When hatchling loggerhead turtles embark on their maiden voyage, they join a well-swum route through the warm circulating current of the North Atlantic gyre. However, navigation errors could prove fatal if the novices lose their bearings and head into the colder waters further north. Yet little is known about the navigation mechanisms that keep the novices on course. In 2001, Ken Lohmann from the University of North Carolina at Chapel Hill, USA, measured the directions that newly hatched loggerheads took as they swam through three magnetic fields that occur at locations in the boundary of the north Atlantic gyre where the current shifts direction and could sweep the turtles out into the cold north Atlantic if they were unable to follow the magnetic field and change to course. He found that the turtles' always took bearings that positioned them safely in the gyre. But it wasn't clear if the young voyagers are programmed only to respond to magnetic fields at specific staging posts on the migration route, or whether they respond to all fields within the gyre. Matthew Fuxjager, Brian Eastwood and Lohmann collected newly emerged hatchlings from beaches in Florida and tested the animals' responses to two magnetic fields found at intermediate points in the gyre, one at the gyre boundary and another located 2000 km north of the gyre, to find out whether the hatchlings respond to a few fields in the gyre or, a wider range of fields within and beyond the circulating current (p. 2504).
Measuring the turtles' orientations as they swam in the magnetic fields found in the north and east of the gyre, the team saw that the hatchlings took the best bearing to keep them on track in the warm waters. However, when the youngsters swam in the magnetic field north of the gyre, they headed off in random directions. ‘This result is consistent with the hypothesis that hatchling loggerheads are not programmed to respond to regional fields that exist outside of the normal migratory pathway,’ say Lohmann and his colleagues, although they point out that the location of the northern field is quite remote from the turtle's natural migration route and the loggerheads may be able to orient in fields that are closer to their usual migration route. The team also adds that turtles that lose their way are unlikely to survive the cold waters, naturally selecting turtles whose sense of direction steers them safely through the gyre and allowing subsequent generations to stay on course as the Earth's magnetic field slowly shifts.