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About 50 animal species, ranging from
birds and mammals to reptiles and insects, use Earth's magnetic field for
navigation.
Yet Earth's magnetic field is very week. It ranges from
approximately 30 to 60 millionths of one tesla. By comparison, magnetic
resonance imaging, or MRI, uses magnetic fields from 1.5 to 3.0 tesla.
So scientists unsure exactly how birds
do it.
New research finds that a photochemical compass may simulate
how migrating birds use
the magnetic field, along with light, to navigate.
One theory for how
it all works has been that photoreceptors in a bird's retina absorb light,
which causes
a chemical reaction that, in turn, produces a short-lived
photochemical species whose lifetime is sensitive to the magnitude and
direction of a weak magnetic field.
The idea is
supported by the fact that blue light photoreceptors have been detected in
retinas of migratory birds when they perform magnetic orientation. However, it
has not been confirmed that a magnetic field as weak as Earth's can produce
detectable changes within a photochemical molecule; nor, has a photochemical
molecule been shown to respond to the direction of such a magnetic field.
Until now.
A new study, funded by the National Science Foundation and
detailed online in the April 30, 2008 issue of the journal Nature,
shows that the photochemical model becomes sensitive to the magnitude and
direction of weak magnetic fields similar to Earth's when exposed to light.
A synthesized photochemical molecule composed of
linked carotenoid (C), porphyrin (P) and fullerene (F) units can act as a
magnetic compass, the researchers found. When excited with light, CPF forms a
short-lived charge-separated state with a negative charge on the ball-like
fullerene unit and a positive charge on the rod-like carotenoid unit. The
charge-separated state lasts only as long as the magnitude and direction the
field stays constant.
Why do scientists care about all this complex
stuff?
Power lines and communications equipment also generate weak
magnetic fields that can disrupt animal navigation, so "it is essential
for humans to understand how animals navigate using Earth's weak magnetic field
and the effects of human activity on animal navigation," said Devens Gust,
professor of chemistry and biochemistry at Arizona State University.
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