Head Turner

What's the point of the hammerhead's head if the hammerhead doesn't hammer?

Story by Adam Summers   ~  Illustrations by Shawn Gould

HOW THE HAMMERHEAD TURNS Investigators had hypothesized that the shark's head might tilt during turns, thereby speeding up the maneuver (see drawing below); in fact, the animal's head stays level (above) and may even stabilize the rest of the body during a turn.

Thirty feet underwater, over a sandy patch next to the Great Barrier Reef, is a perfectly ordinary place for a shark to swim. But the student divers I was leading made a din that should have kept a cautious carnivore at a wary distance, so I was surprised when an eight-foot shark glided past. From the side it looked like the other sharks that regularly patrol the reef. As it turned toward us, however, it revealed a grossly expanded and flattened head, and I realized with a rush that this was my first underwater encounter with the great hammerhead shark (Sphyrna mokkaran).

Perhaps no other morphological oddity has inspired so many fanciful-and sensible-theories about its function as has the cephalofoil (the winglike head) of the eight species of hammerhead. Recent experimental evidence supports some ideas and refutes others, while pointing to a previously unsuspected role for this peculiar feature.

Hammerhead heads come in many widths and shapes. The winghead shark (Eusphyra blochii), which lives in the western Pacific and Indian oceans, looks like the letter T when viewed from above, its head nearly half as wide as the body is long. The bonnethead shark (Sphyrna tiburo), common throughout the Western Hemisphere's warm waters, has a relatively modest foil, less than five inches across. A phylogenetic tree based on comparisons of DNA from different species indicates that the cephalofoil is shrinking (evolutionarily speaking); the wingspans of more ancient hammerhead species are much larger than those of more recent additions to the family.

There are two main schools of thought about the function of the hammerheads' peculiar noggin: it improves sensory perception, or it makes for more efficient hydrodynamics (lift, maneuverability). Or maybe the shark actually "hammers" its prey with the head-behavior once documented in an underwater video by Wesley R. ("Rocky") Strong, a marine biologist and filmmaker. (Still, a hammer with exposed eyeballs at both ends seems a poor tool for regular use as a bludgeon.)

The sensory hypotheses focus on the advantages of widely spaced eyes, nostrils, and electroreceptors: for enhanced binocular vision, better tracking of odors, and more precise detection of the electric currents generated by potential prey. The electroreceptors are particularly intriguing. All shark species have hundreds of minute dark pores on their heads-the open ends of electrically sensitive organs known as ampullae of Lorenzini. Each ampulla, or vesicle, is filled with conductive gel. Exquisitely sensitive neurons project into the gel, firing at rates proportional to the current passing through it. These ampullae could detect the electric field from a 1.5 volt AA battery at about ten yards (hammerheads' prey, however, create electric fields far weaker than would a battery adrift in the sea). The sharks hunt for their most common quarry-animals that are cryptic, often buried, unmoving, nearly scentless, and quite invisible-by sensing two kinds of electric fields: the DC field that results from the osmotic potential between the prey's body tissue and seawater, and the AC fields generated by the contraction of the prey's muscles.

The hydrodynamic hypotheses about the cephalofoil focus instead on the hammerhead's remarkable agility: the animal can make exceptionally fast turns when pursuing prey or fleeing from danger. The idea is that when a hammerhead changes direction, it could tilt (or, as the maneuver is known to pilots, roll) its big winglike head, which is far forward of its center of gravity, and so exert a huge turning force on the body. The same concept led aeronautical engineers to incorporate small wings at the front of some advanced fighter jets; the forward "control surfaces" replace the larger ones that are needed when the control is behind the main wing, thereby enabling faster turns with less drag.

Stephen M. Kajiura, now a post-doctoral fellow at the University of California, Irvine, designed an elegant set of experiments to simultaneously test the sensory and hydrodynamic roles of the cephalofoil. Kaneohe Bay, on the Hawaiian island of Oahu, is a pupping ground for the scalloped hammerhead (Sphyrna lewini) as well as a favorite haunt of the sandbar shark (Carcharhinus plumbeus), a typical, blunt-headed reef shark. In large, screened-in pens, Kajiura compared the ability of the two species to perceive an electric field. At the same time, he videotaped the sharks as they interacted with one of four pairs of electrodes-simulated prey-set in a large, clear acrylic sheet.

When Kajiura activated one of the electrode pairs, the hungry young sharks immediately turned toward it, swam rapidly around it, and bit the acrylic surface. His observations confirmed the conventional wisdom among shark watchers: that hammerheads turn more quickly and make sharp turns more often than reef sharks do. The hammerheads also had a distinct advantage over their more bullet-headed cousins in detecting prey-they could sense the electric field 50 percent farther away than could sandbar sharks of the same size.

Unfortunately for proponents of the hydrodynamic hypotheses, however, Kajiura's experiments showed that hammerheads don't roll their heads to turn. Rather, the sharks stay perfectly level, as if they were turning on rails. In retrospect, that finding is not surprising. During a turn, a shark tries to maintain an electrical picture of the prey. If the shark tilted its head, its reception of the electric signal on one side would sharply decline. By holding its head steady, the shark can more effectively keep its senses focused on the object of its desires-whether that's a nutritious fish buried in the sand or an inedible electrode.

In fact, the width and winglike shape of the cephalofoil may even stabilize the body as the shark turns, twisting the head in the opposite direction from the torque generated by shark's beating tail. As the shark turns, the outside wing of its head travels faster than the inside wing. Because the lift of a wing is proportional to its speed, the outside wing also develops more lift than the inside wing. That lift tends to roll the shark so that its belly is oriented toward the outside of the turn.

The upper lobe of the shark's tail, however, is larger than the lower lobe. Thus, as the tail beats harder to one side (to effect the turn), the first dorsal fin feels the more powerful push of the upper lobe and so tends to roll towards the outside as well. The two opposite effects could cancel each other out, leading to increased stability in the turn. The net result is that even though hammerheads do turn heads, they do not turn with their heads.

Adam Summers is an assistant professor of ecology and evolutionary biology at the University of California, Irvine (asummers@uci.edu). He is happy to report that the hammerhead circled his nearly breathless diving students twice before swimming away without incident