'Magnetic' sharks add bite to compass theory

Marine biologists say they have obtained the first proof that sharks can spot changes in magnetic fields, boosting evidence that the fish have an internal compass to guide them as well as a phenomenal sense of smell.

A Hawaii University team trained six sandbar sharks and one scalloped hammerhead shark to associate food with an artificial magnetic field, they report in a publication of Britain's Royal Society.

The field, derived from a copper coil surrounding the sharks' seven-metre diameter tank, was switched on every time food was placed on the tank floor.

The conditioned sharks were then put through their paces in a series of 11 trials conducted over six weeks in which the field was switched on at random times, but no food was made available.

The sharks still converged on the usual feeding site, proving they had detected the field.

Until now, evidence of sharks' magnetic compass has been circumstantial, based mainly on sightings of tiger sharks and blue sharks that swim in straight lines for long distances across the ocean, a feat that is unlikely to be accounted for by sense of smell alone.

Scalloped hammerhead sharks have also been known to converge on seamounts - mountains that rise from the ocean floor, where there are quirks in earth's magnetic field.

The next step is to find out how sharks detect magnetic fields and to measure their sensitivity.

The study appears on Wednesday in the publication Journal of The Royal Society Interface.

Last month, University of North Carolina researchers unveiled experiments showing that tiny magnetic particles in pigeons' upper beaks help the birds to carry out long-range navigation.