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Long-distance Navigation in Sea Turtles
Adult sea turtles of several species migrate across vast expanses of ocean to arrive at specific nesting areas and feeding sites. Two hypotheses have been proposed to account for this remarkable navigation. The first is that chemical cues emanating from target areas guide turtles to their destinations. The second is that turtles can approximate their position relative to target regions using fea-tures of the earth's magnetic field. Because animals often rely on multiple cues while migrating, the two hypotheses are not mutually exclusive. Satellite tracking experiments have revealed that migrating turtles often swim directly to distant goals, even when traveling perpendicularly to water cur-rents. Because animals usually change course frequently while seeking the source of a chemical plume, the consistency of headings casts doubt on the hypothesis that turtles follow such plumes over great distances. Chemical cues may nevertheless play a role in enabling turtles to recognize a target area in the final stages of a long migration. The magnetic navigation hypothesis is based on the finding that hatchling loggerhead turtles can detect two different features of the geomagnetic field (inclination angle and intensity) that vary across the earth's surface. Hatchlings from Florida, U.S.A., respond to magnetic features found along their migratory route by swimming in directions that may help keep them safely within the North Atlantic gyre, a circular warm-water current system favorable for growth and development. These results suggest that young turtles can derive positional information from features of the earth's field, and that such information may play an important role in guiding trans-oceanic migrations. Adults might also exploit geomagnetic features in long-distance navigation. In principle, turtles nesting on coastlines might locate the appropriate region by returning to an area marked by the intersection of the shoreline and a magnetic isoline (e.g., a particular inclination angle or intensity). Turtles that migrate to remote islands may be able to exploit bicoordinate magnetic maps for position-finding, although secular variation and other factors may limit the conditions under which such a system can be used.