In 2021, scientists tested 20 bonnethead sharks at Florida State and found they use Earth’s magnetic field to navigate: Study suggests sharks may have a magnetic map

Sharks have been swimming the oceans for hundreds of millions of years, yet for decades a simple question about them went unanswered: how do they find their way home over hundreds of miles of open water with no visible landmarks? According to a study published in the journal Current Biology, bonnethead sharks use Earth’s magnetic field as a built-in map. The study titled ‘Map-like use of Earth’s magnetic field in sharks,’ led by shark biologist Bryan Keller, provided strong evidence that sharks can sense magnetic cues well enough to navigate home from a distance.

Why smell was ruled out

Sharks cover vast distances during their travels. Some great whites are known to cross entire oceans, and lemon sharks make their way back to the same tiny Bahamian island year after year. Scientists used to think that smell was behind this navigation system until they realized that smell cannot cover such large distances, meaning smell is unlikely to explain how sharks navigate hundreds of miles back home. That left magnetism as the leading theory, as each point on Earth has a slightly different magnetic signature that animals could learn to read.

Testing 20 juvenile bonnethead sharks

To test this, Keller's team caught 20 juvenile bonnethead sharks, a small species of hammerhead sharks known for their tendency to return to the same estuaries to reproduce, at Turkey Point Shoal, in the Gulf of Mexico off Florida. The sharks were caught between April 2018 and July 2019 and then taken to the Florida State University Coastal and Marine Laboratory for the tests. Researchers placed each shark in a tank, surrounded by coils of wire capable of generating different magnetic fields on command. When placed in a magnetic field matching its capture location, the shark swam randomly in all directions. The critical part of the experiment followed soon after. When the researchers created a field which would correspond to a place located around 375 miles south of the sharks' native location, the sharks started to swim towards the north as if correcting their course and heading back home. However, when the team recreated a field from the same distance to the north, the sharks seemed disoriented and showed no consistent direction at all.

A map that only worked one way

This lopsided result was significant. Keller and his coauthors reasoned that the test site 375 miles south was within the broader range bonnetheads are known to travel through, making it plausible the sharks had encountered similar magnetic signatures there before, unlike the area to the north, which is mostly land these sharks would rarely, if ever, cross. The researchers say this suggests a magnetic map that is at least partly learned through experience as the sharks explore.

What sea turtle researchers make of it

Sharks are not the only animals thought to navigate in this way. Sea turtles are well known to imprint on their home beach's unique magnetic signature and return to it years later. According to a study, 'There and back again: natal homing by magnetic navigation in sea turtles and salmon,' sea turtles and salmon imprint on their home magnetic field while they are still very young and heavily depend on it during their long migrations. Kenneth Lohmann, a sensory neuroscientist at the University of North Carolina at Chapel Hill who studies turtle navigation, said the shark findings echoed what his lab had already found in turtles. However, researchers say it is still unknown how much of a shark's magnetic sense is genetic and how much is learned.

Why this discovery matters beyond curiosity

Understanding how sharks navigate can help scientists identify key ocean routes and habitats that need protection. A separate 2021 study in the journal Nature tracked 18 species of oceanic sharks and rays and found that their global abundance fell 71 percent between 1970 and 2018, largely because of overfishing. More recent assessments have continued to show the decline extending into the 2020s, though at a somewhat slower rate in some regions with stronger fishing regulations. If sharks rely on magnetic cues to navigate, protecting those routes may be as important as protecting the water itself.

Keller, who did the research while at Florida State University and is now at the National Oceanic and Atmospheric Administration, thinks the same kind of magnetic sense could help other migratory sharks as well, especially those that migrate even longer distances, such as great white sharks. It is a reminder that some of the ocean's oldest inhabitants are still capable of surprising us, and that Earth itself may be quietly guiding their journeys home.