The Magnetic Mysteries of Turtles
by Marissa Melton
Each year in early July, when loggerhead turtle eggs start hatching on
the shores of eastern Florida, Ken and Catherine Lohmann pack a van full
of computers, coolers, and lab equipment and rumble down the road to Boca
Raton, Florida. They
are going there to learn how turtle hatchlings find their way to sea.
The turtles are headed for the North Atlantic gyre, a system of
warm-water currents that spans the Atlantic between the eastern
coast of North America and the western coast of Africa. The turtles may
linger there for years, feeding on seaweed, until they
grow large enough to return to American feeding grounds without fear of
predators near the shore.
Navigational skills are crucial. A turtle straying off course at the
north end of the gyre could wash up on the shores of Great
Britain, shocked with a lethal dose of cold North Atlantic water. A
turtle swimming south out of the gyre could be swept up in
south Atlantic currents and carried away from its range.
How the turtles find their way back to the nesting ground has long
been a mystery. But part of the answer, the Lohmanns believe, lies in the
turtles' ability to orient themselves through the magnetic field of the
earth.
In Florida, the Lohmanns and their student assistants work out of a
windowless, cinderblock building on the campus of Florida Atlantic
University (FAU), the use of it granted by colleague Mike Salmon, a
friend and FAU professor. The loggerhead
hatching season lasts through September, but the Lohmanns have only six
to seven weeks to conduct their research before the
UNC-CH fall semester starts. The pace is hectic.
"Every single night's a research night," Catherine says.
"We don't take a night off."
Workdays start around 10 a.m. with data analysis and maintenance
chores, but the research begins late, when hatchlings
begin moving under the sand. Around 5 p.m., the researchers head out to
the beach with their Styrofoa
m coolers.
The city of Boca Raton has people patrolling the beach during the late
spring, when adult loggerheads come ashore to
lay their eggs. The patrols cover each nest with an open cage, which
protects the eggs from vehicles or tourists but allows
hatchlings to escape as they surface.
Using identification dates found on the cages, the Lohmanns can
estimate to within a couple of days when a set of hatchlings
might emerge. As many as a hundred new hatchlings scramble around in the
nest before digging their way out, creating a
depression in the sand. The Lohmanns dig into the depression with their
hands, removing about a dozen turtles and placing
them one by one into the coolers to protect them from light.
Back in the lab, the Lohmanns outfit one of the hatchlings with a
custom-made Lycra "swimsuit" tethered to a movable arm
that is attached electronically to a computer. The swimming tank is
surrounded by a magnetic coil, which changes the
magnetic field features that the turtle experiences. Meanwhile, a
researcher sits at a computer monitor in the next room to
plot the turtle's path. The room in which the turtles swim is dark, to
eliminate
visual cues.
Scientists have known since the late 1960s that baby turtles make
their way toward the sea by pointing themselves toward a
light source--low, bright moon and starlight reflecting off of the
water. What the turtles did after finding the sea, however,
had been a mystery until the Lohmanns began their studies.
"At the point we began," Ken says, "no one knew that
turtles could sense the earth's magnetic field. We just thought it was likely
that they could, because they migrate such long distances." Limited
eyesight, too, was a clue that turtles use some other sense to
get around. "They have eyes that have evolved to see
underwater," Ken says, "and that means that when they lift
their heads
above water, they're extremely nearsighted."
The Lohmanns found that, once in the water, the turtles oppose the
motion of the waves. Swimming straight into a wave means
swimming away from shore and the predators that lurk there. Farther
offshore, the waves begin moving in directions other than
straight away from land. At that point, turtles must use a different
method of orientation.
Most recently the Lohmanns have discovered that turtles might even
carry magnetic "maps" around in their heads, memorizing
the unique magnetic properties of the nesting ground or feeding ground.
Each place on earth has unique magnetic properties,
identified by two characteristics. The first is called "lines of
inclination," which define the Earth's magnetic fields and vary with
surface features. The second characteristic is the intensity of the
magnetic field, which also varies across the earth's surface. Field
intensity is expressed in units of nanoTesla, or nT. In many areas of
the ocean, the lines of inclination and intensity form a grid roughly
corresponding to lines of latitude and longitude.
The Lohmanns tested the turtle's response to magnetic fields by
placing a hatchling in a tank, applying a magnetic field, and
watching which way the turtles swam. When the researchers simulated a
field where the intensity is 43,000 nT, a level found off
the coast of Portugal, the turtles swam west as if to head for the
currents that would carry them back to the eastern shore of
America. When the Lohmanns simulated a field of 52,000 nT, approximating
the coast off the Carolinas, the turtles swam east.
The Lohmanns have shown that simulating an inclination angle found on
the northern edge of the North Atlantic
gyre (the presumed migratory route of the youngest turtles) caused the
hatchlings to swim south. When the turtles encountered an angle
found on the southern boundary, they swam north.
From experiments of this kind, the Lohmanns have concluded that
hatchlings may have, at the very
least, an ability to use the magnetic field to help them remain within
the warm currents of the gyre. There is even some evidence to suggest
that hatchlings
can pinpoint their locations relying solely on magnetic information.
The Lohmanns suggest that adult loggerheads may possess a more
sophisticated mental map. Adult females migrate back to
the nesting grounds from which they were hatched when they are ready to
lay their eggs, which, Ken says, might be
anywhere from ten to thirty years after their own birth. The turtles'
ability to pinpoint a small location and return after so
much time suggests that their magnetic sense is more highly developed
than the baby turtles', perhaps from years of experience traveling
through the ocean's currents and different magnetic conditions.
Perhaps the most difficult part of the Lohmanns' job is the trip out
to get turtles from the nests. Tourists and townspeople
alike often wonder why the scientists are raiding the turtle nests down
at the beach.
"We don't ever send just one person out to the beach to collect
turtles," Catherine says, "because there's got to be somebody
to talk to the public while the other person is collecting."
Even with seven-day work weeks, there's hardly enough time for the
writing, experimentation, and explanation that every
day entails.
"We stay through the last possible moment," Catherine says.
And then, a few weeks after they arrive, they pack up the van to
migrate again--this time north, home. And by the time
they're back in the classroom, their subjects are swimming steadily
toward the refuge of seaweed mats floating over the sea.
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