The average great hammerhead measures up to 3.5 m (11 ft) long and weighs over 230 kg (510 lb). A small percentage of the population, mostly or all females, are much larger. The longest great hammerhead on record was 6.1 m (20 ft). The heaviest known great hammerhead is a 4.4 m (14 ft) long, 580 kg (1,280 lb) female caught off Boca Grande, Florida in 2006. The weight of the female was due to her being pregnant with 55 near-natal pups.
The streamlined body of the great hammerhead with the expanded cephalofoil is typical of the hammerhead sharks. Adult great hammerheads can be distinguished from the scalloped hammerhead and the smooth hammerhead by the shape of the cephalofoil, which has a nearly straight front margin (as opposed to arched), with prominent medial and lateral indentations. The width of the cephalofoil is 23–27% of the body length. The teeth are triangular and strongly serrated, becoming more oblique towards the corners of the mouth. There are 17 tooth rows on either side of the upper jaw with 2–3 teeth at the symphysis (the midline of the jaw), and 16–17 teeth on either side of the lower jaw and 1–3 at the symphysis. The first dorsal fin is distinctive, being very tall and strongly falcate (sickle-shaped), and originates over the insertions of the pectoral fins. The second dorsal fin and anal fin are both relatively large, with deep notches in the rear margins. The pelvic fins are falcate with concave rear margins, in contrast to the straight-margined pelvic fins of the scalloped hammerhead. The skin is covered with closely placed dermal denticles. Each denticle is diamond-shaped, with 3–5 horizontal ridges leading to marginal teeth in smaller individuals, and 5–6 in larger ones. The great hammerhead is dark brown to light gray to olive above, fading to white on the underside. The fins are unmarked in adults, while the tip of the second dorsal fin may be dark in juveniles.
Great hammerhead sharks may mate at the surface, which is unusual behavior for a shark. During mating, the male transfers sperm to the female via his claspers. Great hammerhead sharks are viviparous (give birth to live young). The gestation period for a female shark is about 11 months, and 6-42 pups are born live. The pups are about 2 feet long at birth.
Hammerheads use their cephalofoils for detection of prey using their electro-reception system. This system allows them to detect their prey by electrical fields. Great hammerhead sharks primarily feed at dusk and eat stingrays, invertebrates, and fish, including even other great hammerheads. Their favorite prey is rays, which they pin down using their heads. They then bite at the ray's wings to immobilize them and eat the entire ray, including the tail spine.
- Hammerhead sharks are generally not dangerous to humans, but great hammerheads should be avoided due to their size.
- Great hammerheads are listed as endangered by the IUCN Red List due to their slow reproduction rate, high bycatch mortality and harvest in shark finning operations.
- The great hammerhead is the largest species of of all the hammerhead sharks.