HAMMERHEAD SHARK
FACTS.

Hammerhead sharks are a family of about ten species recognized by the cephalofoil, a flattened, sideways extension of the head that carries the eyes and nostrils toward its outer edges. They range from the scalloped bonnethead, which reaches only 92 centimeters (3 feet), to the great hammerhead, the largest species, for which 4 meters (13 feet) is the more commonly observed maximum. Scalloped hammerheads are known for their daytime schools, with dive guides at Cocos Island counting up to 1,000 on a single dive, whereas great hammerheads are generally solitary.

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.

Top 10 hammerhead shark facts

1. There are about ten species of hammerhead shark, and the small bonnethead is one of them, with the least sideways head expansion in the family.
2. In tests comparing juvenile scalloped hammerheads with sandbar sharks, the hammer did not make the hammerheads’ electrical sense more sensitive, but it gave them a greater lateral search area and enhanced maneuverability.
3. The wider the head, the more the two eyes’ fields overlap: the winghead shark’s binocular overlap is 48°, against 10–11° in lemon and blacknose sharks.
4. Great hammerheads have been seen driving stingrays into the seabed with their heads and pinning them there while they pivot to bite.
5. The largest great hammerheads measured in recent Florida and Australian studies were about 4.5 meters (15 feet) long; the often-quoted 6.1 meters (20 feet) is a reported maximum dating from 1984.
6. A female scalloped hammerhead tagged off Hawaii dived to 1,240 meters (4,068 feet), the deepest dive on record for the species when it was reported in 2022.
7. Bonnetheads eat seagrass, and in feeding trials they digested half of its organic matter, making the bonnethead the first shark shown to have an omnivorous digestive strategy.
8. Scalloped hammerheads tagged in the northern Galápagos have reached Cocos Island, 700 kilometers (435 miles) away, and Malpelo Island, 1,186 kilometers (737 miles) away.
9. A captive bonnethead gave birth without a male in 2001, the first genetic evidence of virgin birth in any shark, ray, or chimaera.
10. The scalloped and great hammerheads are Critically Endangered, yet a 2021 analysis found hammerheads rebuilding in the Northwest Atlantic under strictly enforced US quotas.

How many types of hammerhead shark are there?

There are about ten species of hammerhead shark in two genera, the next rank above species: Sphyrna, which holds all but one of them, and Eusphyra, whose only member is the winghead shark. Nine are assessed on the IUCN Red List. They are the scalloped (S. lewini), great (S. mokarran), and smooth (S. zygaena) hammerheads, which are large and wide-ranging; four small species of the coastal Americas; the Carolina hammerhead; and the winghead.

The count keeps moving. The Carolina hammerhead was first detected as a cryptic species within the scalloped hammerhead, meaning one that cannot be told apart by appearance alone. It was named in 2013, and can be told apart physically only by counting the vertebrae in front of its tail. The bonnethead, meanwhile, is likely a complex of three species. The types chapter explains how to tell them apart.

Why do hammerhead sharks have a hammer-shaped head?

The hammer, or cephalofoil, appears to do several jobs at once, and its primary function remains unclear. Tests suggest that it widens the electrical search area of young scalloped hammerheads, widens forward vision in the wider-headed species, and helps them turn sharply. Great hammerheads also use it to pin stingrays to the seabed.

In trials with prey-like electric fields, juvenile scalloped hammerheads and sandbar sharks responded from the same maximum distance, about 30 centimeters (12 inches). The hammer did not sharpen the sense, but it widened the search and tightened the turns. A computer flow study of model heads suggested that the head provides greater maneuverability but no significant lift when held level, and more drag than a typical shark head. That result weakens the old idea that it works like a wing. The family is young by shark standards, having diverged from the rest of the ground sharks around 10–20 million years ago, and the chapter on why hammerheads have a hammer weighs each idea.

Can hammerhead sharks see in front of them?

Yes, and the wider the head, the larger the zone in front that both eyes see at once. Hammerheads do have a blind area directly in front of the head, which they compensate for by swinging the head from side to side as they swim.

Comparing three hammerheads with two conventionally shaped sharks, researchers measured binocular overlap, the zone both eyes see at once. It was 48° in the winghead, 34° in the scalloped hammerhead, and 13° in the bonnethead, against 10° in the lemon shark and 11° in the blacknose shark. Scalloped hammerheads and bonnetheads also swung their heads from side to side more while swimming (16.9° and 15.6°) than lemon and blacknose sharks (15.1° and 15.0°). The authors read this as a behavioral compensation for the blind area.

How big do hammerhead sharks get?

Most hammerhead species grow no longer than about 2 meters (6.6 feet), but the great, scalloped, and smooth hammerheads reach about 4 meters (13 feet). The great hammerhead is the largest: it has been reported at up to 6.1 meters (20 feet), but the biggest measured in recent field studies in Florida and Australia were about 4.5 meters (15 feet).

So is there a 20-foot great hammerhead? The 2019 Red List assessment notes that the maximum size is reported as 550 to 610 centimeters (18 to 20 feet), citing a 1984 source. It adds that 400 centimeters (13 feet) is a more commonly observed maximum. Recent field measurements fall between those figures. The largest of 238 great hammerheads caught in drumline surveys from Miami through the middle Florida Keys measured 450 centimeters (15 feet), and the largest of 31 tagged in Australia 455 centimeters (15 feet).

Scalloped hammerheads reach about 4 meters (13 feet), or 4.2 meters (14 feet) in the 2019 Red List assessment, and smooth hammerheads 3.7–4 meters (12–13 feet). The largest of the small species, the winghead, reaches 1.86 meters (6 feet), and the scalloped bonnethead only 92 centimeters (3 feet). The size chapter covers every species, pups, and growth.

What do hammerhead sharks eat?

Hammerheads eat fish, squid and octopus, crustaceans, and other sharks and rays, but each species has its own specialty. Great hammerheads feed mainly on bony fishes, rays, and other sharks; scalloped and smooth hammerheads hunt squid and fish; and the bonnethead eats crustaceans along with large amounts of seagrass.

Great hammerheads feed primarily on teleost (bony) fishes and other elasmobranchs (sharks and rays), from small rays to large sharks, and off eastern Australia large individuals rely primarily on other sharks and rays. Scalloped hammerheads take a mixture of fish, crustaceans, and cephalopods, with squid as a primary prey item, and smooth hammerheads feed largely on squid.

Scalloped hammerheads appear to hunt away from the islands where divers see them. In a tracking study at Wolf Island in the Galápagos led by Dr. Alex Hearn, now a professor at the Universidad San Francisco de Quito, none was ever seen feeding at the island. In the Gulf of California, a tagged female dived to at least 980 meters (3,215 feet), possibly pursuing prey such as deep-water squids, although the authors also raise navigation and energy-saving swimming as explanations. The bonnethead, by contrast, eats crustaceans and seagrass, with seagrass making up as much as 62.1% of its gut contents by mass. The feeding chapter covers each species’ diet.

What eats hammerhead sharks?

Young hammerheads are eaten by other sharks, including adult hammerheads, and killer whales have been seen catching smooth hammerheads. For adults, fishing is a major threat: the large species are caught worldwide, as target and bycatch, and kept for their meat and fins.

In Kāne’ohe Bay, Hawaii, scalloped hammerhead pups have been found in the stomachs of adult hammerheads and blacktip sharks. In the Galápagos, a study co-authored by Hearn analyzed DNA in the feces of 107 juvenile blacktip sharks. Scalloped hammerhead was the fourth most common prey detected, at 11.7% by the study’s occurrence measure, although DNA alone cannot show whether the blacktips caught live hammerheads or scavenged them. Off New Zealand, a killer whale was seen to catch a smooth hammerhead about 1 meter (3 feet) long, in the first published observations of killer whales feeding on hammerheads.

Are hammerhead sharks dangerous to humans?

The studies behind this page describe what hammerheads eat and how they behave, but none measures the risk they pose to people, so they cannot settle the question. What they do record is that the scalloped hammerheads divers meet in schools are shy and seldom approach. In two ray kills watched in the Bahamas, the great hammerheads also showed no aggression toward the people nearby. The great hammerhead, the largest species, is nonetheless a powerful predator that deserves the same respect as any big shark.

Hammerheads eat fish, squid, crustaceans, rays, and other sharks. Near Bimini in the Bahamas, two great hammerheads, seen in separate encounters, killed rays with people in the water nearby and chased off scavenging blacktip sharks. Yet, as Demian Chapman and Samuel Gruber noted in 2002, “neither hammerhead exhibited any aggression towards human observers in the immediate vicinity.”

At Darwin and Wolf islands in the Galápagos, guided divers hold position among the boulders and stay clear of the cleaning stations, an approach designed to minimize disturbance to the schools. The diving chapter covers how and where to dive with hammerheads responsibly.

Where do hammerhead sharks live?

Hammerheads live in tropical and warm-temperate seas worldwide, from shallow coastal bays to the open ocean around islands and seamounts, and the smooth hammerhead also ranges into temperate waters. The best-known gatherings are the scalloped hammerhead schools at the Galápagos, Cocos, and Malpelo islands in the Eastern Pacific, while great hammerheads are among the most common sharks in research surveys off South Florida.

Much of what is known about those schools comes from Dr. Alex Hearn’s group, now based at the Universidad San Francisco de Quito, and its partners. At Wolf Island, all but one of the tagged sharks were detected at Shark Point, on the island’s southeastern corner. Later tracking linked Darwin and Wolf with Cocos Island off Costa Rica and Malpelo Island off Colombia. MMF’s scientists meet the schools at Darwin Island, where they work with partners on the Galápagos Whale Shark Project. The sharks also use deep water: in a global analysis we co-authored, 17 tagged adult scalloped hammerheads ranged from the surface to 971 meters (3,186 feet), and the 16 with full depth records had a median depth of 43.3 meters (142 feet).

In Florida, the great hammerhead was among the five most common sharks in drumline surveys from Miami through the middle Keys, with 238 caught in 2009–2021. Great hammerheads tagged at Bimini in the Bahamas and at Jupiter, Florida, made return migrations of about 3,000 kilometers (1,900 miles). The Florida Manta Project’s aerial and boat surveys have recorded hammerheads there too.

Why do hammerhead sharks swim in schools?

No one knows for certain. Scalloped hammerheads, the species known for schooling, gather by day on the up-current side of offshore islands and seamounts, and many leave at night, presumably to feed. Proposed benefits of the daytime schools include rest, cleaning, and social contact that may lead to mating.

At Wolf Island, Hearn’s team found the schools mainly made up of females and saw cleaning behavior frequently, with small fish removing parasites from the sharks’ skin. A follow-up study found that, while sheltering at the island by day, the sharks select both location and depth, where they may carry out essential activities such as cleaning and thermoregulation, the control of body temperature. Not every shark leaves at night, however: three sharks tracked in that study stayed within 500–700 meters (1,600–2,300 feet) of Wolf Island. Earlier work, summarized in a 2019 Cocos Island study co-authored by Hearn, suggested that schools may serve an energy optimization function or as a landmark for nocturnal foraging. They may also ease the social activity that leads to mating.

The schools are mostly female, 80% of 134 sharks tagged at Wolf and Darwin, possibly because young females move offshore at a smaller size than males, as they do in the Gulf of California. The schooling chapter covers the schools and their migrations.

How are baby hammerhead sharks born?

Hammerheads give birth to live young, called pups, which are nourished in the womb by an early placenta formed from the yolk sac. Litter sizes vary between species, for example 4–16 pups in the bonnethead and 20–50 in the smooth hammerhead, and the young of many species grow up in shallow coastal nurseries.

Off Ecuador, scalloped hammerheads are pregnant for 10–11 months and give birth to pups of 47–55 centimeters (19–22 inches). Great hammerheads have 6–42 pups after an 11-month pregnancy, born at 50–70 centimeters (20–28 inches), while the bonnethead’s pregnancy, at about 4.5–5 months, is one of the shortest known in sharks.

In the Galápagos, Hearn’s team at the Universidad San Francisco de Quito’s Galápagos Science Center found a putative, or suspected, scalloped hammerhead nursery at Puerto Grande, a shallow bay on San Cristóbal Island. The researchers describe it as the first oceanic island location of a nursery ground for this species in the Eastern Tropical Pacific. Life in a nursery is hard. In Kāne’ohe Bay, Hawaii, an estimated 7,700 scalloped hammerheads are born each year, but only 7–15% remain in the bay to age one, and starvation appears to be a primary cause. In Florida, researchers identified the first probable great hammerhead nursery on the US Atlantic coast in Biscayne Bay, less than 7 kilometers (4.3 miles) from downtown Miami. The chapter on baby hammerhead sharks covers pups and nurseries.

How long do hammerhead sharks live?

Great hammerheads have been aged at up to 44 years and scalloped hammerheads at 30.5 to 35 years, so the large species are long-lived sharks. Validated maximum ages for the small bonnethead are about 16 years for males and 18 for females.

These ages come from counting growth bands in the vertebrae, much like tree rings. Great hammerheads have reached 44 years in the northwest Atlantic and 39 in the western central Pacific. The oldest scalloped hammerheads aged were 30.5 years in the northwest Atlantic and Gulf of Mexico and 35 years, a female, in Indonesia. For adult scalloped hammerheads, however, the assumption that one band forms each year still needs validation. Bonnetheads have validated maximum ages of 16.0 years for males and 17.9 for females.

Long life comes with late maturity, which is why fished populations recover slowly: female smooth hammerheads mature at an estimated 15 years. The size chapter covers growth and age.

What is the difference between a bonnethead and a hammerhead?

A bonnethead is a hammerhead. It belongs to the same genus as the scalloped and great hammerheads, but it is a small coastal species, reaching 1.5 meters (5 feet). Its rounded, spade-shaped head is the least expanded in the family.

Bonnetheads (Sphyrna tiburo) have a broadly rounded, spade-like head and the least degree of lateral head expansion of all hammerhead species, and live along both the Atlantic and Pacific coasts of North and South America. Mature males develop a prominent bulge along the front edge of the head. The types chapter compares all the species.

Are hammerhead sharks endangered, and how many are left?

Most hammerhead species are threatened with extinction: of the nine on the IUCN Red List, six are Critically Endangered, including the scalloped and great hammerheads. How many are left is unknown for the great and smooth hammerheads, whose assessments report no data on global population size, so scientists measure how fast their numbers are falling instead.

The scalloped, great, winghead, scalloped bonnethead, scoophead, and smalleye hammerheads are Critically Endangered, the bonnethead is Endangered, the smooth hammerhead Vulnerable, and the Carolina hammerhead Data Deficient. For both the great and smooth hammerheads, the assessments state that no data are available on global population size.

The declines are steep. The 2019 assessment estimated median reductions of 76.9–97.3% over three generations (72.3 years) for the scalloped hammerhead. At Cocos Island, Costa Rica, its relative abundance in 34,342 dives logged by dive guides fell by 75% between 1993 and 2019. Local counts are possible where hammerheads gather: at Darwin Island, a study co-authored by Hearn estimated resident populations of about 500, 670, and 400 scalloped hammerheads on field trips in September 2011, September 2012, and October 2013.

Fishing is the main cause. Scalloped hammerheads are caught as target and bycatch in longline, purse seine, and gillnet fisheries and retained for the meat and fins. A pilot DNA study traced 61.4% of scalloped hammerhead fin trimmings sampled in Hong Kong to the Eastern Pacific; see MMF’s overfishing page. Ocean temperature matters too: at Cocos, predicted counts were 105.6% higher in strong La Niña than strong El Niño conditions, a change the authors attribute mainly to the sharks moving elsewhere. MMF’s climate change page uses the study as a case study.

Protection has grown. The three large species were listed on CITES Appendix II in 2013, in effect from September 2014. The great and scalloped hammerheads were listed by the Convention on Migratory Species in 2014, and the scalloped hammerhead became the first shark protected by the US Endangered Species Act, for four of its six populations. A global analysis we co-authored, published in 2021, found that hammerheads were rebuilding in the Northwest Atlantic, owing to strictly enforced quotas throughout their US range. In the Galápagos, MMF’s work helped secure the Hermandad Marine Reserve. The threats chapter sets out what else is being done, and the hammerhead hub and guide contents lead to every chapter. You can support MMF’s shark research in the Galápagos, carried out with Hearn’s team, through the Galápagos silky shark project.