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THE internal compasses of some animals might work by detecting minute changes
in the pace of biochemical reactions in different magnetic fields, researchers
in the US suggested this week. They say their theoretical studies could
contribute to the debate on whether the electromagnetic fields of powerlines
cause diseases such as cancer.

Many creatures, including some birds, amphibians and reptiles, navigate by
sensing tiny changes in the Earth鈥檚 magnetic field. Sea turtles, for instance,
can sense changes as small as a tenth of a microtesla鈥攍ess than 0.2 per
cent of the typical geomagnetic field.

But nobody knows exactly how these biological compasses work. One theory is
that the magnetite molecules found in some tissues act just like miniature
compass needles. Another is that animals sense changes in biochemical reaction
rates caused by differing magnetic fields, which are known to alter the pace of
a wide range of chemical reactions.

But no one had proved such a biochemical compass could work. As well as being
influenced by magnetic fields, reaction rates fluctuate randomly and also change
with temperature. 鈥淎ll organisms, warm or cold-blooded, experience temperature
variations,鈥� says James Weaver, a biophysicist at the Massachusetts Institute of
Technology. 鈥淭he challenge is to understand how nature would be able to sense
small magnetic field differences in the presence of all this competition.鈥�

Weaver and his colleagues decided to find out the mathematical way. They took
equations describing a common reaction involving two reactive molecules called
free radicals. Then they looked at how much the reaction rate would be swayed by
the 鈥渘oise鈥� of random fluctuations and temperature variations in the
environment, as well as by magnetic variations like those sea turtles can
sense.

Sure enough, it turned out that a small group of cells should be able to
sense the magnetic field signal loud and clear above the noise. 鈥淎n evolved
animal sensory system like this should be possible, and it could have a rather
fantastic performance,鈥� Weaver concludes.

He hopes that theoretical work like this could shed light on whether similar
mechanisms might explain the possible, albeit contentious, health risk of
electromagnetic fields from power lines and cellphones. 鈥淭hat鈥檚 a big hot
potato,鈥� says Weaver. 鈥淏ut it would help if we can get simple models from
physics and chemistry that give us some insight into what鈥檚 possible and what鈥檚
苍辞迟.鈥�

鈥淔rom the point of view of a chemical compass, this could be very
interesting,鈥� says Nicholas Day, an epidemiologist at the University of
Cambridge, whose studies have failed to find a link between childhood cancer and
magnetic field exposure in Britain. However, he thinks that only epidemiology
and carefully controlled lab experiments can lay the debate about the cancer
link to rest.