An extraordinary internal compass allows sea turtles to seek out the beaches where they hatched across thousands of miles of ocean, a study suggests.

The turtles achieve their feat of navigation by sensing unique magnetic signatures that act as coastline signposts, scientists believe.

Loggerhead sea turtle eggs. Photo: Yuriko Nakao/Reuters

US lead researcher Roger Brothers, from the University of North Carolina at Chapel Hill, said: “Sea turtles migrate across thousands of miles of ocean before returning to nest on the same stretch of coastline where they hatched, but how they do this has mystified scientists for more than 50 years.

“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.”

The scientists analysed a 19-year database of loggerhead turtle nesting along the eastern coast of Florida, the largest sea turtle colony in North America.

A female turtle can be sure she is nesting in a place favourable for egg development if she nests on the same beach where she hatched

They found a strong association between the distribution of turtle nests and subtle shifts in the earth’s magnetic field.

At certain times in some places, the field shifted so that the magnetic signatures of nearby locations along the beach moved closer together.

As the scientists predicted, when that happened, the nesting turtles packed themselves in a shorter stretch of coastline. Where the magnetic signatures diverged, the turtles spread out and laid their eggs in nests that were more widely separated.

Little is known about how turtles detect the geomagnetic field. It is thought likely that tiny magnetic particles in the creatures’ brains respond to the field, acting like microscopic biological compasses.

Sea turtles are thought to go to such lengths to find the places where they hatched because so many factors have to be right to guarantee successful nesting, including soft sand, the right temperature, few predators and easy accessibility.

“The only way a female turtle can be sure that she is nesting in a place favourable for egg development is to nest on the same beach where she hatched,” said Brothers. “The logic of sea turtles seems to be that ‘if it worked for me, it should work for my offspring’.”