Female sea turtles will swim for thousands of miles to return to their place of birth to lay eggs.  For years, how they found their home has remained a mystery to researchers.  However, according to a new study, these sea turtles find their way home by relying on the unique magnetic signatures along the coast.

For more than 50 years the mystery of the sea turtles has remained unsolved, says J. Roger Brothers, a graduate student of biology at the University of North Carolina at Chapel Hill and lead researcher of the new study.

"Our results provide evidence that turtles imprint on the unique magnetic field of their natal beach as hatchlings, and then use this information to return as adults," Brothers says.

Previous studies have shown that turtles use the Earth's magnetic field to help guide them while they are out at sea, but scientists up until now have been unsure if they also used these fields to help steer them toward the nesting sites chosen by their mothers.

Scientists examined a database of records covering 19 years of loggerhead sea turtle nesting along the eastern coast of Florida, which has become the largest sea turtle rookery in North America.  Discovering that the sea turtles used the magnetic fields to navigate was only part of the issue.

The Earth's magnetic field, which acts as a giant shield that protects the planet from dangerous solar radiation, changes over time.  The iron core of the Earth is surrounded by a layer of molten metal, and as this molten metal moves around, it creates fluctuations in the magnetic field, with some areas growing in strength while others weaken. 

And with these magnetic dynamics in mind, the researchers began to wonder whether or not the sea turtles had changed their nesting locations based on these changes in the magnetic fields across the coast.

"We reasoned that if turtles use the magnetic field to find their natal beaches, then naturally occurring changes in the Earth's field might influence where turtles nest," Brothers says.

A closer examination of data recorded from 1993 and 2011, confirmed this concept.  At certain times, the Earth's magnetic field shifted so that the signals from nearby beaches moved closer together.  During these times, the sea turtles' nests more densely covered these areas. In addition, there were fewer turtle nests and the nests were farther apart in the locations where the magnetic field diverged, just as researchers had predicted.

"Our results provide the strongest evidence to date that sea turtles find their nesting areas at least in part by navigating to unique magnetic signatures along the coast," Brothers says.

Scientists still don't know how the turtles detect this magnetic field, but it is possible that small magnetic particles in their brains help the turtles find their way. And that is something that the researchers would like to continue to investigate.

"The only way a female turtle can be sure that she is nesting in a place favorable for egg development is to nest on the same beach where she hatched," Brothers says. "The logic of sea turtles seems to be that 'If it worked for me, it should work for my offspring'."