Sharks use the Earth's magnetic field as a sort of natural GPS to navigate long journeys across the world's oceans, scientists have discovered.

Marine laboratory experiments with a small species of shark have confirmed long-held suspicions that sharks use magnetic fields as aids to navigation.

This behaviour has also been observed in other marine animals, including sea turtles.

The study, published in the journal Current Biology, also reveals why sharks are able to traverse seas and find their way back to feed, breed and give birth, according to marine policy specialist Bryan Keller, one of the study authors.

He said: "We know that sharks can respond to magnetic fields. We didn't know that they detected it to use as an aid in navigation... You have sharks that can travel 20,000km (12,427 miles) and end up in the same spot."

Researchers have been intrigued by how sharks have been able to perform long-distance migrations for years.

The sharks undertake their journeys in the open ocean, where they come across few physical features such as corals that may serve as landmarks.

Seeking answers, scientists at Florida State University decided to study bonnethead sharks - a type of hammerhead that lives on both American coasts and returns to the same estuaries every year.

They exposed 20 bonnetheads to magnetic conditions that simulated locations hundreds of miles away from where they were caught off Florida.

Great whites likely share the bonnethead's reliance on magnetic fields, researchers say

Researchers found that the sharks began to swim north when the magnetic cues made them think they were south of where they should be.

Robert Hueter, senior scientist emeritus at Mote Marine Laboratory and Aquarium, who was not involved in the study, described the findings as compelling.

But he said a further study is needed to find how the sharks use the magnetic fields to determine their location and whether larger, long-distance migrating sharks use a similar system to find their way.

He said: "The question has always been: Even if sharks are sensitive to magnetic orientation, do they use this sense to navigate in the oceans, and how? These authors have made some progress at chipping away at this question."

The bonnethead's reliance on Earth's magnetic field probably is shared by other species of sharks, such as great whites, that make cross-ocean journeys, researchers say.