Study finds that sharks use Earth's magnetic field to guide them like a map

Sea turtles are known to rely on magnetic signals to find their way across thousands of miles to the beaches where they hatch. Now, in the May 6 issue of Current Biology, researchers report some of the earliest conclusive evidence that sharks also rely on magnetic fields for long-distance trans-oceanic voyages.

Bryan Keller of Florida State University's Coastal and Marine Laboratory, head of the Save Our Seas Foundation project, said, "How sharks successfully navigate during their migration to their target sites has never been addressed." The study supports the theory that they use the Earth's magnetic field to help them find their way; it's nature's GPS.

Researchers already knew that some species of sharks travel long distances year after year to reach very specific locations. They also knew that sharks are sensitive to electromagnetic fields. As a result, scientists have long speculated that sharks are using magnetic fields for navigation. The challenge, however, is to find a way to test this on sharks.

"I was honestly surprised it worked," Keller said." The reason this problem has been around for 50 years is that sharks are hard to study." Keller realizes that the needed research will be easier to do on smaller sharks. They also needed a species that would return to a specific location every year. He and his colleagues eventually chose the narrow-headed hammerhead shark (Sphyrna tiburo)." Narrow-headed hammerhead sharks return to the same estuary every year. This suggests that the shark knows where 'home' is and can navigate back there from a distant location."

The question then was whether narrow-headed hammerhead sharks manage these return journeys by relying on magnetic maps. To find the answer, the researchers used a magnetic displacement experiment to test 20 wild-caught juvenile fish. In their study, they exposed narrow-headed hammerhead sharks to magnetic conditions hundreds of kilometers away from where the sharks were caught. Such a study could directly predict how the sharks should subsequently orient themselves if they do rely on magnetic cues.

If the sharks get their position information from the geomagnetic field, the researchers predict a northward orientation in the southern magnetic field and a southward orientation in the northern magnetic field as the sharks try to compensate for their perceived displacement. They predicted no orientation preference when the sharks were exposed to a magnetic field that matched their capture site. Moreover, the sharks proved to behave as they predicted when exposed to magnetic fields within their natural range.

The researchers suggest that this ability to navigate based on magnetic fields may also contribute to the population structure of sharks. The findings in narrow-headed hammerhead sharks may also help explain the impressive feats of other shark species. For example, one great white shark has been recorded migrating between South Africa and Australia, only to return to the same location the following year.

This image shows how experiments have assessed the ability of bony sharks to navigate using the Earth's magnetic field.

"How cool is it that sharks can swim 20,000 kilometers back and forth in a three-dimensional ocean and return to the same spot?" Keller asked." It's mind-blowing. In a world where people use GPS navigation almost everywhere, that ability is remarkable."

In future research, Keller said he intends to explore the effects of magnetic fields from man-made sources, such as submarine cables, on sharks. They also want to study if and how sharks rely on magnetic guidance not only during long-distance migrations but also in their everyday behavior.