"Magnetic Map" Found to Guide Animal Migration

Bijal P. Trivedi
for National Geographic Today
October 12, 2001

Do you ever wonder why migrating animals such as birds, salmon, and whales, to name a few, never seem to meander off course and get lost? The answer, according to a couple of new studies, may be that those migration routes and navigation skills are hard-wired into the animals' brains.

Studies of loggerhead turtles revealed that hatchlings have the ability to sense the direction and strength of Earth's magnetic field, which they use for navigating along the turtles' regular migration route.

The migration begins and ends on the shores of eastern Florida. It takes the turtles on a circuit around the Sargasso Sea, an elliptical region in the North Atlantic Ocean that's strewn with seaweed known as sargasso. The entire journey takes five to ten years to complete.

"These tiny, defenseless sea turtles embark on this 8,000-mile (12,900-kilometer) migration route around the Atlantic, and they do it alone without following other turtles," said biologist Kenneth Lohmann of the University of North Carolina in Chapel Hill, North Carolina, who led the study.

Long Journey

The loggerhead turtles are less than two inches (five centimeters) long when they emerge from underground nests on the eastern Florida coasts. They crawl straight from their shells and plunge into the Gulf Stream, then into the North Atlantic gyre, a circular current that wraps clockwise around the Sargasso Sea.

The North Atlantic gyre takes the turtles from their Florida nests and east across the Atlantic, past the Azores, south past the Canary and Cape Verde Islands, and finally back toward their birthplace on North American shores.

To determine whether the turtles inherited a migratory map, Lohmann and his colleagues collected baby loggerheads straight from their nests and studied their behavior while exposing them to different magnetic fields.

Each of the 79 loggerheads in the study was outfitted with a blue nylon-Lycra "bathing suit" that was tethered to a tracking system. The turtles were then placed in a shallow circular water tank. Surrounding the tank was a huge electric coil that generated magnetic fields.

Lohmann's team exposed the turtles to magnetic fields that simulated three key locations along the migratory route—northern Florida, the northeastern gyre near Portugal, and the southern gyre—and recorded the direction in which each animal swam.

"We found that turtles followed their migratory route," said Lohmann.

When the turtles were exposed to a magnetic field that mimics the one that occurs near Portugal, for example, the turtles paddled south. In the ocean, the movement in that direction would keep the turtles in warm, nutrient-rich circuit and away from cold waters.

Continued on Next Page >>




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