Discovering how 'sixth sense' works

New Zealand scientists have proved that pigeons use the same magnetic "sixth sense" as fish to find their way home.

Experiments reported in today's issue of British journal Nature show that homing pigeons navigate using tiny deposits of a mineral called magnetite, or lodestone, in their beaks. The mineral is in the same part of the body, and uses the same nerve to connect to the brain, as in trout, stingrays and a bird called the bobolink.

Auckland University biologist Michael Walker, who led the research, says finding the sixth sense in such disparate creatures suggests that it may date back to the origin of vertebrate (backboned) animals 350 million years ago.

Anecdotal evidence suggests dogs, cats and other land animals can also travel long distances to find their way home, but Dr Walker says there is no evidence that they have the magnetic sense. "I'm not going to exclude it. [But] when it really matters is when you are flying or swimming long distances, and humans and other terrestrial animals don't do that as a rule. The reason is that in the fluid environments of air or water, you are subject to passive displacement by wind or currents."

Dr Walker and colleagues Michael Davison, Martin Wild and German post-doctoral student Cordula Mora put pigeons in a wooden tunnel and set up a magnetic field that could be switched on or off in the middle of the tunnel.

They taught the pigeons to jump on to one platform when they detected the magnetic field, and another when the field was turned off. The birds were rewarded with food for the right choices.

Next, the scientists gave the birds local anaesthetics to block information going from their beaks to their brains. Most pigeons lost the ability to detect the magnetic field. Finally, the scientists cut several nerves connecting the pigeons' beaks to their brains, and found the birds lost the ability to feel the magnetic field only when the cut was made to the trigeminal nerve - the nerve that conveys the magnetic sense to the brain in trout, stingrays and bobolinks.

Dr Walker plans to look for the same sense in other species.