Studying Responses to Magnetic Fields

Studying the Magnetic Map of Juvenile Green Turtles

To investigate whether turtles have a "magnetic map" capable of helping them navigate to particular geographic goals, we captured juvenile green turtles several years of age in feeding grounds near Melbourne Beach, Florida, U.S.A., and subjected them to magnetic fields replicating those found at locations either north or south of the capture area. Turtles were tested outdoors in a circular, water-filled arena surrounded by a large coil system (see below), which was used to control the magnetic field in which each turtle swam. Each turtle was tethered inside the arena to an electronic tracking unit that relayed the turtle's position to a computer located inside a nearby house.

Producing magnetic fields: the magnetic coil system

To produce a magnetic field closely resembling that of the Earth's, it is necessary to construct a special magnetic coil system. The picture on the left shows the magnetic coil (boxlike structure) and orientation arena (blue tub) used in the study. The wooden frame of the coil supported an elaborate array of wires. Electrical current running through the wires generated weak magnetic fields around turtles swimming in the arena. Turtles were restricted to an area in the center of the coil where the field was highly uniform.

Tracking Tethered Turtles

Inside the orientation arena (blue tub in picture above), each turtle was placed into a soft nylon-Lycra harness and tethered to a rotatable lever-arm in the center of the arena. The arm continuously tracked the direction that the turtle swam. A digital encoder (electronic tracking device) relayed the direction to a computer located in a nearby house. A video camera mounted above the arena enabled researchers to observe each turtle without disturbing it. Each turtle swam for 10 minutes in one of two magnetic fields. Half of the turtles swam in a field that exists approximately 340 km (210 miles) north of the test site. The other half swam under identical conditions except that they were exposed to a magnetic field that exists at an equivalent distance south of the test site.

Most turtles swam steadily while tethered, establishing and maintaining consistent courses during their trials. They did not appear to notice that they were not making any progress. At the end of the trials, all turtles were returned unharmed to the ocean and released near the place where they had been captured.

To see a video of a tethered turtle swimming click here.

last edited 04/28/2004