April 30, 2004
Seafaring captains of old probably could have learned a thing or two from the sure-fire navigation sense of Atlantic green sea turtles, whose pinpoint accuracy across vast ocean distances has now been attributed not only to an inner compass but also to an inner map.
The finding, reported Thursday in the journal Nature, helps to explain a major enigma in the field of animal behavior: how juvenile and adult sea turtles can return year after year to the same exact feeding grounds after their extensive voyages.
University of North Carolina biologist Kenneth Lohmann, the study's lead author, said prior knowledge of green sea turtle navigation had been limited primarily to understanding how hatchlings use wave direction as a cue and an inner magnetic compass as a simple guide to reach the sea after birth. But a straight-line crawl to deeper water is a far different matter than swimming hundreds of miles across the open ocean.
"This study shows for the first time that the older turtles acquire the ability to use magnetism in a far more sophisticated way than the hatchlings do," he said. "It's as if these older turtles have their own GPS based on the Earth's magnetic field."
Lohmann and his colleagues assessed their study subjects' navigational sense by successively placing about 25 captured Atlantic green sea turtles in cloth harnesses and tethering them to a computerized tracking system within a tub resembling an over-sized backyard wading pool. An elaborate coil system surrounding the tub controlled the magnetic field.
When exposed to a magnetic field equivalent to one existing about 210 miles north of the test site in Melbourne Beach, Fla., the turtles oriented themselves roughly southward, as if trying to swim back to Melbourne Beach. When the researchers reconfigured the magnetic field so it was equivalent to an area about 210 miles to the south, the tethered turtles instead swam northward.
The results suggest that the sea turtles can distinguish among magnetic fields of different geographic locations and use this inner magnetic map to navigate toward a specific target. Intriguingly, particles of the mineral magnetite have been found in the heads of both birds and turtles. Magnetite particles align in response to magnetic fields, although researchers haven't yet established a definitive link between the contained particles and signal processing by the animals' central nervous systems.
"It's possible that the turtles literally have little compass needles somewhere in their brains or central nervous systems," Lohmann said.
The new research may impinge upon conservation practices, he added, since underwater electric cables, offshore oilrigs, and seawalls reinforced with iron beams can all distort the local magnetic field and potentially interfere with turtle navigation. On the other hand, if sea turtles rely on such a magnetic field, their inner navigational map could eventually provide conservationists with a new tool for turtle re-introductions.
"One possibility is that young turtles imprint on the magnetic field that exists in their birthplace," he said. If such a hypothesis is borne out, "turtle eggs could be permitted to develop in the magnetic field that exists in whatever location that you want the turtles to return to years later."
Kim Durham, the rescue program director for the Riverhead Foundation for Marine Research and Preservation, said more research is needed to understand how many Atlantic green sea turtles use New York's coastal waters as feeding areas, she said, but preliminary findings suggest the bays may be important areas for juvenile development. Foundation researchers also are hoping to determine the length and route of typical Atlantic green sea turtle odysseys by affixing satellite tags, but observations already have demonstrated that the turtles can travel from New York to North Carolina within a single month.
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