Animal magnetism

Robert Zimmerman's article "Deep Impressions" (3/04), regarding the interior of the Earth, got me wondering: How would a reversal of the Earth's magnetic field affect the behavior of birds and other animals with "natural compasses" in their heads and bodies? The geologic record indicates that reversals in magnetic polarity happen quickly and frequently, relative to the pace of organic evolution.

Paul D. Neuwald

Martinez, California

ROBERT ZIMMERMAN REPLIES: No one can predict the exact consequences. But according to James L. Gould, an ethologist at Princeton University, if the polarity reversal is slow, there should be little effect on migrating birds. But if the transition is fast--a few years or less--many birds would have trouble remaining oriented. That birds have continued to thrive through any number of reversals, Gould suggests, is probably the best reason to think that magnetic-pole flips are not rapid.

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