How Sea Turtles Find Their Way

ByABC News
October 11, 2001, 9:23 AM

Oct. 12 -- The moment they emerge from their underground nests, tiny loggerhead sea turtles face an enormous journey. They scramble to the sea and begin navigating 8,000 miles across the open ocean and back home again.

How do they find their way? New studies suggest that like many migrating birds and honeybees, the thumb-sized turtles are guided by very slight differences in the Earth's magnetic field.

Taking subtle cues from the field, which is most intense at the poles, these turtles keep themselves within a circular swathe of warm ocean current known as the Atlantic gyre, which stretches from the eastern U.S. to the coasts of Spain and Africa. Swimming outside it, the turtles would face colder waters and certain death. After swimming in the gyre for years the turtles make their way home to Florida beaches.

It's unlikely the turtles find their way using visual cues since the open ocean offers few landmarks. Temperature is also an unlikely indicator since water temperatures vary due to factors like the Gulf stream even within the Atlantic gyre.

"People have wondered for years now how young turtles navigate during that first migration because it seems utterly impossible," said Kenneth Lohmann, associate professor of biology at the University of North Carolina in Chapel Hill. "We're finally on the verge of understanding how they do it."

Mini Ocean and Turtle Bathing Suits

To prove that the loggerhead turtles use magnetic fields for navigation, Lohmann and his team constructed a miniature ocean in a four-foot-wide fiberglass tank filled with seawater and surrounded it by a miniature magnetic field created by a carefully charged network of copper coils.

Then, one by one, they dressed 79 newborn turtles in tiny bathing suits and lowered them inside the tank.

Attached to the Velcro/Lycra bathing suits were harnesses rigged to a fishing line. The fishing line connected to a mechanical arm that measured every small turn the turtle in the tank made.

By manipulating the tank's magnetic field to mimic the magnetic angles and intensities that the turtles would experience at sea, Lohmann found the turtles immediately swam in a direction that would have kept them inside the warm current.