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Friday, December 1, 2023

Sharks Use the Earth’s Magnetic Field Like a Compass

Every year, great white sharks travel over 12,000 miles from South Africa to Australia, charting a nearly perfect straight line across the ocean. And every year, they turn around and travel back. There are no street signs to guide them and, for much of the journey, no stable landmarks by which they can set their course. Currents and water temperatures change. The sun sets at night, the stars disappear during the day. But the sharks carry on.

For decades, scientists have speculated that sharks must be using the Earth’s magnetic field as a sort of atlas, but it was hard to prove because sharks are notoriously difficult to study. It’s not easy to keep them in captivity, and some species are large—a great white, for example, stretches up to 20 feet long and can weigh in at over 2,000 pounds. It’s tricky to design an experiment big enough to test them in a controlled laboratory setting. Now, in a study out today in Current Biology, a team of researchers describe how they did it.

To test the long-held theory of sharks’ magnetic navigation, Bryan Keller, a researcher at the Florida State University Coastal and Marine Laboratory, had to build an apparatus that could mimic specific magnetic fields. He constructed a 10-foot wooden cube with a large tank at the center. Then he coiled over a mile of copper wiring around the cube at precise intervals. When connected to power, the copper conducted electrical current and created a magnetic field. By adjusting the power, Keller could create a stronger or weaker field, mimicking specific conditions the sharks might encounter in the ocean. If the sharks oriented themselves in a certain way based on the strength and angle of the magnetic field, that would be an indication that they were using that information to understand their position on the planet and to figure out which direction to swim.

This method has been used to study other animals, like sea turtles. And Keller, the study’s lead author, says that scientists already knew that sharks are capable of detecting magnetic fields. But, he says, “this is the first instance where it’s shown that they use that ability to infer location.”

Yet there was a limitation. The cube’s magnetic field was much too small to track the movements of famous navigators like the great white. “So in order to study these animals with this methodology, we needed a shark that was small but was still migratory,” says Keller. He opted for 20 wild juvenile bonnetheads—each under 2 feet long. Bonnetheads migrate thousands of miles every year, commuting between their summer habitats in estuaries and bays on the Florida coast and the Gulf of Mexico, where they spend the winter.

Keller tested the bonnetheads in three artificially generated magnetic fields. One mimicked the angle and strength of the one they’d naturally encounter in their Florida home, one was like the field they’d encounter at a point 600 kilometers south along their normal migratory route, and another was like a point 600 kilometers north in Tennessee, a place where the sharks have never been. The field from their home area didn’t elicit any specific response from the animals. Similarly, when they were exposed to the field mimicking the northern location, the bonnetheads didn’t react. But when they were exposed to the magnetic field like the one they’d find 600 kilometers south, they consistently oriented themselves with their heads pointing north. Keller concluded that the sharks use this information to decide which direction to travel, the way a hiker might use a compass.

“It’s a really interesting and clear demonstration that sharks are using the Earth’s magnetic field as a kind of map,” says Kenneth Lohmann, a professor of biology at the University of North Carolina who was not involved in the study. Lohmann has documented similar abilities in salmon and in sea turtles. He says this study suggests that the ability to navigate using magnetic sensing may be widespread among marine animals that migrate seasonally.

“It’s kind of equivalent to a young child being made to learn their home address,” Lohmann says. When they’re small, the sharks learn the magnetic “address” of their native estuary or bay. That information helps them return later, even after traveling thousands of miles. (They may not have responded to the magnetic information from Tennessee, he supposes, because that’s outside of the area they know.)

Salmon use scents, in addition to magnetic data, to detect their spawning grounds, and sharks may do the same, particularly at the end of their journeys. “For fine-scale movement, I think olfaction plays a huge role,” says Keller, but he doesn't think it’s powerful enough to guide them hundreds of miles.

Yet exactly how any animal senses magnetic fields remains “a real mystery,” says Lohmann. One theory is that they have magnetite crystals, which sense true north, embedded somewhere in their brains or nervous systems. Another is that magnetic fields affect receptors in their visual systems, superimposing colors or light patterns over their vision, like an augmented-reality headset. Perhaps north appears as a reddish tint, and an animal simply follows that color.

Sharks also have pores in their snouts filled with ampullae of Lorenzini, receptors which detect electrical currents in the water; sharks find prey by electrically sensing their heartbeats. Perhaps these receptors also sense magnetic fields, or pick up on them indirectly by noticing how they interact with electrical currents. Nobody can make definitive claims yet. And, Lohmann says, “there’s no reason to think that there is only one mechanism that all animals use.”

Studies like Keller’s are important because they help fill in a piece of a long-standing puzzle about how sharks achieve their vast migrations, and give humans a better understanding of how our marine technologies affect them. “It has really big implications for management and conservation of these species,” says Kyle Newton, a biologist at the Washington University of St. Louis, who studies how stingrays navigate using magnetic fields.

It’s something that’s particularly important to understand as offshore wind farms become more popular—and might disrupt these fields. Turbines turn energy from the wind into power that’s conducted back to shore through underwater cables. And just as Keller’s cube used power to mimic the Earth’s magnetic field, power cables underwater also create their own little magnetic fields in the ocean. Those anomalies could confuse animals, encouraging them to swim away from the correct route or luring them to forage in an environment that doesn’t have the right prey.

It’s not clear yet whether any disruptions are actually happening; these anomalies are small and might not have any effect at all, Newton says. Or they may bother some animals more than others. But he feels people need to study the possibility so that we don’t end up derailing these important migrations. Since people can’t feel magnetic signals, says Newton, “it’s easy for us to overlook this stuff. It’s just not on our radar.” But if we understand the stimuli that other animals can sense, we can be careful not to do lasting damage to those cues.

Update 5-10-2021 1:18 PM: This story was updated to correct the name of Kyle Newton's university.

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