Sharks are the ocean's world travellers, swimming thousands of kilometres a year to their favoured locations.

But how do they pull off these impressive feats of navigation without looking out for landmarks?

A US team has shown that sharks use the Earth's magnetic fields as a map when making long-distance migrations to specific locations.

The findings are published today in the journal Current Biology.

"Even when they're far away, the animals know where they are and where to swim to get home," said study co-author Bryan Keller at Florida State University. 

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Magnetic map readers?

Many species of sharks, skates and rays travel far and wide in the ocean, with some species returning to the same locations each year.

In 2005, a great white shark called "Nicole" made a record-breaking return trip from South Africa to Australia, swimming over 20,000 kilometres in just nine months.

For half a century, researchers have suspected that sharks are sensitive to magnetic fields, which could be used for navigation.

Other animals like sea turtles, lobsters and newts are known to be magnetically sensitive, but no-one has been able to confirm whether sharks are too.

Homeward bound

To find out, Mr Keller and colleagues captured 20 juvenile bonnethead sharks – a small coastal species that return to the same estuaries each year – from off the coast of Florida.

They brought the sharks back to the lab and placed them in holding tanks.

Surrounding the tanks were wooden frames with copper wires running along their edges.

By tweaking the current running through these wires, the researchers were able to recreate the magnetic fields the sharks would experience in three different locations.

When the team adjusted the current to match conditions 600 kilometres south of where the sharks were collected from, they swam in a northward direction, indicating that they were able to read the magnetic field like a map to guide them home.

They also checked what happened when the magnetic field values north of Florida, in the middle of the state of Tennessee, were reproduced.

In this case, the sharks showed no particular swimming preference.

The researchers reasoned this might be because the sharks' magnetic map-reading abilities were learned from locations they frequent.

Since they would never have experienced a land-based magnetic field before, the animals would not be able to rely on it to navigate.

And when the researchers exposed the sharks to the magnetic field conditions of the site where they were captured from, they didn't swim in a particular direction as they were already "home".

"This suggests that sharks have an amazing ability to detect and navigate using the Earth's magnetic field," said Nathan Hart, a neurobiologist at Macquarie University who was not involved in the study.

All in the genes

The researchers also wanted to explore whether magnetic fields could help explain another mystery: why sharks from the same species are genetically distinct in different locations?

The team compared mitochondrial genes (passed down from the mother) and nuclear genes (inherited from both parents) in bonnethead sharks from various locations in the northwest Atlantic Ocean.

They also tracked the difference in magnetic field values, sea surface temperatures, and coastal distances between the locations.

It turned out that the magnetic fields across the locations accounted for more variation in the sharks' mitochondrial DNA than temperature and distance. 

This suggests that on an evolutionary scale, females that initially colonised an area may have selected it based on similarities between the magnetic fields of that location and the one they originally came from, Mr Keller said.

This tendency to stick to specific locations means that sharks don't cross paths with far-flung members of the same species, leading to genetically distinct groups scattered throughout the Atlantic Ocean.

A built-in compass?

While it's clear that sharks rely on magnetic navigation to cruise the seas, the next step is figuring out how they pick up on these magnetic cues.

Even though there are no shortage of theories on how animals, including sharks, can detect magnetic fields — from the presence of light-sensitive pigments in the eye to tiny crystals of magnetite in the nose and head — scientists are still scratching their heads. 

For instance, migratory birds also use the Earth's magnetic fields to guide their flights paths, but they are only able to pick up on them during the day, suggesting that light plays a role.

However, other animals, such as turtles, are able to read magnetic maps in complete darkness.

"This is definitely still an unsolved mystery in biology," Professor Hart said.

"Given how many animals detect and orient to the Earth's magnetic field, it's an important one to solve."

Mr Keller said that it would be also interesting to explore whether magnetic fields generated by human activities — such as underwater cables and offshore wind farms — throw off sharks' sense of direction. 

"If a shark is using magnetic navigation to find a target and it detects an anomaly, it could get confused."