MOLECULES ON A WING

“This is a proof of principle that a chemical reaction can act as a magnetic compass,” said Peter Hore of the University of Oxford, who with fellow chemist Christiane Timmel led the research. Hore is testing similar molecules, called cryptochromes, isolated from the eyes of migratory birds.

The seasonal comings and goings of birds have mystified people for millennia. Recent scientific findings have seemed almost as incredible. By reversing the magnetic fields around captive birds as they prepared to migrate, scientists could induce them to take off in the wrong direction. The conclusion was that birds have a “sixth sense” that can detect magnetic energy the way eyes detect light and ears detect sound.

But how? Two hypotheses have dominated. One centres on the discovery that birds (and other organisms, including salmon) make and store in their bodies a version of iron called magnetite, which orients itself to magnetic fields.

In birds, magnetite is often concentrated in the beak. Studies have shown that when the beaks of these birds are exposed to powerful magnetic fields—or are numbed with an anesthetic—the birds lose their ability to navigate properly.

But many scientists have suspected that another mechanism is also crucial—one that can tell a bird not only which way is north but also how far it is from the equator by detecting the angle of magnetic field lines. Those lines emerge from Earth’s magnetic poles perpendicular to the planet’s surface, then arch overhead to meet over the equator, at which point they run parallel to the surface. If a bird could detect the angle of those lines relative to the surface, it could know, in effect, its latitude.

Scientists had theorised that a molecule with the right characteristics might change its behaviour depending on the inclination of the magnetic field around it. It might react with another chemical more quickly, for example. In the new work—conducted in a chamber that blocks Earth’s magnetic field and creates fresh ones of various strengths—the team made a three-part molecule that, in response to light, gives up electrons at one end and passes them to the other end. There they linger for a millionth of a second or so before returning. Significantly, the precise amount of time each electron spends in its temporary home at the far end of the molecule varies with the angle of the surrounding magnetic field.

If cryptochromes or other chemicals in a bird’s eye behave as the new molecule does, they could provide the foundation of a bird’s magnetic sense. Their shape would probably vary slightly, depending on how much time electrons spent at the far end, or those lingering electrons might affect the shape of another, nearby molecule in the eye. And shape determines biological function.

So depending on how far north or south a bird is from the equator, these molecules could be expected to send different signals to its brain, telling the flier whether it is veering east or west and pinpointing its latitude.

No one knows how a bird would perceive this input. “It could be a bright or dark spot that would move around” in the bird’s field of vision, Hore said. Others doubt that birds have, or need, anything more than their magnetite mouths. Joe Kirschvink, an expert in magnetoreception at the California Institute of Technology, noted among other things that Hore’s experiment worked only at very cold temperatures—”a major stumbling block to the suggestion that optical effects in any organism can be used as the basis of a physiological compass,” he said.
-Rick Weiss (The Washington Post)