Why Do Hammerheads Have Hammer-Shaped Heads? Cephalofoil

What is a cephalofoil?

A cephalofoil is the flattened, sideways extension of a hammerhead’s head. The eyes and nostrils sit at its outer tips, and the relatively small mouth lies underneath. The name combines the Greek word for head with “foil”, as in hydrofoil, a flat wing that moves through water.

The size of the hammer varies a lot between species. In the great hammerhead, the largest species, the head is about 23–27% as wide as the body is long, so a 4 m (13 ft) shark has a head roughly 1 m (3 ft) wide. The winghead shark has an extreme version, close to half its body length, while the small bonnethead has a rounded, shovel-shaped head.

No single function explains the hammer. Researchers have tested several ideas, and most now think the shape brings a mix of sensory and hunting advantages.

A wider net for electric signals

All sharks can sense the weak electric fields produced by the muscles and nerves of other animals, using jelly-filled pores called ampullae of Lorenzini. In hammerheads, these pores are spread across the underside of the broad head.

A 2002 study compared young scalloped hammerheads with sandbar sharks of similar size. Both species reacted to prey-like electric fields from about the same distance, around 30 cm (12 in), so the hammerheads were not more sensitive. But they had two advantages: the wide head swept a broader strip of seabed, raising the chance of finding hidden prey, and the hammerheads could pivot sharply over a signal, keeping one tip of the head in place while swinging the body around.

This is why hammerheads often cruise just above the bottom, swinging their heads from side to side like a person using a metal detector. It is a very good way to find stingrays and flatfish buried in sand.

Better vision in front

Placing the eyes at the tips of a wide head changes what a shark can see. A 2009 study measured the visual fields of three hammerhead species and two requiem sharks. The overlap between the left and right eyes in front of the head, known as binocular vision, was about 48 degrees in the winghead shark and 34 degrees in the scalloped hammerhead, compared with only 10–11 degrees in lemon and blacknose sharks. The small-headed bonnethead, at 13 degrees, was no different from the requiem sharks.

Binocular vision helps animals judge distance, which is useful when striking at moving prey. The wide head does leave a blind spot directly in front of the snout. The study found that hammerheads swing their heads a little more than other sharks as they swim, which helps them scan that area.

Smelling in stereo

The nostrils of a hammerhead sit far apart, near the ends of the head, and most species have grooves along the front edge that channel water toward them. A 2005 study found that this lets hammerheads sample a larger volume of water, and the wide spacing could help them detect which side a smell is stronger on, much like hearing in stereo.

The same study, however, found that scalloped hammerheads do not have more smell-sensing tissue than sandbar sharks, despite their much wider heads. So hammerheads may be better at following a scent trail without having a keener nose overall.

Does the head help hammerheads turn?

For years, a popular idea was that the cephalofoil works like the front wings of some aircraft, helping hammerheads make sharp, stable turns. Hammerheads do turn very tightly. But a 2003 study of young bonnetheads and scalloped hammerheads found that they do not bank their bodies when turning, as they would if the head were acting as a steering wing. Their sharp turns seem to come mainly from flexible bodies. The head may still provide some lift and stability while the shark swims.

A tool for pinning stingrays

The great hammerhead adds a hunting use. Stingrays are its favorite prey, and in the Bahamas it has been seen knocking a southern stingray to the seabed with a blow from its head, then holding the ray down with the cephalofoil while biting pieces off the edges of its disc. Another great hammerhead used its head to press an injured spotted eagle ray to the bottom.

Pinning the ray stops it from escaping while the shark disables it. Stingray spines do not always stop the attack: one great hammerhead caught off Florida had 96 spines in and around its mouth.

How hammerhead heads differ between species

There are about ten hammerhead species, and the head shape is the main way to tell them apart.

Head size seems to match lifestyle. Big-headed species roam reefs and coasts hunting fish and rays, while the bonnethead feeds over seagrass beds on crabs and shrimp. A 2018 study even showed that bonnetheads digest much of the seagrass they swallow.

How did the hammer evolve?

Hammerheads are a relatively young branch of the shark family tree. Fossil teeth show the group existed by the Eocene, more than 34 million years ago, and their closest living relatives are the requiem sharks, such as the bull shark.

DNA studies place the winghead shark, with the widest head of all, on the earliest branch of the hammerhead family. That suggests the first hammerheads already had large heads, and that smaller hammers, like the bonnethead’s, evolved later. In other words, the hammer did not slowly grow bigger over time: it started big and was then reshaped to suit each species’ way of life.