Sea turtles use Earth's magnetic field to travel thousands of miles to the beach where they were born, scientists have shown.

The idea that these creatures have an inbuilt compass that utilises the magnetic field was first proposed a few years ago. It was thought that sea turtles and a number of other animals learn the magnetic field of the homing area then use this information to get back there.

However, the theory – known as geomagnetic imprinting hypothesis - was extremely difficult to test.

Scientists from the University of North Carolina have now provided evidence that hatchlings seek out the unique magnetic signatures along the coast and use it to return to the same beach to nest as adults.

Study author J. Roger Brothers 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."

In their study published in the Cell Press journal Current Biology, the researchers used 19 years' worth of data on sea turtles in Florida and compared it to changes in Earth's magnetic field.

Speaking to IBTimes UK, Brothers said: "We reasoned that if the turtles are using the Earth's field to find their neonatal beaches, then these naturally occurring changes in the field might influence where they nest.

"Results confirmed that prediction and show a strong relationship between changes to the sea turtle nesting distribution and these subtle changes in the Earth's field."

sea turtle
Turtles changed where they nested according to how the magnetic field had shifted J. Roger Brothers

At some times and places, the field shifted so locations along the beach moved closer together – resulting in all the nesting turtles packing themselves in along a shorter stretch of the beach. Conversely, they spread out when magnetic signatures diverged.

The scientists do not know why or how the sea turtles use the magnetic field, but believe it could be to do with tiny magnetic particles in the brain.

"We really don't know how the turtles detect magnetic fields – or any other animal for that matter – but the most likely explanation for sea turtles is that tiny magnetic particles in the brain respond to the earth's field and help to provide a physical basis for the magnetic sense. Presumably that's the same or might be the same material we use in compass fields. But there is no conclusive evidence either way."

It is thought Earth's magnetic field is weakening at a rate of about 5% every decade. The field changes all the time, but this rate is more than the average, leading some experts to suggest the field is about to flip – an event that takes place every 200,000 years or so.

Discussing whether this weakening will impact how turtles use the field to find their nesting spot, Brothers said it will likely impact on sea turtles, in that they will not be able to find the exact geographic area they are looking for, but is not of any major concern.

"Presumably they will be able to find somewhere to nest and at least some of their offspring will survive into a period of time where it stabilises a little more and this geomagnetic imprinting can again return to be a reliable mechanism for finding their beaches."