The three green dots show crucial turns in the loggerheads' migration routes. The researchers recreated magnetic conditions at these points in their turtle Jacuzzi.
Courtesy Kenneth Lohmann.

It's Nylon. It's Lycra. And it's made so you can string a turtle to an arm that records the young aquanaut's swimming direction. Courtesy Kenneth Lohmann

'round the far-away Sea of Sargasso,
swam a squadron of turtle fantastico,
in circles and jerkels and ovals you see,
year after year they swam 'cross the sea.

Kings of the pond,
lords of the lake,
'twas truly an ocean,
but could they mistake
what's here for what's there?
for scarce were the signs on that ocean I swear.

Consider the plight of the loggerhead sea turtle, on its 10-year migration around the Atlantic Ocean. Even though they gobble uck like sponges and jellyfish, we're not grossed by their diet (it also contains mussels, clams and shrimp).

No, we're wondering about this little problem of navigation. Like most animals, the loggerhead is particular about its environment.

If the water gets too cold, it croaks.

So while circumnavigating the Sargasso Sea, in the sunny subtropical Atlantic, loggerheads gotta worry about getting caught in a frigid current headed for Scandinavia.

Turtles don't buy global positioning systems, and there are no road signs on the open ocean. So how's a turtle supposed to stay found while migrating in a lazy, clockwise loop around the Atlantic? (That's how loggerheads born in Florida spend their first five or 10 years. Then they spend another decade along the U.S. Atlantic Coast before returning to Florida to breed.)

Canada geese can play follow-the-leader, so only the head honker needs a map. But loggerheads seem to migrate independently, and that means they must all know where they are -- or at least where to make the key turns in their journey.

No dunderheads, these loggerheads
Now comes word from the University of North Carolina that loggerheads navigate by "reading" the intensity and inclination of Earth's magnetic field.

Intensity is the field's strength -- which weakens toward the equator. Inclination -- the angle at which the field lines intersect Earth's surface -- is almost zero at the equator, and steeper near the poles. When combined, the two kinds of data seem to tell more than just latitude.

They've taken a turtle,
a simple critter you think,
Hooked him to a string,
plopped him in a sink,

They switched on the magnet,
gave him no shove,
and so he swam left,
as seen from above.

At a turtle's pace
Kenneth Lohmann and colleagues put just-hatched loggerheads into a four-foot diameter tank enclosed by a big coil of wire that could simulate Earth's magnetic field.

Lohmann had already shown that turtles could "read" the intensity and inclination of Earth's magnetic field. The new study showed that they can read both at once, at levels like those found in the Atlantic.

To measure the turtles' behavior in various magnetic conditions, Lohmann dressed them in tiny Lycra-and-nylon (we kid you not!) swimsuits, then tethered them to a levered gadget that recorded the animal's swimming direction.

The turtle Jacuzzi, used to measure swimming direction under simulated Earth's magnetic field. Courtesy Kenneth Lohmann.

Then the researchers cranked up the magnetic-field-maker to simulate a location where the loggerheads must veer a certain way, and watched as the little swimmers indeed swam in a direction true to their migratory path.

They swam like little motors,
With flippers for bitty rotors,
The turtles swam east, west, south or north,
Reading the fields that we did put forth.

Smarter 'n a bird?
The research builds on existing knowledge that some animals can determine direction based on Earth's magnetism. "Birds are known to have magnetic compasses, they can maintain specific magnetic headings," says Lohmann. The new study, he adds, showed that an animal can also get positional information from the magnetic field.

Here's the big dif: directional information simply says: "north is this way." Positional information can say, for example, you're nearing Spain. Have some tapas, then veer off to the south.

Loggerhead turtle hatchlings struggle across a beach, headed for the surf.
Courtesy NOAA.

While there are hints that birds and some other animals may get positional data from the magnetic field, "Our study is the clearest demonstration so far that animals use magnetic fields as a landmark during long migrations," says Lohmann.

The idea that turtles can read magnetic maps may seem splendid, but wouldn't they have been caught high and dry (or more accurately cold and wet) when Earth's magnetic field reverses polarity? Such a swaperoo could leave the loggerheads begging for directions to Bermuda from Edinburgh...

"If the field shifts quickly, we predict that would create a problem for them," says Lohmann, who adds that most field reversals apparently take a few thousand years to complete.

Nothing is static in nature, and he notes that while climate change also cause extinctions, most organisms adapt. "There have been numerous periods of climate change over the course of evolutionary time," he says, "some have been very rapid, yet those have not prevented animals from evolving adaptations to specific climates when conditions stabilize."

-- David Tenenbaum