Out for a swim. A green sea turtle ready to show off its magnetic talents.

Magnetic Maps Guide Migrating Turtles

Migrating sea turtles are known for finding their way home after traveling far and wide. After years of wondering how they know which way to go, researchers have found that turtles use Earth's magnetic field as both a compass and a map.

It's no secret that turtles can figure out which way is north from Earth's magnetic field, along with the position of the sun and other stars. But by itself, that's not enough to reach a particular destination. Researchers suspected that turtles must also keep track of their location using some kind of internal map. Now, in the 29 April issue of Nature, Ken Lohmann of the University of North Carolina, Chapel Hill, and his team report that green sea turtles, Chelonia mydas, create a map from Earth's magnetic field. Turtles, they show, are sensitive to subtle differences in the intensity and angle of Earth's magnetic field between various locations.

The team captured two-dozen juvenile turtles from their feeding grounds near Melbourne Beach, Florida, and swiftly plunked them into a large tub of water in the backyard of their rented summerhouse, a makeshift lab. They slipped the turtles into bathing suits--cloth harnesses that allowed the turtles to swim in any direction, but kept them from actually going anywhere. A large coil around the tub generated a magnetic field.

The team fooled the turtles into thinking they'd been moved away from home, by tweaking the field ever so slightly to mimic sites some 340 kilometers to the north or south. The turtles that thought they'd been dropped off up north started swimming south in the tank to get back. Conversely, turtles that thought they were south of their feeding grounds started swimming north. The results suggest that green sea turtles know where they are based on the magnetic field, and they use that information to plan their routes. Lohmann has found similar results in spiny lobsters and suspects that many other animals navigate by magnetic maps.

“It's an amazing achievement,” says Mike Salmon, a turtle biologist at Florida Atlantic University. “They've figured out how sea turtles navigate, that's been one of the big questions for a long time.” Salmon agrees this study could explain how navigation works in many animals.

Related sites
A description of Ken Lohmann's research including a video of a swimming turtle
Mike Salmon's Web site