If Rudyard Kipling was writing a `Just So' story for the hammerhead shark,he'd surely have come up with a tale of a three way struggle that produced the fish's distinctive profile. Although Kipling's story would be highly entertaining, Stephen Kajiura has got a much more realistic theory. All sharks are endowed with an electrical sense, that allows them to sense the weak electrical fields that most creatures emit. Kajiura wondered if the hammerhead's odd shape gave it a head start over other electrosensing sharks. Tempting hammerheads and more classically shaped sharks with decoy dipoles,Kajiura has discovered that the shark's extended head allows it to search for food over much larger areas than its more streamlined relatives().
There are a variety of possible explanations for how the fish's distorted features evolved, ranging from wide angle binocular vision and stereo smell to electroreception. Sharks are one of the few species that specialise in electroreception, and as no other species on the planet has evolved such a distinctive head shape, Kajiura wondered if it had evolved to somehow enhance the fish's electrical sensitivity. He decided to compare the electrical sensitivity of young hammerheads with the sensitivity of a narrow headed shark to see if it might be the clue that explains why the shark went out on this evolutionary branch.
Kajiura set up a seawater enclosure where he could keep the young sharks happy before testing their senses. Collecting the young sharks was also relatively straightforward, but he remembers that building the dipoles to fake a small fish's electric field was far from simple, until he optimised the dipole's strength by applying a 6 μA current across a 1 cm dipole.
Ready to start testing the fish's electrical reflex, Kajiura tempted individuals towards the dipoles with a whiff of tempting squid essence, before switching on one of the dipoles at random, and videoing each shark attack to see how close the shark had to swim before the dipole sparked its interest. But when he compared the fish's sensitivities, both species seemed to sense the field at the same level. The head's distinctive shape apparently hadn't enhanced its electrical sensitivity! But Kajiura knew that the shark's extended head could bring another benefit; it simply covers more ground. Put in terms of football fields, Kajiura explains that while the electrosensitive head of a 60 cm sandbar can only cover an area the size of half a football field everyday, a better endowed hammerhead's head can cover almost three times as much ground in the same time.
Looking back at the hours of videotaped hunting approaches collected over two field seasons, Kajiura also realised that the hammerheads are significantly more agile than the stockier sandbars. Which has sent him off on his next hypothesis, does the hammerhead help this shark to turn on a pinhead?
