Late on a January night in 1993 I found myself on a beach on the Pacific coast of Costa Rica, kneeling in the sand beside a leatherback sea turtle. Like a giant mango with wings, the huge black turtle had hauled herself up the beach in great stentorian gasps of air and was laying her eggs in a pit she had laboriously scooped out with her hind flippers.

Knowing basic facts about her ecology and physiology, I was in awe. How her kind, the largest living reptiles, had been around for 120 million years. How she lived solely on jellyfish, a thing more water balloon than animal. How she could collapse her lungs and dive to depths that would cause you or me to implode. How she had traveled thousands of miles around the Pacific Ocean, only to return there to the very beach she was born on years before.

That navigational and homing ability astonished me more than any other. How did she navigate around a trackless wilderness larger than the world's total land area and find her way back to that same short ribbon of sand? One hypothesis was just starting to be floated in those days: that to aid their long-distance migrations leatherbacks and other sea turtles appear to use the Earth's magnetic field (see Figure 1).

When I learned recently that our planet's magnetic shield is rapidly weakening and may be ready to reverse its polarity, causing compasses to point south, I immediately wondered what that would mean for leatherbacks and the many other species that use the magnetic field to orient themselves and find their way around. Could they withstand a significant dwindling of the field's strength or even a reversal? Or might extinctions, perhaps mass extinctions, be in the offing?

Animal magnetism

One of the first concrete signs that animals can tap into the magnetic field was observed, as in many a great discovery in science, by chance. It was the fall of 1957, and Hans Fromme, a researcher at the Frankfurt Zoological Institute in Germany, noticed that several European robins he kept in a cage were becoming restless and were fluttering up into the southwestern part of the cage. Nothing unusual there: it was known that migrating birds in cages become edgy at that time of year, and European robins in Germany migrate southwestwards to Spain to overwinter.

What made it striking was that the birds were in a shuttered room. They could see neither visual landmarks, nor their fellow, non-captive robins, nor the sun or stars, which were known to serve them as navigational aids. Clearly they were acting on something invisible, and Fromme deduced it must be the Earth's magnetic field.

Numerous experiments undertaken by him and others since then have shown that many living things avail themselves of the magnetic field. Organisms as diverse as hamsters, salamanders, sparrows, rainbow trout, spiny lobsters, and bacteria all do it. "I would go so far as to say that it's nearly ubiquitous," says John Phillips, a behavioral biologist at Virginia Polytechnic Institute and State University who himself has detected this ability in everything from fruit flies to frogs. (There's no scientific evidence that humans have this "sixth sense," though curiously, our brains do contain magnetite, the mineral thought to aid other animals' brains in detecting the field.)

How do we know organisms have this ability? A standard method to test for it is to throw a magnetic curve ball, as it were, at experimental subjects. In an effort, for example, to determine if the blind mole rat, a subterranean rodent that builds a home of branching tunnels with no exits to the surface, can sense the magnetic field, Tali Kimchi and Joseph Terkel of Tel Aviv University built an eight-armed maze within a device in which they could alter the magnetic field. They then tested two groups of rats—one in the Earth's magnetic field and the other in a field shifted by 180°—to see whether they had directional druthers for siting their sleeping nests and food chambers. The first group showed a significant preference to build their beds and pantries in the southern part of the maze, while the second group opted for the northern sector.

So they can sense it, but can they use it like we do a compass, to orient themselves? In another experiment, Kimchi and Terkel trained 24 blind mole rats to reach a goal box at the end of a complex labyrinth. Then, when all had mastered the task, they had half the rats do it again under the natural field and half under a reversed field. Lo and behold, the latter rats' performance fell far short of that achieved by their magnetically unmanipulated fellows.

Undersea superhighways

Other animals take things a step further than the blind mole rat, using the magnetic field like we do the Global Positioning System, to determine their location on the surface of the Earth and using that to negotiate unseen pathways during migration.

Kenneth and Catherine Lohmann of the University of North Carolina at Chapel Hill and their team have shown through many experiments that during their 8,000-mile migration around the Atlantic Ocean, young loggerhead sea turtles can detect not only the field's intensity but its inclination, the angle at which magnetic field lines intersect the Earth. The turtles use these two pieces of information, which vary at every point on the planet's surface, as navigational markers that help them advance along their migratory route (see Figure 2).

Sometimes this navigational ability can serve its practitioners only too well. A mystery long bedeviling marine biologists is why otherwise healthy whales beach themselves, often in large groups. In the early 1980s, a British biologist named Margaret Klinowska first noticed a correlation between where whale strandings tended to occur along the coasts of England and where magnetic lineations written into the seafloor intersect those coasts. (These lineations, or anomalies, are different from those produced by the main magnetic field.) Joe Kirschvink of the California Institute of Technology and his colleagues later showed a similar association on the east coast of the U.S.

Whales, it seems, follow these magnetic lineations during migration (see Figure 3). "If that's your game plan, and you get off track, and you follow a sharp magnetic anomaly that curves and runs into the coast, bang, you end up on the beach," says Kirschvink. Because whales are very social, if the leader makes this mistake, so does its entire pod, hence the mass strandings.

Rising to the occasion

If whales can run into trouble when the field is reasonably strong, what might happen to them and other creatures that rely on it if the field becomes feeble or even flips? Hans Fromme had found in Frankfurt that when he placed his European robins into a steel chamber and reduced the strength of the ambient magnetic field by a third, the birds' flutterings were no longer directional. This suggested that the birds needed the magnetic field to be a certain intensity to be of use. But Fromme's.................

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