Turtles have problem of longitude licked

Birds, fish and many other creatures are believed to pick up on changes in the Earth’s magnetic field to determine their north and south location along lines of latitude.

But evidence of longitudinal east and west positioning from magnetic cues has never been seen in an animal before.

Understanding how loggerhead turtles achieve the feat could lead to new developments in navigational technology, say the scientists who report their findings in the journal Current Biology.

Early human navigators solved the longitude problem using accurate clocks to compare times in different locations with reference to the sun.

The loggerhead turtles’ more advanced approach involves sensing subtle magnetic signatures – changes in the intensity of magnetic field lines and their inclination, the angle at which they intersect with the Earth – that vary slightly in different locations.

By monitoring the Earth’s magnetic field the turtles plot both north-south and east-west positions, as if using satellite navigation.

They use the ability during trans-Atlantic migrations from the coast of Florida along a circular route round the Sargasso Sea that last several years.

Scientists simulating conditions at the furthest extents of the Atlantic Ocean then placed hatchlings in a circular water-filled arena surrounded by a computerised coil system used to control the magnetic field.

Researcher Nathan Putman, a PhD student at the University of North Carolina, said: “The most difficult part of open-sea navigation is determining longitude or east-west position. It took human navigators centuries to figure out how to determine longitude on their long-distance voyages.

“This study shows, for the first time, how an animal does this.”