Why the hammerhead shark got its hammer

It's one of evolution's most eccentric creations: a head shaped like a hammer. Now, a study suggests that the hammerhead shark may have evolved its oddly shaped snout to boost the animal's vision and hunting prowess.

For over a century, scientists have speculated why hammerheads evolved such an odd shape and whether having eyes so far apart would enhance their vision. In 1942 a leading authority on sharks, Gordon Walls, suggested the position of the shark's eyes prevented it from having binocular vision. But others have argued exactly the opposite, saying the animals must have enhanced eyesight.

Now, hammerhead sharks have had their first eye examination, and it has laid the debate to rest. Sharks with wider heads have better binocular vision – all the better to track fast-moving prey like squid with far more accuracy than sharks with close-set eyes.

The research also shows that hammerheads – among other sharks – have a 360-degree view of the world in the vertical plane, allowing them to simultaneously see prey above and below them.

Sharks at the optometrist

Michelle McComb of Florida Atlantic University in Boca Raton and colleagues compared the visual fields of three species of hammerhead – the winghead, the bonnethead and the scalloped hammerhead – with those of two other species of shark.

After implanting electrodes into the sharks' eyes, the researchers moved a beam of light across them until the eyes no longer demonstrated electrical activity. This allowed them to measure each eye's field of vision, which they summed to calculate each species' "binocular overlap".

"To our surprise, we found that the degree of overlap increased as the head of the hammerhead species widened," says McComb. The shark with the widest head, the winghead, had 48 degrees of binocular overlap; the others ranged from 10 to 32 degrees.

Eyes on the ball

That overlap helps hammerheads to perceive depth as they hunt, says Demian Chapman of the Institute for Ocean Conservation Science at Stony Brook University in New York. "I've seen hammerheads chasing stingrays in the Bahamas," he says. "The rays are fast and can turn rapidly, so if the hammerhead is to catch one, it has to be able to keep its eye on the ball."

Shark biologist Samuel Gruber of the Bimini Biological Field Station in Miami, Florida, says the paper has changed his view of the debate.

Previously, researchers have theorised that the hammerhead's head may improve its sense of smell, boost its ability to locate prey using electric fields, improve its manoeuvrability or help it to pin down struggling rays so it can bite off their wing-like fins.

Journal reference: The Journal of Experimental Biology, vol 212, p 4010

Source: New Scientist