Open-sea migration of magnetically disturbed sea turtles

Green turtles (Chelonia mydas) that shuttle between their Brazilian feeding grounds and nesting beaches at Ascension Island in the middle of the Atlan tic Ocean are a paradigmatic case of long-distance oceanic migrants. It has been suggested that they calculate their position and the direction of the ir target areas by using the inclination and intensity of the earth's magne tic field. To test this hypothesis, we tracked, by satellite, green turtles during their postnesting migration from Ascension Island to the Brazilian coast more than 2000 km away, Seven turtles were each fitted with six power ful static magnets attached in such a way as to produce variable artificial fields around the turtle that made reliance on a geomagnetic map impossibl e. The reconstructed courses were very similar to those of eight turtles wi thout magnets that were tracked over the same period and in the previous ye ar, and no differences between magnetically disrupted and untreated turtles were found as regards navigational performance and course straightness. Th ese findings show that magnetic cues are not essential to turtles making th e return trip to the Brazilian coast. The navigational mechanisms used by t hese turtles remain enigmatic.