Great Hammerhead Shark Facts

The hammer-shaped head is called a cephalofoil, and it does several jobs at once. Its wing-like shape gives hydrodynamic lift for the sharp, agile turns the shark needs to chase manoeuvrable prey. It places the eyes at the far outer edges, giving exceptional wide-field vision — the shark can effectively see above and below itself at once. Most importantly, the broad underside is packed with ampullae of Lorenzini, gel-filled pores that sense the tiny electrical fields of an animal's muscle activity, turning the head into a sweeping metal-detector for prey hidden under the seabed. Great hammerheads are large, roughly 11 to 20 feet and 500 to over 1,000 pounds, with a distinctively tall, sickle-shaped dorsal fin and countershaded colouring; the front edge of their cephalofoil is nearly straight, unlike the scalloped heads of smaller relatives.

The great hammerhead lives worldwide in warm tropical, subtropical, and warm-temperate seas, off the Americas, Africa, Asia, and Australia — Florida and the Gulf of Mexico, the Bahamas, Brazil, South Africa, the Indian Ocean, and the Great Barrier Reef among them. It favours coastal drop-offs and continental shelves where upwelling feeds abundant prey, patrolling shallow sandy flats and lagoons but also crossing open ocean on long, nomadic migrations that track temperature and prey.

The great hammerhead takes bony fish, smaller sharks (including other hammerheads), squid, octopus, and crustaceans — but its specialty, and the likely reason for its head shape, is hunting stingrays. It cruises just above the bottom, sweeping the cephalofoil side to side like a detector until it senses the electrical signal of a ray buried in the sand. Then it strikes, using the broad flat underside of its head to pin the thrashing ray to the seabed while it bites chunks from the ray's wings. Remarkably, great hammerheads seem largely immune to stingray venom — individuals are often found with dozens of barbs embedded in their jaws with no apparent harm.

Unlike scalloped hammerheads, which famously school in the hundreds, the great hammerhead is a solitary nomad, usually seen alone. It is an obligate ram ventilator — it must keep swimming to push oxygen-rich water over its gills, so it never truly sleeps, resting instead by shutting down parts of the brain while continuing to swim (unihemispheric sleep). Social clashes are rare, but it will defend a food source aggressively and, when threatened, put on a stiff-bodied, exaggerated display to look as large as possible.

Great hammerheads reproduce by placental viviparity, a system close to mammals': after the embryo's yolk runs out, the yolk sac fuses to the uterine wall to form a placenta through which the mother feeds the young. Gestation lasts about 11 months, and litters are large — 6 to 42 pups — usually born in sheltered coastal nurseries. To protect the mother during birth, each pup's cephalofoil is soft and folded back against the body, stiffening only once it hits seawater. The pups are independent at once. Slow growth and late maturity make the species' replacement rate very low, and maximum lifespan is around 20 to 30 years.

The great hammerhead is Critically Endangered, with steep global declines. The main cause is the shark-fin trade: its large, high-value fins are heavily targeted by illegal and unregulated fishing. Even when not targeted, it dies at very high rates as bycatch on longlines and gillnets, because as an obligate ram ventilator it fights hard when hooked and often dies of stress before it can be released. Strong enforced protections, marine protected areas, and curbing the fin trade are needed to keep the species from sliding toward extinction.

• World Wildlife Fund (WWF) - Shark Conservation
• IUCN Red List of Threatened Species - Great Hammerhead
• Oceana - Great Hammerhead Shark Biology
• Florida Museum of Natural History - Great Hammerhead Species Profile