25 February 2011 Last updated at 00:25

Sea turtles' migration mystery is 'solved'

By Camille Ebden BBC News

Loggerhead sea turtles are able to navigate oceans when still hatchlings.

Until now, how species such as loggerhead sea turtles manage to migrate thousands of miles across oceans with no visual landmarks has been a mystery.

Now researchers from the University of North Carolina believe they have found the answer.

Loggerhead sea turtles appear to be able to determine their longitude using two sets of magnetic cues.

It is the first time this ability has been shown in any migratory animal.

This research is published in the journal Current Biology.

Suprising discovery

Although several species of turtles are known to use magnetic cues to determine latitude, it was believed that this wasn't possible for longitude.

However, the loggerhead turtles have managed to surprise researchers by developing a method that involves using the strength and angle of the Earth's magnetic field.

Nathan Putman, the lead author of the research, emphasised that "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."

Loggerhead hatchlings, however, are able to manage this feat as soon as they reach the sea from their nests.

On reaching the sea, the hatchlings are able to establish the correct course to the open ocean.

The young loggerheads then spend several years successfully navigating complex migratory routes over thousands of miles of ocean.

Loggerhead Sea Turtle

• Scientific name: Caretta caretta
• Loggerheads in the North Atlantic cover more than 9,000 miles
• Loggerheads are found in the Atlantic, Pacific and Indian oceans.
• Considered "endangered" by the International Union for the Conservation of Nature (IUCN).

To carry out the research loggerhead hatchlings from Florida were placed in circular water containers and tethered to electronic tracking systems to monitor their swimming direction.

The hatchlings were then exposed to magnetic fields which replicated the fields they would come across in two locations on the same latitude but different longitude along their migratory route.

The turtles reacted to each magnetic field by swimming in the directions that would, in the real location, take them along their circular migratory route.

The researchers say this shows that the hatchlings are able to determine longitude using information from the magnetic field.

Nathan Putman explains that "along the migratory route of loggerheads, nearly all regions are marked by unique combinations of intensity (field strength) and inclination angle (the angle that field lines intersect the surface of the Earth)".

"Thus, turtles can determine longitudinal position by using pairings of intensity and inclination angle as an X, Y coordinate system".

Further applications

Dr Kenneth Lohmann, director of the laboratory where this research was carried out, said the research "not only solves a long-standing mystery of animal behaviour but may also be useful in sea turtle conservation".

The research might even have a role to play in the development of human navigational technologies, according to Nathan Putman.

"There may be situations where satellite might not be available, where this system of using two aspects of a magnetic field could be very useful".