Baby hammerhead sharks are called pups, and they are born live rather than hatched from eggs, in litters that range from 4 to 16 pups in the bonnethead to 20 to 50 in the smooth hammerhead. Great hammerhead pups are among the largest, at 50 to 70 centimeters (20 to 28 inches), and young scalloped hammerheads grow up in coastal nurseries such as Hawaii’s Kāne’ohe Bay, staying up to a year or more. For the bonnethead, the Red List gives a birth size of 35 to 40 centimeters (14 to 16 inches), although near-term embryos in Florida averaged only 21.5 to 29.7 centimeters (8.5 to 11.7 inches).
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.
For the scalloped hammerhead (Sphyrna lewini), the species MMF scientists encounter in the Galápagos, life is divided between adult aggregation sites at oceanic islands and seamounts, and nursery areas mainly along the continental coast. The large schools at Darwin and Wolf islands represent the offshore part of that cycle, made up of subadults and adults. This chapter of the guide follows the rest: pregnancy, birth, the nursery, and the first year of life.
What is a baby hammerhead shark called, and how big is it at birth?
A baby hammerhead is called a pup, and its size at birth depends on the species and region. The smallest birth size suggested in the Red List assessments is about 23 centimeters (9 inches) for the scalloped bonnethead (S. corona), a figure that is only suspected. The largest is 50 to 70 centimeters (20 to 28 inches) for the great hammerhead (S. mokarran), according to the 2019 assessment. For the other small species, the assessments give about 30 centimeters (12 inches) for the smalleye hammerhead (S. tudes) and 34 centimeters (13 inches) for the scoophead (S. media). For the bonnethead (S. tiburo), they give 35 to 40 centimeters (14 to 16 inches). Along Florida’s Gulf coast, however, near-term bonnethead embryos, those close to birth, averaged only 21.5 to 29.7 centimeters (8.5 to 11.7 inches) total length, depending on latitude, below the Red List’s birth range. For the great hammerhead, a study in the Arabian Gulf back-calculated a mean birth size of 71.2 centimeters (28 inches) from growth marks in the vertebrae.
Scalloped hammerhead pups vary by region. Off Ecuador, size at birth was estimated at 47 to 55 centimeters (19 to 22 inches) total length, and in Indonesian waters birth occurred at about 40 centimeters (16 inches). In Kāne’ohe Bay, Hawaii, newborns measured 32 to 41 centimeters (13 to 16 inches) precaudal length, measured from the snout to the base of the tail rather than its tip. A newborn is recognizable by its open umbilical wound, which healed within about two weeks in captive pups.
Pups are not simply miniature adults. In museum specimens of scalloped hammerheads 32 to 130 centimeters (13 to 51 inches) long, head area grew less than it would if the head kept its proportions. Pectoral fin length and some other fin measurements, by contrast, grew faster than body length. The authors link these changes to the move from coastal water into the open ocean. Within the juvenile sizes studied, smaller sharks therefore had proportionally more head area, although the study did not follow the change into adulthood. The size chapter covers growth to adulthood, and the hammer chapter explains what the head is for.
Do hammerhead sharks lay eggs or give live birth?
Hammerheads give birth to live young, and their embryos are nourished first by yolk and then through a placenta that forms from the yolk sac. Hammerheads have evolved a derived form of embryonic nutrition in which the yolk sac merges with the wall of the uterus to form an early placenta-like organ. In the bonnethead, embryos first live on their yolk, and the fetal side of the placenta attaches to the uterine wall near mid-gestation, allowing nutrients to pass from mother to embryo. Embryos shorter than about 10 centimeters (4 inches) still depend on yolk alone. The Red List assessments describe the bonnethead, the winghead (Eusphyra blochii), and the smalleye hammerhead as placental livebearers.
The placenta is also useful to scientists. Because a placental shark builds its first tissues from its mother’s resources, researchers can use a newborn’s tissue chemistry to infer the origin of the energy it accumulated during gestation. In the Eastern Pacific, this approach has been used to link nurseries to adult females from particular areas.
Fertilization is internal. Males transfer sperm with their claspers, the paired external male copulatory organs, which lengthen and calcify as a male matures. At Wolf Island in the Galápagos, observers filmed a scalloped hammerhead pressing its claspers into a sand patch and then shaking them. They believe it is the first record of a shark chafing its claspers, and suggest that it might help keep these heavily parasitized organs healthy for mating.
How many babies do hammerhead sharks have?
Hammerhead litters range from a few pups in the small coastal species to as many as 50 in the smooth hammerhead. The figures by species:
| Species | Litter size | Pregnancy | Size at birth (total length) |
|---|---|---|---|
| Scalloped hammerhead | 14–41 (Indonesia) | 10–11 months (Ecuador) | 47–55 cm (19–22 in), Ecuador |
| Great hammerhead | 6–42 | 11 months | 50–70 cm (20–28 in) |
| Smooth hammerhead | 20–50 | 10–11 months | 49–63 cm (19–25 in) |
| Winghead | 6–25, average 11 | 8–11 months | 32–50 cm (13–20 in) |
| Smalleye hammerhead | 5–12 | 10 months | About 30 cm (12 in) |
| Bonnethead | 4–16 | About 4.5–5 months | 35–40 cm (14–16 in) |
In Indonesian waters, pregnant scalloped hammerheads carried 14 to 41 embryos, with a mean of 25, and larger females carried more. A 2018 review gives 13 to 23 pups in the Eastern Pacific and 30 to 40 in the northwestern Gulf of Mexico.
Bonnethead embryo size varies with region even where litter size does not. Along Florida’s Gulf of Mexico coast, bonnethead litters averaged 10 to 11 pups at all three latitudes studied, while near-term embryos were largest in the north, where the water is coldest.
How long are hammerhead sharks pregnant?
The large hammerheads are pregnant for 10 to 11 months, while the bonnethead’s pregnancy lasts only about 4.5 to 5 months, one of the shortest known in any shark. Off Ecuador, the smallest scalloped hammerhead embryos, about 11 to 14 centimeters (4 to 6 inches) long, were found in October and the largest, about 44 to 55 centimeters (17 to 22 inches), in June. That pattern is consistent with a 10- to 11-month pregnancy running from September to June or July, although the authors caution that it rests on only 12 pregnant females. The great hammerhead breeds on a likely two-year cycle, and how often smooth hammerheads breed is unknown. For the scalloped hammerhead, the data suggest a two-year cycle in the North Atlantic and an annual cycle in the South Atlantic.
The bonnethead is the outlier. Its short pregnancy is repeated every year, and in the eastern Gulf of Mexico bonnetheads give birth in July in Florida Bay but in September in northwest Florida, although the length of pregnancy is the same. Bonnetheads in the eastern Gulf of Mexico also mature at only 2 to 3 years of age. Together, these traits give bonnetheads population growth rates considerably higher than those calculated for other sharks and rays with similar methods.
In the field, pregnancy is often judged by eye. The seasonal aggregations in the Galápagos are made up mainly of adult females, many of whom are thought to be pregnant based on their expanded girth. Great hammerheads at Bimini in the Bahamas have been observed with distended abdomens, appearing to be pregnant.
Where are baby hammerhead sharks born?
Scalloped hammerheads give birth in shallow coastal nurseries, often turbid bays, estuaries, and river mouths, where newborns appear each year. For most other hammerheads, nurseries are known mainly from the juveniles found in them, so where the mothers give birth is less certain.
A nursery is defined by its young rather than by births. The standard criteria, greater abundance than elsewhere, extended residence, and repeated use across years, describe where young sharks live. A 2018 review identified the scalloped hammerhead as the only large hammerhead whose nursery grounds have been well described, although a probable great hammerhead nursery has since been found in Florida.
In Kāne’ohe Bay on O’ahu, Hawaii, scalloped hammerhead pups are born between May and September. Along Mexico’s central Pacific coast, births occur from May to December, peaking from June to August, within 2 kilometers (1.2 miles) of river mouths in shallow, turbid, soft-bottomed water. On the western Yucatán Peninsula in the southern Gulf of Mexico, the nursery lies in relatively shallow and turbid waters fed by river discharge. In Indonesia, births occur mainly in October and November, and a river estuary in Fiji has also been identified as a nursery.
In Kāne’ohe Bay, pups spend the day in a loose school hovering about 1.5 meters (5 feet) off the bay floor. The authors suggest that these daytime aggregations in the most turbid parts of the bay may be a defense against predation.
Whether females return to give birth where they were born, known as philopatry, is still debated. Along Mexico’s Pacific coast, mitochondrial DNA, which passes only from mother to offspring, differed between nurseries 1,000 to 2,000 kilometers (620 to 1,240 miles) apart but not between nearby ones. The authors read this as females returning to their home region, whereas a study spanning Costa Rica to Ecuador found no support for female philopatry along that coastline. The schooling chapter covers this debate and the adults’ migrations.
Is there a hammerhead shark nursery in the Galápagos?
Probably yes. A team including Dr. Alex Hearn of the Universidad San Francisco de Quito’s Galápagos Science Center has identified a mangrove bay on San Cristóbal Island as a putative scalloped hammerhead nursery, one suspected but not yet confirmed. Hearn designed and supervised the study, and the site is the first such nursery for the species at an oceanic island in the Eastern Tropical Pacific.
The question had been open for years. In an earlier review of the Galápagos Marine Reserve’s sharks, Hearn and his colleagues noted that many female hammerheads leave Darwin and Wolf islands between March and June. Four tagged sharks were detected at Cocos Island, 700 kilometers (435 miles) to the northeast. Because newborn hammerheads had not been found in significant numbers in the Galápagos, they suggested that these departures might be migrations to pupping grounds.
Tracking by the same group showed that long-distance trips from the northern Galápagos were made mostly by females that were probably mature, mostly between March and May. The authors suggested that these females may travel to Cocos, and possibly the coast of Costa Rica, for the final stages of pregnancy and birth. At Malpelo Island, Colombia, a study co-authored by Hearn found that tagged hammerheads left in March and April, when many females appeared pregnant. It also reported that 84 of 125 females caught illegally inside the protected area in February 2002 were pregnant.
Puerto Grande, a shallow, mangrove-fringed bay on the northwestern coast of San Cristóbal, changed that picture. After pups were found there in 2017, the team caught hammerhead pups every time they set their beach seine, a net drawn through the shallows, 66 in all between 2017 and 2020. The 54 they measured averaged 52 centimeters (20 inches) total length and ranged from 42 to 78 centimeters (17 to 31 inches), the sizes of newborns and young of the year (pups in their first year). Baited underwater cameras recorded pups only at Puerto Grande and at none of the other San Cristóbal bays surveyed. Four pups fitted with acoustic tags stayed in the bay for 13 to 93 days and were detected on at least 89% of those days. Because the site appears to satisfy, at least partially, the standard nursery criteria, the authors call it a putative nursery, and longer tracking is needed to confirm how long pups stay. They also note that Puerto Grande may be one of the few places in the region where young hammerheads are not vulnerable to fishing.
It may not be the only site. In 2019 the Galápagos National Park Directorate found a site on Santa Cruz Island where hammerhead pups may also occur regularly. Juvenile scalloped hammerheads 44 to 89 centimeters (17 to 35 inches) long, all with healed umbilical wounds indicating they were younger than a year, have also been sampled in shallow mangrove bays on Santa Cruz.
Chemistry links Galápagos pups to the adult females offshore. Newborns in the Galápagos Marine Reserve carry stable-isotope signatures (chemical patterns that reflect diet) similar to those of the adult females living in the archipelago, suggesting they are direct descendants of those females. Newborns on the coasts of Ecuador and Panama, by contrast, match females from several locations across the Eastern Tropical Pacific. Genetic sampling of young sharks from Costa Rica to Ecuador found no parent–offspring pairs with Galápagos adults, but the authors suggest their sample may have been too small, and that birth on the mainland coast remains possible. MMF’s Galápagos program, including its part in the Galápagos Whale Shark Project at Darwin Island, works in these same waters.
Where do great and smooth hammerheads give birth?
Much less is known. A 2021 study noted that pupping by great hammerheads has never been witnessed, and the same study described the first probable great hammerhead nursery on the US Atlantic coast, in Biscayne Bay near Miami. A lack of newborns in nearshore habitats has been used to argue that great hammerheads mainly give birth offshore. At a site in Biscayne Bay less than 7 kilometers (4.3 miles) from downtown Miami, researchers caught nine juveniles under 200 centimeters (6.6 feet) between June 2018 and January 2020. Three of them were under 100 centimeters (39 inches) and probably young of the year, and two still had recreational fishing hooks embedded in their jaws. The authors suggest that protecting nursery habitat or reducing fishing pressure there may improve the species’ conservation. The aerial and boat surveys of MMF’s Florida Manta Project have also recorded hammerheads.
Farther north, a female great hammerhead tracked from Bimini spent two consecutive summers off South Carolina, and the authors suggest that coastal South Carolina and Georgia might be important summer pupping sites. In the Arabian Gulf, newborns of 67 to 74 centimeters (26 to 29 inches) with open umbilical scars were landed in June and July, pointing to suspected births in May and June. That study, however, reported that no pregnant or adult females have been found in Saudi Arabian waters.
For the smooth hammerhead, Mossel Bay on South Africa’s south coast is considered a putative pupping ground, where young sharks sampled in December 2013 and December 2014 belonged to genetically distinct cohorts. The authors found the same maternal lineages contributing in both years, and suggest that the fathers may account for the differences.
How many baby hammerheads survive their first year?
Probably only a small fraction, although the best estimate measures how many pups stay in a nursery rather than how many survive. In Kāne’ohe Bay, a 28-month tagging study estimated that 7,700 hammerheads are born in the bay each year (standard deviation 2,240), and that only 7 to 15 percent of them remain from birth to age one. That loss combines natural deaths, fishing, and emigration, which the study could not separate.
Hunger appears to be a large part of it. Most pups recaptured within 60 days had lost weight, which led the authors to conclude that malnutrition and starvation make a major contribution to this mortality. Pups have high metabolic rates, and their most common prey, snapping shrimp, are low in calories, suggesting that a significant percentage of pups may starve. Newborns fare worst: only 41% of pups with open umbilical scars had food in their stomachs, against 87.8% of those with healed scars. A study of daily rations concluded that food is limiting in the bay, which contradicts one of the most common explanations for why sharks use nurseries. The feeding chapter covers what young hammerheads eat.
If the bay does not offer plentiful food, it may offer safety instead. Adult hammerheads, which are known to prey on pups, occur in the bay from at least March to September, but the bay holds fewer large sharks and other top predators than the open water outside. The authors conclude that nursery grounds are a valuable refuge from predators rather than a source of abundant prey.
How fast do baby hammerheads grow, and when do they leave the nursery?
Young scalloped hammerheads in Kāne’ohe Bay can remain for up to a year or more, and young smooth hammerheads in the Mexican Pacific rely on coastal food for about two years before moving offshore. Both leave long before they can breed, which for a female scalloped hammerhead can take more than a decade.
In Kāne’ohe Bay, scalloped hammerhead pups grew about 9.6 centimeters (3.8 inches) a year in precaudal length. A separate study modeled early growth in great hammerheads in the Arabian Gulf. Their growth rate after birth was estimated at 83.3 centimeters (33 inches) a year in total length, slowing to 22.7 centimeters (9 inches) a year by about age three. Off Miami, one recaptured juvenile great hammerhead grew 25 centimeters (10 inches), from 103 to 128 centimeters (41 to 50 inches), in 146 days.
In the Gulf of California, at least part of the scalloped hammerhead population moves from coastal to offshore waters at about 115 centimeters (45 inches) total length. The sexes do not leave at the same size. Females move offshore at a smaller size than males and feed more on open-water prey, and the study’s author argued that this lets them grow faster to the larger size needed to carry embryos. In the same region, the smallest mature female measured 217 centimeters (7.1 feet), whereas males matured from 163 centimeters (5.3 feet).
Reaching that size takes years. In Indonesian waters, scalloped hammerheads were estimated to mature at 8.9 years for males and 13.2 years for females, although the male estimate rests on only 23 sharks and should be used with caution. Female great hammerheads mature at 5.5 to 8.3 years, depending on the region, and female smooth hammerheads at an estimated 15 years.
Can a hammerhead shark have a baby without a male?
Yes. In 2001 a captive female bonnethead gave birth to a pup that had no father, the first genetic evidence of virgin birth in any shark, ray, or chimaera. On December 14, 2001, one of three adult female bonnetheads held in an aquarium gave birth to a normally developed, live female pup. All three had been held without males for three years, since they were caught in the Florida Keys as immature sharks less than a year old, which made stored sperm an improbable explanation. DNA profiling then showed that the pup carried no genetic material from a father.
In virgin birth (parthenogenesis), the embryo develops from the mother’s genetic material alone, and the type found in this bonnethead leads to a loss of genetic diversity. The authors suggest that it may occur where females have difficulty finding mates, as might happen in wild populations reduced by overexploitation.
Why it matters for conservation
The large hammerheads mature late, and their young depend on coastal waters that are often heavily fished, so a depleted population can rebuild only slowly. A global genetic study of the scalloped hammerhead concluded that regional populations, if depleted, will not recover swiftly through immigration, but slowly through reproduction. It added that prudent management must include not only protection of adults but also access to regional nurseries. The scalloped hammerhead is listed as Critically Endangered on the IUCN Red List, and the threats chapter explains why.
Fishing pressure often falls on the young. Off Ecuador, much of the scalloped hammerhead catch landed in 2003–2009 was immature. In the southern Gulf of Mexico, newborns and young of the year made up 1,109 of 1,885 scalloped hammerheads recorded in small-scale fishery landings. Coastal nurseries are also exposed to a changing ocean. An assessment of 132 Eastern Tropical Pacific sharks, rays, and chimaeras, co-authored by Hearn, found that coastal species relying on inshore nursery areas are the most vulnerable to climate change. MMF’s pages on overfishing and climate change cover both threats.
Protection is already in place in some of these places. Mexico bans shark fishing on its Pacific coast from May to July, according to a 2022 study, a closure that covers part of the scalloped hammerhead birth season. Scientists have proposed a minimum catch size and spatial closures for young smooth hammerheads and longer fishing restrictions at nurseries such as La Reforma, Sinaloa. In the Galápagos, Puerto Grande lies within the Galápagos Marine Reserve. Recovery is possible: hammerheads have the reproductive potential to recover from population depletion if mortality is reduced. A global analysis published in 2021, co-authored by an MMF scientist, also found that hammerhead populations are rebuilding in the Northwest Atlantic, owing to strictly enforced quotas throughout their US range.
In the Galápagos, MMF’s shark research includes the silky shark tracking study led by Dr. Alex Hearn at the Universidad San Francisco de Quito. Dr. Chris Rohner leads MMF’s part, and the project page explains how to support the work.