The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a “hammer” shape called a “cephalofoil”. Most hammerhead species are placed in the genus Sphyrna; some authorities place the winghead shark in its own genus, Eusphyra. Many, not necessarily mutually exclusive, functions have been proposed for the cephalofoil, including sensory reception, maneuvering, and prey manipulation.
Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, hammerheads usually swim in groups (called ‘shivers’). Some of these schools can be found near the Cocos Islands by Costa Rica.

The nine known species range from 0.9 to 6 m (3.0 to 20 ft) long. The average hammerhead shark weighs about 500 pounds, but some may grow up to 1000. All the species have a projection of their face on all sides of the head that gives it a resemblance to a flattened hammer.
It was determined recently that the hammer-like shape of the head evolved to enhance the animal’s vision. The positioning of the eyes give the shark good binocular vision, as well as 360-degree vision in the vertical plane, meaning they can see above and below them at all times. The shape of the head was previously thought to help the shark find food, aiding in close-quarters maneuverability and allowing sharp turning movement without losing stability. However, it was found that the unusual structure of its vertebrae allowed it to make the turns correctly, more often than its head. The hammer would also shift and provide lift.
Hammerheads are one of the most negatively buoyant of sharks. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively. These sharks have been able to detect an electrical signal of half a billionth of a volt. The hammer also allows the nostrils to be placed farther apart, increasing its ability to detect chemical gradients and localize the source.
Hammerheads have disproportionately small mouths and seem to do a lot of bottom-hunting. They are also known to form schools during the day, sometimes in groups of over 100. In the evening, like other sharks, they become solitary hunters.
Hammerheads are notably one of the few animals that acquire a tan from prolonged exposure to sunlight. Tanning occurs when a hammerhead is in shallow waters or close to the surface for long periods.

Since sharks do not have mineralized bones and rarely fossilize, it is their teeth alone that are commonly found as fossils. The hammerheads seem closely related to the carcharhinid sharks that evolved during the mid-Tertiary Period. Because the teeth of hammerheads resemble those of some carcharhinids, it has been difficult to determine when hammerheads first appeared. It is probable that the hammerheads evolved during the late Eocene, Oligocene or early Miocene.
Using mitochondrial DNA, Andrew Martin constructed a phylogenetic tree of the hammerhead sharks that showed the winghead shark as its most basal member. As the winghead shark has proportionately the largest “hammer” of the hammerhead sharks, this suggests that the first ancestral hammerhead sharks also had large hammers.

The hammerhead sharks exhibit a viviparous mode of reproduction with females giving birth to live young. Like other sharks, fertilization is internal with the male transferring sperm to the female through one of two intromittent organs called claspers. The developing embryos are at first sustained by a yolk sac. When the supply of yolk is exhausted, the depleted yolk sac transforms into a structure analogous to a mammalian placenta (called a “yolk sac placenta” or “pseudoplacenta”), through which the mother delivers sustenance until birth.
In 2007, the bonnethead shark was found to be capable of asexual reproduction via automictic parthenogenesis, in which a female’s ovum fuses with a polar body to form a zygote without the need for a male. This was the first shark known to do this.

Hammerhead sharks are known to eat a large range of things, including fish, other sharks, squid, octopus, and crustaceans. Stingrays are a particular favorite. They are also known to eat their own young.