Scalloped hammerheads are the hammerhead sharks best known for schooling: by day, hundreds of them, mostly females, hold position on the up-current side of offshore islands in the Eastern Tropical Pacific. These islands include Darwin and Wolf in the Galápagos, Cocos Island off Costa Rica, and Malpelo off Colombia, and at night the sharks move into open water, most likely to feed. Why they school is still not settled, but tracking shows that the schools are fluid: individual sharks move between islands hundreds of kilometers apart, leave for months at a time, and return.
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.
Do all hammerhead sharks swim in schools?
No. The scalloped hammerhead (Sphyrna lewini) is the species best known for schooling, whereas the 2019 Red List assessment describes the great hammerhead (Sphyrna mokarran) as “a generally solitary, coastal and semi-oceanic pelagic shark”. The types chapter of this guide explains how to tell the species apart.
Scalloped hammerhead schools are best known from the offshore islands of the Eastern Tropical Pacific, including San Benedicto in Mexico’s Revillagigedo Archipelago, and from seamounts in the Gulf of California, but they form elsewhere too. At Mikomoto Island, off southeastern Japan, tagged females showed “high seasonal residency predominantly from August to November”. In a marine park south of Perth, Western Australia, drone surveys counted up to 62 scalloped hammerheads per survey, most likely immature. Not every shark could be identified to species, however, so smooth hammerheads may also be present. Off Grand Cayman in 2022, a deep-water camera filmed at least 11 juveniles swimming close together below 200 m (656 feet).
Where do hammerheads school in the Galápagos?
At the southeastern corners of Darwin and Wolf, the two small islands at the archipelago’s northern tip, where the prevailing current meets the island. The tourism page describes how divers are asked to behave around these schools.
In 2010, Dr. Alex Hearn and colleagues published a detailed tracking study of the Wolf Island schools. Hearn was then at the University of California, Davis, and the Charles Darwin Foundation; he is now at the Universidad San Francisco de Quito’s Galápagos Science Center. His team fitted 61 sharks with acoustic transmitters (tags whose signals are logged by underwater receivers), and all but one were detected at Shark Point on the island’s southeastern corner. Sharks were detected at each of the southeastern sites for a median of 8–11 days, whereas only half were ever detected at the western sites, for a median of a day or less. A follow-up study by James Ketchum, Hearn, and colleagues actively tracked seven sharks at Wolf. By day, the sharks’ core areas (the central space in which each was most active) averaged just 0.25 km² (62 acres) and never exceeded 0.6 km² (150 acres).
Diver surveys from 2007 to 2012 recorded an average of 125.7 scalloped hammerheads per diver-hour in the far north, against fewer than three in the central islands. Stereo-video surveys, which use paired cameras to count and measure fish, were carried out at Darwin and Wolf in the cool seasons of 2013 and 2014. They revealed what the authors reported in 2016 as the largest reef fish biomass (the combined weight of fish in an area) ever recorded, and scalloped hammerheads alone made up 48.0% of it. When the schools are largest is less settled, as the section on seasons below explains.
Why do scalloped hammerheads school?
Nobody knows for certain. The schools appear to be daytime resting groups rather than hunting parties, and the leading explanations, from cleaning and temperature control to energy saving and social contact, remain hypotheses.
Hearn’s team never saw a hammerhead feeding at Wolf, but they frequently saw cleaning behavior at Shark Point and Darwin Arch, where small fish remove parasites from the sharks’ skin. They suggested that the up-current hotspots may also be the best place from which to launch nightly foraging trips. To make these daily movements and pick out the same few sites, they added, the sharks “would appear to require the ability to detect and interpret some environmental property”. Ketchum and colleagues concluded that the sharks select both location and depth while refuging, that is, while resting at a central site by day. They chose the up-current side of the island and the top of the thermocline (the boundary between warm surface water and cooler water below). There, the authors suggested, the sharks “may carry out essential activities such as cleaning and thermoregulation”, the regulation of body temperature. A 2019 study at Cocos Island, with Hearn as second author, summarizes earlier work reporting that sharks in these aggregations appeared uninterested in feeding. That work suggested that the schools save energy, serve as landmarks for night foraging, and ease the social activity that leads to mating.
Two more recent observations add detail. On a sand patch off Wolf Island, scalloped hammerheads were seen rubbing parts of their bodies against the seabed on 24 occasions, the first report of this “chafing” behavior in the wild for any hammerhead. Most scratched the edge of the mouth, likely because parasites are common there. Genetic testing of 134 adult females from Darwin and Wolf, meanwhile, found only one “possible” pair of full siblings, suggesting that these aggregations are not family groups.
Why are hammerhead schools mostly female?
One explanation is that young females leave coastal waters for offshore islands and seamounts at a smaller size than males, so that adult females come to dominate the offshore aggregations. Of 134 scalloped hammerheads tagged at Wolf and Darwin, 80% were female; at Cocos Island, only one male was identified among 84 tagged sharks.
That explanation comes from A. Peter Klimley’s 1987 study of scalloped hammerheads in the southern Gulf of California. There, females of 105–120 cm (41–47 inches) total length were already present in offshore schools, while males first appeared at 150–165 cm (59–65 inches). Offshore, the young females fed more on open-water prey, and with greater success, than males of the same size. Klimley argued that this lets females grow faster to the larger size needed to carry embryos, and so reach maturity at a similar age to males. In his study, the smallest mature female measured 217 cm (7.1 feet) total length, whereas males matured from 163 cm (5.3 feet). Small males, he suggested, may stay inshore because the benefit of faster growth “might not exceed the cost of higher risk of mortality due to predation”. The size chapter covers size at maturity in more detail.
How many hammerheads are in a school?
Often hundreds, but a single school is hard to count. Researchers describe partially sex-segregated schools of more than 200 individuals. At Cocos Island, dive guides have recorded up to 1,000 scalloped hammerheads on a single dive, although a count per dive does not establish how many sharks were in any one school.
The Cocos figures come from 34,342 dives logged by 46 dive guides between 1993 and 2019, in which scalloped hammerheads were seen on 77.7% of dives, with an average of 42.1 per dive when present. Counting a school of hundreds by eye is necessarily rough, and the guides approximated their counts when they met large groups. At Darwin Island, a 2025 study co-authored by Hearn combined acoustic tags with underwater visual surveys on field trips in September 2011, September 2012, and October 2013. It estimated a resident population of about 500 scalloped hammerheads in 2011, 670 in 2012, and 400 in 2013. The total number of sharks using the site was higher, up to an estimated 1,740 in 2012. The authors suggest that Darwin therefore hosts many transient individuals, mostly females, moving between the island and offshore feeding or breeding grounds or other aggregation sites.
Where do hammerheads go at night?
Into open water, often several kilometers from the island, where many make deep dives, most likely to feed, before returning around dawn.
At Wolf Island, one 2-m (6.6-foot) female was typically first detected at Shark Point around 6 am and left around 6 pm. At Cocos, most detections fell between dawn and dusk, with a mean detection time of 11:30 am, and sharks at Malpelo and San Benedicto follow the same day-night pattern. Hearn’s team followed one female by boat over three nights in November 2008. Each night she moved to an area 5.6 km (3.5 miles) east of Wolf and returned before dawn. Near the island she generally stayed above 50 m (164 feet), but she dived to 100–300 m (330–980 feet) as she headed out to sea in the evening and back before daybreak.
Scalloped hammerheads can dive much deeper still. A female tagged off Hawaii Island in 2020 made a daytime dive to 1,240 m (4,068 feet), the deepest dive on record for the species when it was reported in 2022. For much of the time, however, scalloped hammerheads stay far shallower. A global analysis of tag data that we co-authored found a median depth of just 43.3 m (142 feet) in the depth records of 16 adult scalloped hammerheads. The feeding chapter covers the deep dives and what the sharks eat on these night trips.
Do hammerheads stay at the same island all year?
Some return to the same island for years, but few remain there continuously. Scalloped hammerheads are present mostly by day, individuals frequently leave for weeks or months, and detections at the offshore islands decline for part of each year, most consistently from about March to May.
At Cocos Island, 84 tagged sharks had an average residency index (the proportion of monitored days on which a shark was detected) of 0.52, falling to 0.24 for the 27 sharks tracked for 120 days or more. Sharks were notably absent from March through May, and five returned after absences of nine months or longer. At San Benedicto, 11 tagged adults remained in the area for 1 to 27 months, with a mean of 8.9 months. In the Galápagos, hammerheads were present at Darwin and Wolf throughout the year, but the number detected dropped from March to May every year and rose again in June.
The peak season is less consistent. In Hearn’s 2010 study, diver counts at Wolf were highest in January and lowest in May, whereas surveys across 2007–2012 found more hammerheads from May through October. At Malpelo, the largest schools were seen from January to March, with many tagged sharks leaving between March and early April. The studies define the seasons differently, but they share one signal: a departure from the islands around March to May.
Do hammerheads travel between the Galápagos, Cocos Island, and Malpelo?
Yes, but only a small share of tagged sharks were detected making these open-ocean crossings; most movement is between neighboring islands.
Within the northern Galápagos, movement between Wolf and Darwin, 38 km (24 miles) apart, is routine: 12 of the first 14 sharks tagged at Wolf crossed to Darwin within six months. The fastest crossing took 11 hours and 56 minutes, but most took 1 to 10 days, suggesting that most crossings were not direct. In contrast, of 134 sharks tagged at the two islands over four years, only two were ever detected in the cooler central archipelago.
The first sign of longer links came when two of 18 hammerheads tagged in the Galápagos were detected at Cocos Island, within hours of each other, in April 2007. Over four years of tagging, three Galápagos sharks reached Cocos, about 700 km (435 miles) away, and one reached Malpelo, 1,186 km (737 miles) away, in 26 days. From Malpelo, five of 69 tagged sharks were recorded at Cocos, 627 km (390 miles) away, and one continued to the Galápagos, although such movements involved only about 7% of the sharks tagged there. At Cocos, five of 210 sharks tagged in the Galápagos and four of 83 tagged at Malpelo arrived, seven of them in April or May, and most stayed only one to five days. The authors suggest that these sharks “may use Cocos as a navigational waypoint or stopover” on seasonal journeys toward the coasts of Central and South America. Long-distance trips from the Galápagos were more common in the warm season and occurred mostly from March to May, the same months in which detections at the islands decline.
How far do hammerhead sharks migrate?
Hundreds to thousands of kilometers. Tracked journeys include about 3,350 km (2,080 miles) by a juvenile scalloped hammerhead, round trips of about 3,000 km (1,900 miles) by great hammerheads, and more than 6,600 km (4,100 miles) by a smooth hammerhead.
In the Gulf of California, a juvenile female scalloped hammerhead tagged at 95 cm (37 inches) total length traveled about 3,350 km in 10.5 months, north to the central gulf and back. She was recaptured 11.7 km (7.3 miles) from where she was tagged. Partway through, she moved from shallow coastal water into deeper offshore water, which the authors interpret as the change from “a nursery-inhabiting juvenile inshore to a migratory sub-adult offshore”.
Great hammerheads tagged at Bimini, Bahamas, and Jupiter, Florida, provided what the authors describe as the first evidence of return migrations, seasonal residency, and long-term site fidelity in the species. Sharks returned from as far north as Virginia, a round trip of about 3,000 km. The three long-distance migrants tagged at Bimini departed in April or early May, as the water warmed above 26 °C (79 °F). Not every great hammerhead migrates. Of sharks tagged in the Florida Keys, some made repeated round trips north into the Gulf of Mexico or along the Atlantic coast, while others stayed year-round, a pattern known as partial migration. MMF’s Florida Manta Project has also recorded hammerheads during its aerial and boat surveys.
The 2019 Red List assessment calls the smooth hammerhead (Sphyrna zygaena) “the most oceanic of the hammerhead species”. Off Baja California, a study that read mercury isotopes in the sharks’ tissues as chemical signatures of where they had fed estimated that young smooth hammerheads rely on coastal food for about two years before migrating offshore. In the eastern Atlantic, one adult female traveled 6,610 km (4,107 miles) in 150 days southeast along the coast of West Africa, crossing the equator. That 2018 study reported it as the longest migration recorded for the species. Off the US East Coast, juvenile smooth hammerheads moved between a summer area in the New York Bight and a winter area off Cape Hatteras, North Carolina. One juvenile’s track, followed for 441 days, summed to 7,319.5 km (4,548 miles), but that figure adds up the distances between estimated positions rather than measuring a single migration.
Not every hammerhead follows the scalloped hammerhead’s daily routine. Bonnetheads (Sphyrna tiburo), a small coastal hammerhead, showed no consistent day–night pattern of movement when tracked in a Florida estuary. The population also shifted away from parts of the estuary where salinity dropped sharply, returning when it recovered.
How hammerheads navigate across open ocean is still an open question. In experiments with wild-caught bonnetheads, magnetic map cues elicited homeward orientation, which the authors suggest “may help explain” the genetic structure of bonnethead populations.
Do female hammerheads return to where they were born?
Possibly, at a regional scale. Many adult females appear to leave the offshore islands to give birth near the coast, but genetic studies disagree on how faithfully they return to their own birthplace.
The annual departure of females from the islands has long been linked to pupping. At Malpelo, 84 of 125 female hammerheads fished illegally inside the protected area in February 2002 were pregnant. The authors say this observation supports “the idea that sharks are leaving the island in March and April to move to pupping areas”. Ketchum and colleagues called the long trips to Cocos, made mostly by females that were probably mature, “possible evidence of partial migration of females for breeding”. Hearn and colleagues noted that newborn hammerheads had not been found in significant numbers in the Galápagos, so the seasonal departure may be a migration to pupping grounds. Hearn’s group has since described a putative nursery at Puerto Grande on San Cristóbal Island, one suspected but not yet confirmed, and the first for the species found on an oceanic island in the Eastern Tropical Pacific. The reproduction chapter covers nurseries in detail.
Whether females return to the nursery where they were born (natal philopatry) is less clear. A global study of mitochondrial DNA, which is inherited only from the mother, found that nurseries linked by continuous coastline are well connected, but “oceanic dispersal by females is rare”; it judged natal philopatry to be low. In the Mexican Pacific, mitochondrial DNA differed between estuaries 1,000–2,000 km (620–1,240 miles) apart, but not between those 400–500 km (250–310 miles) apart. That pattern suggests that females return to their home region rather than their exact birthplace. Yet a study spanning about 2,000 km (1,240 miles) of coast from Costa Rica to Ecuador, plus the Galápagos, found that scalloped hammerheads there form a single genetic stock, with “no convincing evidence of philopatry”. At the scale of ocean basins, the 2019 Red List assessment stated that “males move across ocean basins, while females only move regionally”. A newer genetic analysis, however, concluded that neither sex migrates effectively across the Indian Ocean and that scalloped hammerheads “do not cross ocean basins”.
How does El Niño affect hammerhead schools?
Strongly. At Cocos Island, divers see far fewer scalloped hammerheads, and far fewer schools, in warm El Niño conditions than in cool La Niña conditions, a response the researchers think mainly reflects the sharks shifting their distribution.
Analyzing 27 years of the Cocos dive logs, Osgood and colleagues found that predicted hammerhead counts were 105.6% higher during strong La Niña than strong El Niño conditions. The predicted probability of observing a school of more than 50 sharks was also 118.0% higher. Each 1 °C (1.8 °F) increase in sea surface temperature was associated with a 10.3% reduction in predicted counts, the strongest response to temperature of any shark they monitored. An earlier analysis of the same logs recorded a major increase in hammerhead sightings in 1999–2000, the La Niña years that followed the 1997–98 El Niño. A decade of tourist-submersible dives at Cocos found that the probability of seeing scalloped hammerheads at 300–400 m (980–1,310 feet) rose from 20% in 2010 to 37% in 2019. Those authors suggest that the species’ depth range is being compressed between warming surface water and the oxygen-poor layer below.
Osgood and colleagues read the temperature effects as “primarily distributional change rather than overall population declines”. Over the full 27 years, however, the scalloped hammerhead’s relative abundance at Cocos fell by 75%, a continuing decline that they suggest reflects ongoing fishing; the threats chapter covers it. Looking ahead, a habitat model co-authored by Hearn projects that suitable habitat for scalloped hammerheads may shift south toward the Chilean Pacific by 2050. MMF’s climate change page uses Cocos as a case study.
Why it matters for conservation
Scalloped hammerheads are easiest to protect where they school and hardest to protect between those sites, because a shark that spends its days inside a marine reserve can spend its nights, and its migrations, outside it.
At Cocos, a 2019 study reasoned that nightly excursions may take sharks beyond what was then the protected area’s 22 km (14 mile) radius. There they may be exposed to longline fishing, which generally operates from dusk to dawn. In the Galápagos, Peñaherrera-Palma, Hearn, and colleagues reported satellite tracking of eight hammerheads in the Galápagos Report 2015–2016. In the cold season (June–October), 90% of the area the sharks used lay inside the Galápagos Marine Reserve, but in the warm season (December–April) only 30% did.
Governments across the region have expanded protection around the aggregation sites. Ecuador created the 40,000 km² (15,400 square mile) Darwin and Wolf Marine Sanctuary in March 2016, and MMF’s work helped secure the Hermandad Marine Reserve, a 60,000 km² (23,000 square mile) expansion declared in 2022. In 2017, Colombia expanded the protected area around Malpelo to 26,770 km² (10,336 square miles), and Mexico created the Revillagigedo National Park, “the largest no-take area in the waters surrounding the American Continent”. Klimley, Hearn, and colleagues argue that the region’s tracking data justify these reserves, and that protection should be expanded in El Niño years, when predators must range farther from the islands. They add that it “may be more feasible to protect species such as the scalloped hammerhead shark that aggregate within spatially confined biotic hotspots”.
Because the sharks cross borders, no single country can protect them alone. The Galápagos aggregations belong to a single genetic stock shared with the mainland coast, and the authors call for “coordinated, multinational management cooperation”. Important Shark and Ray Areas are habitats identified as important to these species. Across the Central and South American Pacific, an assessment that we co-authored found that they overlap with marine protected areas by only 15.6%, and with no-take areas by 7.3%. Genetic evidence also suggests that a depleted regional population will not recover swiftly through immigration, but slowly through reproduction, which is one more reason to protect the females that gather at these islands.
To support MMF’s shark research in the Galápagos, carried out with Dr. Hearn’s team at the Universidad San Francisco de Quito, visit the Galápagos silky shark project.