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Working in Florida, scientists have found what they believe is the strongest
evidence yet that baby loggerhead turtles "read" the Earth's magnetic field to
help them navigate the massive clockwise current that sweeps the northern
Atlantic Ocean.
Built into the humble creatures' genes is a remarkable ability to monitor
global magnetism and to recognize magnetic fields that mark geographic regions
where the turtles must change direction to stay on course. The mechanism is not
infallible, University researchers say, and some hatchlings perish off track in
colder waters, but it appears to operate for most of them most of the time.
"To me, one of the great wonders of the world is that baby sea turtles enter
the ocean and then swim across the Atlantic and back all by themselves," said
Kenneth J. Lohmann, associate professor of biology. "People have wondered for
years how young turtles navigate during that first migration because it seems
utterly impossible.
"How can they find their way along an 8,000-mile migratory route that they've
never before encountered? We're finally on the verge of understanding how they
do it."
A report on the findings appears in the Oct. 12 issue of the journal
Science. Besides Lohmann, authors are his wife, Catherine M.F. Lohmann,
lecturer in biology at Carolina; former graduate student Shaun D. Cain, now at
the University of Washington; and former undergraduate Susan A. Dodge, now an
elementary school teacher and technology expert.
Florida loggerheads' migratory path follows an enormous circular current system
known as the North Atlantic gyre. Water in the gyre is relatively warm, and
food is abundant. But outside the current, conditions are less favorable, and
turtles that stray outside the route often die from the cold.
To determine whether loggerhead hatchlings could distinguish among magnetic
fields found in different parts of the gyre, the biologists fashioned tiny
bathing suits for the animals that were tethered by a fishing line to a
lightweight mechanical arm that swung in the direction the turtles swam. The
creatures, which were later released unharmed into the ocean, did not notice
that they weren't making progress.
Researchers tested the turtles, which had never been at sea before, in a
saltwater-filled, fiberglass pool surrounded by a computerized coil system. The
coil produced magnetic fields closely resembling those found in different
locations along the migratory route.
When exposed to a magnetic field like the one that exists in northern Florida,
most of the reptiles swam eastward, a direction that would carry them out to
the north-flowing Gulf Stream. When exposed to a field found off the coast of
northern Spain, most turned south. And when subjected to magnetism like that at
the southern edge of the North Atlantic gyre between Africa and South America,
most headed northwest, a direction that would send them homeward to the U.S.
coast.
"Our results provide direct evidence that young sea turtles can in effect
exploit regional magnetic fields as open-ocean navigational markers," Lohmann
wrote. "The turtles emerge from their nests ready to respond to specific fields
with directed movements. These responses are appropriate for keeping young
turtles within the gyre system and facilitating movement along the migratory
route."
Two previous studies the Lohmanns published described experiments showing that
the turtles could detect two specific features of magnetic fields known as
inclination angle and intensity. For the first time, he said, the latest
experiments combine those two elements to show conclusively that the hatchlings
can detect fields they encounter while migrating and respond to them in ways
that help keep them on track.
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