Oct. 16, 2001 CHAPEL HILL – Working in Florida, scientists have found what they believe is the strongest evidence yet that baby loggerhead turtles "read" the Earth’s magnetic field to help them navigate the massive clockwise current that sweeps the northern Atlantic Ocean.
Built into the humble creatures’ genes is a remarkable ability to monitor global magnetism and to recognize magnetic fields that mark geographic regions where the turtles must change direction to stay on course. The mechanism is not infallible, the University of North Carolina at Chapel Hill researchers say, and some hatchlings perish off track in colder waters, but it appears to operate for most of them most of the time.
"To me, one of the great wonders of the world is that baby sea turtles enter the ocean and then swim across the Atlantic and back all by themselves," said Dr. Kenneth J. Lohmann, associate professor of biology. "People have wondered for years how young turtles navigate during that first migration because it seems utterly impossible.
"How can they find their way along an 8,000-mile migratory route that they’ve never before encountered? We’re finally on the verge of understanding how they do it."
A report on the findings appears in the Oct. 12 issue of the journal Science. Besides Lohmann, authors are his wife, Dr. Catherine M.F. Lohmann, lecturer in biology at UNC, former graduate student Dr. Shaun D. Cain, now at the University of Washington, and former undergraduate Susan A. Dodge, now an elementary school teacher and technology expert.
Florida loggerheads’ migratory path follows an enormous circular current system known as the North Atlantic gyre. Water in the gyre is relatively warm, and food is abundant. But outside the gyre, conditions are less favorable, and turtles that stray from the route often die from the cold.
To determine whether loggerhead hatchlings could distinguish among magnetic fields found in different parts of the gyre, the biologists fashioned tiny bathing suits for the animals that were tethered by a fishing line to a lightweight mechanical arm that swung in the direction the turtles swam. The creatures, which were later released unharmed into the ocean, did not notice that they weren’t making progress.
Researchers tested the turtles, which had never been at sea before, in a saltwater-filled, fiberglass pool surrounded by a computerized coil system. The coil produced magnetic fields closely resembling those found in different locations along the migratory route.
When exposed to a magnetic field like the one that exists in northern Florida, most of the reptiles swam eastward, a direction that would carry them out to the north-flowing Gulf Stream. When exposed to a field found off the coast of northern Spain, most turned south. And when subjected to magnetism like that at the southern edge of the North Atlantic gyre between Africa and South America, most headed northwest, a direction that would send them homeward to the U.S. coast.
"Our results provide direct evidence that young sea turtles can in effect exploit regional magnetic fields as open-ocean navigational markers," Lohmann wrote. "The turtles emerge from their nests ready to respond to specific fields with directed movements. These responses are appropriate for keeping young turtles within the gyre system and facilitating movement along the migratory route.
"Similar mechanisms might function not only in sea turtles but in diverse ocean migrants such as fish and marine mammals, as well as in some migratory birds."
Two previous studies the Lohmanns published described experiments showing that the turtles could detect two specific features of magnetic fields known as inclination angle and intensity. For the first time, he said, the latest experiments combine those two elements to show conclusively that the hatchlings can detect fields they encounter while migrating and respond to them in ways that help keep them on track.
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