Every December, great white sharks swimming off the coast of California make a beeline for a mysterious spot in the middle of the Pacific roughly halfway to the Hawaiian islands. The sharks travel roughly 1,000 miles to the so-called white shark cafe. Tracking data has revealed that their routes are remarkably direct considering their paths traverse apparently featureless open ocean. Tiger sharks, salmon sharks and multiple species of hammerheads also make lengthy journeys to and from precise locations year after year.
Pete Klimley, a retired shark researcher who worked at the University of California, Davis calls the ability of some animals to find their way to pinpoint locations across the globe “one of the great mysteries of the animal kingdom.”
Now, new research published today in the journal Current Biology provides new support for a longstanding hypothesis that sharks use the Earth’s magnetic field to navigate during their long-distance migrations. Scientists caught bonnethead sharks off the coast of Florida and put them in a tank surrounded by copper wires that simulated the magnetic fields sharks would experience in locations hundreds of miles from their home waters. In one key test, the bonnetheads were tricked into thinking they were south of their usual haunts and in response the sharks swam north.
Iron and other metals in Earth’s molten core produce electrical currents, which create a magnetic field that encircles the planet. The north and south poles have opposing magnetic signatures and invisible lines of magnetism arc between them. The idea that sharks can navigate by sensing these fields rests on the fact that Earth’s geomagnetism isn’t evenly distributed. For example, the planet’s magnetism is strongest near the poles. If sharks can somehow detect the subtle perturbations of Earth’s magnetic field, then they might be able to figure out which way they’re heading and even their position.
Sharks are known to have special receptors—tiny jelly-filled pits called ampullae of Lorenzini that are clustered around their noses—which can sense changes in voltage in the surrounding environment. In theory, these electroreceptors, which are usually used to detect the electrical nerve impulses of prey, could pick up Earth’s magnetic field. Prior experiments have shown that, one way or another, sharks can indeed perceive and react to magnetic fields, but figuring out whether sharks can use them to navigate long distances or as a kind of map is another matter.
To test whether sharks can use the Earth’s magnetic field to orient themselves, researchers caught 20 roughly two-foot-long bonnethead sharks off Florida’s Gulf Coast at a spot called Turkey Point Shoal. Bonnetheads are a small species of hammerhead known to travel hundreds of miles and then return to the same estuaries they were born in to breed every year.
Picking a small species was crucial, says Bryan Keller, a marine biologist at Florida State University and the study’s lead author, because he and his co-authors needed to put the sharks in a tank and then build a structure that could produce electromagnetic fields that they could manipulate horizontally as well as vertically around the sharks.
Using two-by-four lumber and many feet of copper wire rigged up to a pair of adjustable electric power supplies, the team made a roughly ten-foot-wide cube that could create magnetic fields with variable poles and intensity. This allowed the team to mimic the geomagnetic conditions of three different locations on Earth to see how each impacted the sharks’ behavior.
The three magnetic locations the sharks were exposed to consisted of the place they were caught (the control treatment), a location about 370 miles north of where they were caught (the northern scenario) and a location 370 miles south (the southern scenario) of where they were caught.
As the researchers expected, when the bonnetheads were placed amongst magnetic fields of a similar intensity and arrangement to their home range they didn’t exhibit any apparent preference for swimming in one direction over another inside their tank.
Next, the northern scenario simulated something that no shark would ever experience in the wild: the magnetic conditions of Tennessee. This test was aimed at figuring out if the sharks could orient themselves toward home in a totally unnatural geomagnetic context that they would have had no occasion to ever experience. Alas, the movements of the sharks in the northern treatment showed no statistically significant heading. Keller says this non-result wasn’t terribly surprising, since the bonnetheads would never need to find their way home from Tennessee in nature.
But in the southern scenario, in which the magnetic fields were tweaked to approximate a location about 100 miles west of Key West, the sharks tended to orient themselves northward—towards home.
“To orient towards home, these sharks must have some kind of a magnetic map sense,” says Keller. “If I put you in the middle of nowhere you couldn't point toward your house unless you knew where you were in relation to it, and that’s a map sense.”
Klimley, who was not involved in the paper and is one of the progenitors of the notion that sharks use geomagnetism to navigate, says the experiments “show that if you give sharks a magnetic environment that’s different from what the sharks have in their home range, they will head for home.”
But other researchers aren’t convinced that the word “map” is appropriate to describe the sharks’ apparent ability to orient themselves by detecting magnetic fields.
“This is a good study but what I don’t buy into is that it demonstrates the use of a magnetic map,” says James Anderson, a researcher studying sharks’ sensory systems at California State University, Long Beach who was not involved in the paper. Anderson says Keller’s study shows that bonnetheads could orient themselves toward home, but adds, “a magnetic map implies the animal knows not just where it is and where it’s going but also its end destination—for example, ‘I need to go north for 500 miles to get to seamount X.’ And I’m not sure they’ve shown that here.”
The paper also drew support for its findings regarding sharks’ magnetically-guided navigation from the genetic makeup of various subpopulations of bonnetheads scattered along the perimeter of the Gulf of Mexico and Florida’s Atlantic Coast. Keller and his co-authors calculated the genetic distance between more than ten populations of bonnetheads using samples of their DNA.
When populations are separated by some barrier like physical distance or an obstacle that prevents them from mixing and breeding with each other, genetic differences tend to accumulate over time and ultimately lead to increasingly divergent DNA.
When Keller and his co-authors looked at the bonnetheads’ mitochondrial DNA, which is inherited only from the individual’s mother, the team found that physical distance and differences in temperature didn’t provide the best statistical explanation for the genetic distances they saw between populations. Instead, the populations with the greatest genetic distances between them tended to have home areas that also had very different magnetic signatures.
Because female bonnetheads return to the same estuary they were born in to give birth, and because mitochondrial DNA is only inherited from momma sharks, these results support the idea that these females’ sense of what feels like home may be partly defined by local magnetic fields.
“This highlights the possibility that females might choose pupping grounds partly based on magnetic signatures,” says Keller.
Great white shark researcher Salvador Jorgensen of the Monterey Bay Aquarium says he thinks the finding that sharks use Earth’s magnetic fields to orient and navigate is likely to apply to a majority of shark species, including the big, toothy ones he studies. “I’m intrigued by this study because we recognize the same individuals returning to the same seal rookeries on the Central California coast for 15 to 20 years with pinpoint accuracy,” says Jorgenson, who was not involved in the paper. “And that’s after travelling thousands of miles to and from the white shark cafe or Hawaii.”
Scientists’ expanding sense of how sharks perceive their environment may even one day help researchers understand if humans are blocking or confusing the animals’ navigation as offshore infrastructure continues to grow in scope and complexity.
“One of the things that makes this work important is that they’re putting in wave farms and offshore wind farms and all of these projects have big high-voltage cables leading to shore,” says Klimley. “Those cables put off their own electric fields and if that’s how sharks navigate, we need to find out how that undersea infrastructure might impact migratory sharks.”