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"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.

"These turtles have never been exposed to water, yet they were able to process magnetic information and change their swimming direction accordingly," said Lohmann. "It seems they inherited some sort of magnetic map." The report appears in the current issue of the journal Science.

The researchers do not know how the turtles sense the magnetic field or what part of the brain is involved.

The results of the study have broad implications for conservation efforts. If populations of turtles from different locations inherit different instructions that guide their migration, then these populations are clearly unique, said Lohmann. This means that a void created when a certain population at one location becomes extinct cannot be filled by introducing turtles that are endemic to another part of the world.

"This suggests that we need to pay more attention to conserving specific populations rather than simply focusing on the species in general," said Lohmann.

If fish carry a similar "magnetic map," this could explain why low fish populations in one region do not benefit from a spillover of the same species from another location.

Wired for Navigation

In a second report published in Science, scientists have discovered a collection of nerve cells in the brains of subterranean Zambian mole rats that enable the animal to process magnetic information used in navigation.

The mole rats dig tunnels up to 200 meters (220 yards) long and build their nests in the southernmost tip of their burrows. As the direction of the magnetic field changes, so does the location of the moles' nests.

As in the loggerhead turtle study, the German and Czech researchers who conducted the mole rat study have not yet determined how the mole rats detect the magnetic fields.

Lohmann described the mole rat study as "an excellent step forward," tying a specific region of the brain with navigational ability.

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