When it comes to remarkable feats of migration, you’ll often hear about birds or salmon. But many sharks also undertake impressive journeys across the oceans, from great white sharks—some of which travel from South Africa to Australia and back—to lemon sharks that can find their way home to a tiny island in the Bahamas.
For decades, scientists have wondered how these fishes pull this off. Many species have a superior sense of smell, but although it may help them to orient during the final stretch, it’s unlikely that smell alone could guide them across large distances. That’s why many experts believe sharks navigate by sensing Earth’s magnetic field, perhaps using the same electromagnetic sensory organs that help them track down prey.
Since each location on Earth has a different magnetic signature, some have hypothesized that sharks may have some kind of “magnetic map” in their heads that tells them where they are. (Read about the most fascinating shark discoveries of the last decade.)
To test this theory, Bryan Keller, a shark biologist now at the National Oceanic and Atmospheric Administration brought 20 juvenile bonnethead sharks, a hammerhead species chosen because they faithfully return home to breed, into a laboratory at Florida State University.
In a new paper, published in the journal Current Biology this week, Keller and colleagues confirm that bonnethead sharks can indeed use Earth’s magnetic field to navigate.
“There have been other papers demonstrating that sharks detect and respond to magnetic fields,” says Keller, “but this is the first study to show that they have a map-like sense.”
Learning how sharks navigate may help us understand where they go and better protect these areas, many of which are heavily affected by overfishing and pollution: Populations of 18 oceanic shark and ray species have declined by 70 percent since 1970.
Fooling young sharks
For their experiment, the team placed the young sharks in a single tank surrounded by a cube wrapped in copper wire. “When you change the amount of power running through those wires, it changes the magnetic field,” Keller says. If the sharks indeed have a magnetic map in their heads, exposing them to a different field should redirect their movements, he says. (Learn about six sharks you’ve probably never heard of.)
And that’s exactly what happened, at least some of the time. When the young sharks were exposed to the magnetic field of the place they were caught, off Florida’s Gulf Coast, they swam in various directions. But when they were exposed to a magnetic field mimicking a location about 375 miles south of where they had been caught, many of them tried to swim north.
That certainly suggests that the animals were using some kind of magnetic map that informed them they were way south of where they were supposed to be, Keller says.
But he didn’t stop there.
He also exposed the sharks to a magnetic field mimicking a place the exact same distance to the north. When he did that, though, the sharks didn’t seem to have a clue where they were and didn’t know which way to turn.
Way off the mark
Why would the sharks’ map only work in the south? One reason, Keller points out, might be that sharks in this population never go north of where they were caught, since there is only land there.
To the south, on the other hand, lies the Gulf of Mexico, which the juveniles are likely to have had a chance to explore. This implies to Keller and his colleagues that the sharks may have to learn their magnetic maps, piecing them together as they swim around.
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“The findings suggest that sharks and sea turtles use the magnetic field in similar ways,” says Kenneth Lohmann, a sensory neuroscientist at the University of North Carolina at Chapel Hill who studies sea turtle navigation, (Read how sea turtles find the beach where they were born.)
So far, he explains, the navigational mechanisms uncovered in sea turtles have turned out to be quite similar to those found in other long-distance ocean migrants, such as salmon.
“In the ocean, there is very limited information that can be used to guide migrations,” says Lohmann, who wasn’t involved in the research. “So, Earth’s magnetic field is a particularly useful cue.”
A more elaborate map ?
In sea turtles, research has shown that some ability to read the magnetic field is inherited, but some is also learned, so that a combination of the two helps the reptiles find their way.
“With sharks, it’s not yet possible to determine what is inherited and what is learned, but this study provides a good foundation,” he says. (Read more about amazing animal migrators.)
Henrik Mouritsen, a sensory neuroscientist at the University of Oldenburg in Germany, agrees it is too early to say whether sharks learn their maps, but notes that in birds, whose magnetic sense is better studied, they are “definitely learned.”
In sharks, he adds, “I wonder whether and how their special electric sensors are involved. For me, the real exciting thing would be the explanation of how they do it.”
One really interesting experiment to do next, Keller says, would be to test whether bonnetheads living along the U.S. East Coast, which travel north as well as south from the place they are born, have a more elaborate map than those living in the Gulf of Mexico.