- The mystery of the migrating sea turtle may have been
solved. Scientists have shown that the marine reptile can use the earth's
magnetic field as both a compass and a map.
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- An experiment by marine biologists working off the coast
of Florida has demonstrated the phenomenal ability of the green sea turtle
to sense subtle variations in the earth's magnetic field.
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- Turtles migrate thousands of miles over open ocean and
the research suggests the animals can sense small perturbations in the
north-south direction of the magnetic field - both to work out where they
are and where they should be going.
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- Green sea turtles, Chelonia mydas, are especially accurate
in terms of their migration, with some of them regularly commuting from
the same feeding and breeding sites off the coast of Brazil to Ascension
Island, the tiny dot of land just six miles wide in the middle of the South
Atlantic.
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- The scientists, led by Kenneth and Catherine Lohmann,
a husband and wife team at the University of North Carolina in Chapel Hill,
investigated the green turtle's homing instincts using a giant, two-storey
magnetic coil they built around an aquarium in which the field could be
artificially controlled.
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- The Lohmanns and their colleagues used the coil to reproduce
the magnetic fields that are known to exist at various locations along
the Florida coast so they could expose captive turtles to fields from two
widely different geographical locations.
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- "We captured juvenile turtles using a 400m net stretched
parallel to the coastline. Each turtle was placed in a cloth harness and
tethered to a tracking device in the pool of water inside the magnetic
coil," Dr Catherine Lohmann said.
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- The scientists varied the magnetic field of the experiment
to mimic the fields that exist at two sites, one just off the north coast
of Florida and the other at the state's southern tip. The test site where
the turtles were caught was midway between.
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- "Half of the turtles swam in a magnetic field that
exists at a location about 350km (217 miles) north of where they actually
were. The other half swam in a field that exists at an equivalent distance
to the south," she said.
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- The study, published in the journal Nature, found that
the turtles exposed to the northern field responded by swimming south while
those in the southern field swam north.
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- In each case, therefore, the turtles swam in a direction
that would have taken them home if they had been at the place where the
magnetic field exists, Dr Lohmann said.
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- Kenneth Lohmann said the experiment shows the turtles
are using the Earth's magnetic field in a more complex way than if it were
used as a mere compass pointing north-south.
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- Scientists have shown other migratory animals, notably
birds, use a variety of natural cues that act as a map and compass, and
those include the earth's magnetic field.
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- As yet, biologists have not located the sensory organ
that can detect changes in a magnetic field. One idea is that it might
be a general property of nerve cells that are full of electrically charged
particles.
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- © 2004 Independent Digital (UK) Ltd http://news.independent.co.uk/world/science_technology/story.jsp?story=516317
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