By Rick Weiss
Washington Post Staff Writer
Monday, February 16, 1998; Page A02

Loggerhead turtles routinely cross the Pacific Ocean from Japan to Baja California and then return unerringly to the precise Japanese beaches from whence they came. New research sheds light on how they accomplish this navigational feat.

Scientists knew that the first directional cues perceived by newly hatched loggerheads are the parallel lines of waves washing ashore. Newborns scrambling from the beach to the sea instinctively maintain a nautical bearing perpendicular to those waves, which keeps them from circling around to land.

At sea, however, waves may come from any direction, so turtles need a different navigational tool. Studies have shown they can detect Earth's magnetic field, but do they use it as a compass needle?

Researchers at the University of North Carolina and Florida Atlantic University tracked loggerheads in a large tank exposed to a magnetic field. The turtles were allowed to swim for 30 minutes against artificial waves. When the waves stopped, the turtles maintained their heading unless the scientists reversed the magnetic field, in which case the turtles started swimming in the opposite direction.

The animals probably "set" an internal magnetic compass while orienting against the waves, then switched to the magnetic orientation system at sea, the team concludes in the January Animal Behaviour. When the waves were stopped after just 15 minutes, the turtles wandered aimlessly without regard to magnetic field, suggesting they need more than 15 minutes orienting against waves to entrain themselves to the surrounding magnetic field.