, and sandbar sharks used as comparisons in the experiments above. Hammerheads have since evolved behavioral, physiological, and anatomical adaptations not seen in their relatives, of which the hammer is only the most visible.

Great, scalloped, and smooth hammerheads also have extremely large brains relative to their body mass, with enlarged forebrain and cerebellum regions; the cerebellum coordinates movement. In an analysis of 84 species of sharks, rays, and chimaeras, they had the most highly folded cerebellums. Enlargement of those brain areas has been linked to sociality, complex coordination of the senses with movement, long-distance migration, and agile prey capture.

Why it matters for conservation

The same specialized traits that made hammerheads so successful may now make them more vulnerable to fishing. Researchers have argued that hammerheads’ schooling in predictable places, their slow life histories, and their severe physiological stress when caught have increased their vulnerability to human exploitation, producing what they call “a de facto evolutionary trap”. On the IUCN Red List, the scalloped hammerhead and the great hammerhead were listed as Critically Endangered in 2019, and the winghead in 2025. The threats chapter covers why, from overfishing and the fin trade to climate change.

The hammerhead’s senses may also offer part of the solution. Experimental fishing hooks made from lanthanide metals, rare-earth elements that produce an electric field in seawater, catch significantly fewer scalloped hammerheads than standard hooks. More research is needed, however, before such hooks can be used widely. Recovery is possible, too: a global analysis published in 2021, with an MMF co-author, reported that hammerhead shark populations are rebuilding in the Northwest Atlantic, owing to strictly enforced quotas throughout their US range.

In the Galápagos, home to the scalloped hammerhead schools of Darwin and Wolf, MMF works alongside Dr. Alex Hearn’s group at the Universidad San Francisco de Quito. There, MMF’s Dr. Chris Rohner leads MMF’s part of a regional study of silky shark movements, which the university leads. You can support that work through the Galápagos silky shark tracking project, or read more about MMF’s work in the Galápagos.