Researchers Study Turtle Navigation
Thursday, October 11, 2001 2:13 p.m. EDT

By PAUL RECER AP Science Writer


WASHINGTON (AP) - From the moment they hatch in sandy nests on
Florida beaches, tiny loggerhead turtles can navigate
across the Atlantic Ocean and back home again using the subtle
signals of the Earth's magnetic field, a new study shows.

The study, appearing Friday in the journal Science, reports on a
laboratory experiment that showed the baby sea turtles
used natural magnetic field patterns as navigation guides to stay
within the Atlantic gyre, an ocean-wide current of warm
water.

The gyre forms a huge circular ocean current, moving clockwise from
the U.S. East Coast, across the North Atlantic and
then south along the coasts of Spain and Africa, before turning west
to complete the circle.

After hatching, baby loggerhead turtles swim out into this current
and flow with it for years, nudged along by warm waters
rich in food. Their migration, lasting for years, carries them on an
8,000-mile path around the Atlantic Ocean.

If they leave the gyre, the turtles die, killed by the surrounding
frigid waters, said Kenneth J. Lohmann, a University
of North Carolina turtle researcher and first author of the study.
``They need to stay in the warm waters to survive.''

Just how the turtles are able to navigate their way across the
Atlantic and back while staying within that current has
long been a mystery, but a laboratory experiment by Lohmann and his
North Carolina colleagues suggest that the animals
use magnetic markers to find their way.

In the study, the Lohmann team constructed a four-foot wide tank,
filled with saltwater and surrounded by copper coils
that can create precisely controlled magnetic fields.

The researchers put tiny bathing suits, attached to a harness, on 70
turtle hatchlings and let the animals swim around
the tank as copper coils created carefully regulated magnetic fields.
The harnesses were connected to a mechanical arm,
like a weather vane, that pointed the direction the animal was
swimming.

``We used the coils to reproduce the magnetic fields that exist in
different parts of the Atlantic Ocean,'' said Lohmann.
``Each turtle was exposed to one magnetic field which stimulated a
particular part of the ocean.''

The researchers found that whenever the turtles were in a magnetic
field like that which exists naturally in the Atlantic,
they immediately swam in a direction that would have kept them in the
warm current.

For instance, when they were exposed to a magnetic conditions
matching those along the coast of Spain, the turtles turned
south as if following the course taken by the warm current. Magnetic
conditions mimicking those in the mid-Atlantic caused
the turtles to swim west-northwest, a direction that would keep them
within the warm current and return them to Florida from
the middle of the ocean.

Each of the turtle hatchlings was released into the ocean after one
test swim in the laboratory tank.

Lohmann said the study shows the turtles are born with the ability to
use subtle magnetic fields as open-ocean guides.
He said the turtles apparently use both the intensity and direction
of the magnetic fields to navigate.

Colonies of loggerhead turtles, a threatened species, also exist
along the coasts of Australia and Japan. Those groups
follow another gyre, a warm current that circles the Pacific. Since
those turtles are genetically imprinted to follow the
Pacific gyre, it is unlikely that they would survive if they were
introduced into the U.S. East Coast, said Lohmann.

``These results imply that turtles from different populations are not
interchangeable,'' said Lohmann. ``If one population
goes extinct, we'll not be able to restore it by bringing in turtles
from another part of the world.''

Kenneth P. Able, an expert on navigation by animals, such as birds,
at the University of Albany, said the new Lohmann
study ``is a significant advance beyond what has been done before''
in demonstrating how the loggerhead navigates across
the ocean.

``It shows that the turtles are very, very sensitive to both magnetic
intensity and inclination (direction) and can use
that information for positioning,'' he said.

Michael Salmon, a turtle researcher at Florida Atlantic University in
Boca Raton, called the Lohmann experiment a
``very clean, very simple and very straightforward way of
demonstrating'' the turtles' ability to navigate.

---

On the Net:

Loggerhead turtles: www.unc.edu/depts/oceanweb/turtles

Science (after 2 p.m. EDT): http://www.eurekalert.org

Copyright © 2001 Associated Press. All rights reserved. This material
may not be published, broadcast, rewritten,
or redistributed.