No. 84: Nov-Dec 1992
When magnetite particles were found in organisms from bacteria to bats, it was assumed that here was the long sought magnetoreceptor which animals used for magnetic navigation. But so far, biologists do not have the slightest notion how such magnetite particles can be turned into a "magnetic sense," which sends the brain information on the direction of the geomagnetic field or, perhaps, draws a magnetic map of sorts.
A completely different sort of magnetreceptor is now under investigation, one that humans may also unknowingly possess. It utilizes special photoreceptors that employ an electron-spin resonance process which is modulated by the geomagnetic field. Some of our very sensitive magnetometers use similar phenomena. The biological version of such a receptor would be connected to the brain, as the eye is, and send signals as to the direction of the earth's magnetic field.
Sounds interesting, but is there any basis for thinking such a sophisticated gadget could have evolved? It seems that some experiments with newts by J.B. Phillips and S.C. Borland support the idea. The newts were first trained to orient themselves in a certain direction with respect to the geomagnetic field.
"When tested under one of four artificial field alignments (magnetic north at geographic north, east, south or west), the newts kept their training directions constant relative to the magnetic rather than the geographic system of reference, but they selected different angles with respect to the magnetic field when they were illuminated by either short (about 450 nm) or long-wavelength light (about 500 nm). When tested under 475-nm light, or in the dark, they were completely disoriented."
The experiments demonstrated that light was crucial to the newts' magnetic sense, and that photoreceptors had to be involved.
(Wehner, Reudiger; "Hunt for the Magnetoreceptor," Nature, 359: 105, 1992.)
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