Great Hammerhead Shark (Sphyrna mokarran): Facts and Size

What does a great hammerhead look like?

The great hammerhead’s head is flattened and stretched sideways into a wide blade called a cephalofoil, with an eye and a nostril at each tip and the mouth underneath. In adults the front edge of this blade is almost straight, with a small notch in the center. That straight edge is the easiest way to tell it apart from the scalloped hammerhead, whose curved head has a wavy, scalloped front edge, and from the smooth hammerhead, whose curved head has no central notch. The head is about 23–27% as wide as the shark is long, so a 4 m (13 ft) shark carries a head roughly 1 m (3 ft) across.

The first dorsal fin is very tall and strongly sickle-shaped, much taller than in other large hammerheads, and it is often the first thing divers notice. The pelvic fins are also curved, with concave rear edges. The back is dark brown, gray or olive and the belly is white. Young great hammerheads have a more rounded front to the head, which straightens as they grow.

The teeth are triangular and strongly serrated, with about 17 rows on each side of the upper jaw and 16–17 on each side of the lower jaw. As in other sharks, worn or lost teeth are replaced throughout life.

How big do great hammerheads get?

The great hammerhead is the biggest of the roughly ten hammerhead species. The IUCN notes that most of the largest individuals actually measured are around 4 m (13 ft), while maximum lengths of 5.5–6.1 m (18–20 ft) have been reported. Large adults often weigh more than 230 kg (500 lb), and the biggest individuals are females.

The heaviest great hammerhead on record was a pregnant female caught off Boca Grande, Florida, in 2006. She measured about 4.4 m (14.5 ft) and weighed about 580 kg (1,280 lb), a figure boosted by the pups she was carrying. Claims of great hammerheads weighing 900 kg (2,000 lb) or more have never been verified.

Why does it have a hammer-shaped head?

Scientists think the cephalofoil does several jobs at once. Its underside is dotted with ampullae of Lorenzini, jelly-filled pores that detect the tiny electric fields given off by living animals. Spreading these sensors across a wide head lets the shark sweep a broad strip of seabed as it swings its head from side to side, which is ideal for finding stingrays buried in sand.

Studies of other hammerhead species show that eyes placed at the tips of the head give a wider overlap of vision in front of the shark than in most other sharks, and widely spaced nostrils may help a hammerhead tell which side a smell is coming from. The great hammerhead also uses its head as a tool to press prey against the bottom. The full explanation, with the evidence for each idea, is in Why do hammerheads have hammer-shaped heads?

Where do great hammerheads live?

Great hammerheads live in tropical and warm-temperate seas worldwide, roughly between 40°N and 37°S. In the western Atlantic they range from North Carolina to Uruguay, including the Gulf of Mexico, the Caribbean and the Bahamas. In the eastern Atlantic they occur from Morocco to Senegal and in the Mediterranean, where they are now rare. They are found all around the rim of the Indian Ocean, including the Red Sea and the Persian Gulf, in the western Pacific from the Ryukyu Islands of Japan to Australia, New Caledonia and French Polynesia, and in the eastern Pacific from southern Baja California to Peru.

They are mainly coastal and semi-oceanic sharks. They can hunt in water less than 1 m (3 ft) deep on reef flats and usually stay shallower than about 80 m (260 ft), but they have been recorded down to 300 m (980 ft). Favorite places include coral reefs, lagoons, continental shelves and drop-offs near islands. Genetic studies suggest two separate populations, one in the Atlantic and one in the Indo-Pacific.

Great hammerheads are strong travelers. In 2011 researchers reported that a 2.5 m (8.2 ft) shark tagged in the middle Florida Keys had moved at least 1,200 km (750 mi) northeast into the open Atlantic in 62 days. A later tagging study found that many great hammerheads return year after year to Bimini in the Bahamas and to Jupiter on Florida’s east coast, mostly between October and March, and recorded a round trip of about 3,000 km (1,900 mi). Other sharks found off the state are covered in sharks in Florida.

What do great hammerheads eat?

The great hammerhead is a powerful predator with a varied diet. It eats crabs, lobsters, squid and octopus, many kinds of bony fish, including tarpon, groupers, sea catfish, jacks, flatfish, boxfish and porcupinefish, and smaller sharks. It is also cannibalistic and will eat other hammerheads.

Its specialty, though, is rays. Stingrays are its favorite prey, and the venomous spines that stingrays use for defense do not seem to stop it: one great hammerhead caught off Florida had 96 stingray spines lodged in and around its mouth. Great hammerheads hunt mostly at dawn and dusk, cruising just above the bottom and swinging their heads in wide arcs to pick up the electric signals of rays hidden in the sand.

Observers in the Bahamas have watched a great hammerhead knock a southern stingray to the seabed with a blow from its head, pin it down, then pivot to bite off pieces of each side of the ray’s disc until it could no longer escape. Another was seen biting a spotted eagle ray in open water, then using its head to hold the injured ray against the bottom. These observations suggest the cephalofoil is also a prey-handling tool.

Behavior and movement

Great hammerheads are usually solitary and nomadic. Other reef sharks tend to keep out of their way. When threatened, a great hammerhead may perform a warning display, swimming stiffly with its pectoral fins lowered, before moving off.

One of its oddest habits was confirmed in 2016. Sharks fitted with motion-recording tags spent up to 90% of their time swimming tilted on their side at angles of 50–75 degrees. In this “rolled” position the very tall first dorsal fin works like a wing, providing lift, and the researchers calculated that it cuts drag, and so the energy needed to travel, by about 10% compared with swimming upright.

Mating has rarely been seen. Unlike most sharks, which mate on or near the seabed, great hammerheads have been observed mating near the surface: in one account from the Bahamas, a pair rose through the water while circling each other and mated as they reached the surface.

Reproduction and life cycle

Like other hammerheads, the great hammerhead gives birth to live young. The embryos first feed on yolk, and then the empty yolk sac turns into a placenta-like link with the mother. Pregnancy lasts about 11 months, and females breed only once every two years. Pups are born from late spring to summer in the Northern Hemisphere and around December and January in Australian waters.

Litters usually contain 6–42 pups, each 50–70 cm (20–28 in) long, and the record Boca Grande female was reported to be carrying 55. The young look like small adults but have more rounded heads, and they receive no care from their mother. Males mature at about 2.25–2.7 m (7.4–8.8 ft) and females at about 2.1–3 m (6.9–9.8 ft), with female age at maturity estimated at roughly 5.5–8 years in different regions.

Lifespan, predators and threats

Great hammerheads are long-lived. Counting growth bands in the vertebrae, checked against radiocarbon from Cold War bomb tests, has given maximum ages of 44 years in the northwest Atlantic and 39 years in the western Pacific, and a typical lifespan is about 20–30 years.

Young great hammerheads are eaten by bigger sharks, including bull sharks. Adults have few natural predators. The real danger comes from people. Great hammerheads are caught on longlines, in gillnets and purse seines and by trawls, both on purpose and as bycatch, and their large fins are highly valued in the shark fin trade. The meat is rarely eaten, but the skin, liver oil and remains are sometimes used. Many die before they can be released: studies estimate that about 56% are dead when US bottom longlines are hauled in. Beach protection nets in South Africa and Australia also kill them. The scale of global shark catches is explained in How many sharks are killed each year?

Conservation status

The IUCN Red List has listed the great hammerhead as Critically Endangered since 2019, up from Endangered in 2007, with a decreasing population. The assessment concluded that the global population has most likely declined by more than 80% over three generations, a period of about 71–74 years. Declines appear steepest in the Indian Ocean, West Africa and the Mediterranean, while in the northwest Atlantic numbers may be slowly recovering after US management measures introduced from 2005.

Protection is growing but patchy. Atlantic tuna fisheries managed by ICCAT have banned keeping hammerheads since 2010, and the Mediterranean followed in 2012. The species was added to Appendix II of CITES in 2013, so international trade in its fins requires permits showing they come from legal, sustainable fisheries. Florida protects great hammerheads in state waters. The reasons behind the decline, and what could help, are covered in Why are great hammerheads Critically Endangered?

Are great hammerheads dangerous to people?

The International Shark Attack File lists only 18 confirmed unprovoked bites by all hammerhead species combined, none of them fatal, and few can be pinned on the great hammerhead itself. Divers usually describe great hammerheads as shy or indifferent, although they sometimes approach closely, especially at dive sites where sharks are attracted with bait. Because of its size and cutting teeth, the great hammerhead could seriously injure a person and should always be given space and respect. The real risk is explained in Are hammerhead sharks dangerous?