The Earth’s magnetic field covers the entire planet like an invisible three-dimensional film. This all-encompassing force doesn’t drive human behavior (at least, as far as we know
), but it’s an important factor in some animals’ ability to navigate—and that group has just expanded to include sharks, according to a new study in the journal Current Biology.
The new research suggests that bonnethead sharks, just like certain species of migratory birds, sea turtles, eels, and others, use the Earth’s magnetic field to help them find their way.
“We know that sharks, rays, and skates”—a group of fish known as ‘elasmobranchs’—“are sensitive to the electromagnetic field,” says lead author Bryan Keller, a PhD student in oceanography at Florida State University. But researchers hadn’t yet managed to demonstrate whether they use that sensory ability for navigational purposes.
“What we were specifically interested in testing, was if that ability allows them to infer map-like information from the Earth’s magnetic field,” says Keller.
The team of researchers, funded in part by the Save Our Seas Foundation, a nonprofit that supports marine conservation research, looked at bonnethead sharks, a relatively small (and therefore lab-friendly) coastal species that had been shown to travel back to specific locations on a seasonal basis. The researchers captured 20 young bonnetheads in the Gulf of Mexico off the coast of Florida and brought them back to a lab, where they were placed in a tank inside a magnetic coil system that exposed the sharks to magnetic fields that resembled different geographic locations. They used software to track the sharks’ responses, observing which direction in the tank they were trying to swim towards.
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“The main test we were interested in is exposing the shark to a magnetic field that represents the location far south of their preferred habitats where they spend their summers, up in the northern Gulf,” says Keller. For this test, they oriented the sharks about 600 kilometers (373 miles) south of where they were captured. Instead of swimming randomly or in circles around the edge of the tank, as they did in a control test that mimicked the magnetic field where they were captured, in this case the sharks swam in a kind of half-moon formation, moving leftwards and then rightwards in an apparent attempt to push north.
In a test that looked at the opposite—whether they would try to move south if placed farther north than expected—the sharks did not demonstrate such a trend. The authors speculate that this could be because the sharks had no experience with this more northern magnetic field, suggesting that their “magnetic map” was something they learned to access through experience, rather than an innate ability they were born with. “But really, in order for us to make that claim, we need to do some more research,” says Keller.
This study is “a big step forward in our general understanding of the navigation capabilities of these animals,” says Yannis Papastamatiou, an assistant professor of biology at Florida International University who was not involved in the research. “It’s not an experiment in the field,” he says, “but it’s still pretty convincing evidence.” Papastamatiou notes that researchers have used similar experimental techniques to show that turtles can navigate using magnetic fields.
The evidence here would have been even stronger if a second change resulted in a different direction, wrote Catherine Lohmann, a biologist at the University of North Carolina at Chapel Hill, in an email to Popular Science. “I would have liked to see the responses to a western location, for example.”
The magnetic field might be especially useful for marine animals because they don’t have as much access to landmarks, stars or other guides, wrote Lohmann, who was not involved in the research. Overall, the research “adds to the larger idea that magnetic maps may actually be common in marine migrants.”
And if they’re widespread in the sea, she suggested, “then they also may be widespread among animals in other kinds of habitats.”