Why Are Hammerheads' Heads Shaped Like That?

There are 10 species of sharks in the Sphyrnidae family, all of them but one falling into the genus Sphyrna, a Greek word that means “hammer”—an apt description for the head shape of these carcharhiniforms. But why are their heads, called cephalofoils, shaped like that? How does a hammer-shaped head, with eyes on each end, help the shark survive?

According to John S. Sparks, Curator-in-Charge of the Department of Ichthyology at the American Museum of Natural History and co-curator of its newest exhibition, Life at the Limits, there are two hypotheses scientists are working with. One, he says, is that “with its eyes out to the side, [the shark] can maximize its visual capacity, in terms of the area that it can see,” which might help when the animal is hunting.

The other has to do with how the hammerhead hunts: The sharks look for prey by rooting around on the ocean bottom with their heads, which are studded with sensory organs called ampullae of Lorenzini (the hammerhead isn't special, though; all sharks have these organs). “Every organism puts off a weak electric field, and [with these electroreceptors] the sharks can sense them,” Sparks says. “It’s thought that having a broader head allows for more of these ampullae, so the shark can better sense prey.”

There may be some evidence to back up this hypothesis, Sparks says. Not all hammerheads’ cephalofoils are created equal: Some have small hammers, while some, like the great hammerhead, have huge hammers. The scientists behind one grant that Sparks reviewed "have found that sharks with larger hammers are more accurate at detecting prey in the substrate.”