By Rick Weiss
Washington Post Staff Writer
Monday, May 5, 2008
Four decades after scientists showed that migratory birds use Earth's magnetic field to orient themselves during their seasonal journeys, researchers have at last found a molecular mechanism that may explain how they do it.
If the hypothesis is true, the planet's magnetic field lines -- which arch around Earth from north to south -- may be plainly visible to birds, like the dashed line in the middle of a road.
The work, described online yesterday in the journal Nature, was conducted in a test tube and does not prove that birds actually use the mechanism. And researchers aligned with a competing model say they are not convinced.
But by identifying for the first time a molecule that reacts to very weak magnetic fields, the experiments prove the plausibility of a long-hypothesized method of avian navigation that has had a credibility problem because no one had ever found a molecule with the required sensitivity.
"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. Devens Gust, a chemist at Arizona State University who worked with Hore and Timmel, said the molecules "seem to have the right structural and chemical features to allow them to show this effect."
The seasonal comings and goings of birds have mystified people for millennia. Some early observers, noting that certain species routinely disappeared each year as others appeared, presumed that one species was somehow being transformed into the other. As late as the 18th century, an anonymous essayist who described himself simply as "a Person of Learning and Piety" concluded that many birds probably spend winters on the moon.
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.
Two hypotheses have dominated. One centers 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 theorized that a molecule with the right characteristics might change its behavior 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. Light looks like light. Sound sounds like sound. What would magnetic information "feel" or "look" like?
"It could be a bright or dark spot that would move around" in the bird's field of vision, Hore said. As in a video game, the goal might be to keep that spot centered.
But maybe not.
"I think it would be annoying to have this dot moving around," said Thorsten Ritz, a biophysicist at the University of California at Irvine, who nonetheless called the new work "breathtaking." Perhaps as a bird veered off course it would feel the way airplane passengers do in a quick descent, he suggested.
Others doubt that birds have, or need, anything more than their magnetite mouths.
"Hore is a great chemist, and this is an impressive demonstration of a weak field effect. However, I'm not sure it has any biological relevance," said Sönke Johnsen, who studies bird navigation at Duke University.
Joe Kirschvink, an expert in magnetoreception at the California Institute of Technology, was even more dismissive, noting 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.
Hore and Ritz said similar molecules are expected to work at warmer temperatures.
And in the end, both camps may be right.
"Maybe there is a compass in the eye of birds," Ritz said, "and a map in their beaks."