Sharks Use Earth's Magnetic Field as a GPS to Navigate the Ocean

When humans need to get somewhere, we can look at a map or plug the destination into a GPS that will calculate our route. 

But how do migratory animals, who travel long distances without technological assistance, find their way? It turns out, some of them may have a built-in GPS system of their own. 

A study published in Current Biology this May provided evidence for the first time that at least one species of sharks uses the earth’s magnetic field to direct their long-distance journeys. 

"It had been unresolved how sharks managed to successfully navigate during migration to targeted locations," Save Our Seas Foundation project leader and study author Bryan Keller said in a press release. "This research supports the theory that they use the earth's magnetic field to help them find their way; it's nature's GPS."

Finned Migration

Several marine animals rely on the magnetic field to find their way, among them sea turtles, salmon, anguillid eels, and spiny lobster, Keller tells Treehugger. 

“How the animals perceive the magnetic field and what components of the magnetic field are used for navigation vary by species,” Keller says. 

But for sharks and similar species of fish, the relationship between magnetism and navigation has remained something of a mystery. It has long been known that many elasmobranchs—the subclass of cartilaginous fish that include sharks, skates, and rays—have the ability to detect and react to the earth’s magnetic field.

Several shark species are also renowned for their ability to return to the same precise location year after year. Great white sharks, for example, swim all the way between South Africa and Australia. A 2005 study showed that the sharks were able to make the more than 12,427-mile round-trip journey in nine months, returning to the exact same South African tagging site. 

“[G]iven that many of these species are migratory and that these movements are often incredibly precise to target locations, the use of the magnetic field as a navigational aid is perhaps the only logical explanation for behaviors observed in the wild,” Keller says.

However, while the explanation was logical, it had never before been demonstrated. Instead, researchers had observed associations between sharks’ swimming paths and local magnetic minimums and maximums between seamounts and feeding grounds. To actually prove the sharks were using their magnetic-detection abilities to find their way, Keller explains, scientists needed a shark species that met two criteria:

1. It had to be small enough to participate in laboratory experiments.
2. It had to exhibit a trait known as site fidelity.

“This means the sharks have the capability to remember a specific location and to navigate back to it,” Keller tells Treehugger. “There are not many species that are both small and have described site fidelity, furthering the difficulty of this work.”

Enter the bonnethead. 

Bonnetheads in Motion

Bonnetheads (Sphyrna tiburo) are one of the smaller species of hammerhead shark, reaching an average of three to four feet in length, according to the Florida Museum. They tend to spend their summers close to the Carolina and Georgia coasts, preferring the Florida coast and the Gulf of Mexico during the spring, summer, and autumn. During the winter, they migrate closer to the equator. Amidst their travels, they always return to the same estuaries every year, Keller explains. 

To determine whether or not this return is influenced by the earth’s magnetic field, Keller and his team captured 20 juvenile bonnetheads in the wild and tested their abilities in the lab. They did this by building something called a merritt coil system—a 10-foot-by-10-foot frame wrapped in copper wire, as Keller explained in a video abstract. Running an electric charge through the wire creates a 3.3 foot-by-3.3 foot magnetic field at the center of the system. 

“When you change the power supply to the cables, you can change the magnetic fields within the cube to represent different locations,” Keller explained in the video.

The researchers manipulated the current to match the magnetic field in three separate locations: the location the sharks were taken from, a location 373 miles north, and a location 373 miles to the south. When the sharks were placed within the magnetic field to the south of their original location, they swam in a northerly direction. 

This result, Keller said in the video, “is pretty exciting, because that means the animals are using the unique magnetic field at this location to orient towards their target location.”

The sharks in the northern magnetic field did not alter their direction, but Keller said this was not unexpected. Sea turtles, which also use the earth’s magnetic field to navigate, do not respond consistently when placed in a magnetic field outside of their natural range, and the northern magnetic field put the sharks somewhere in Tennessee, where they had “obviously never visited,” Keller said.

Far to Go

While sharks’ use of an internal GPS has so far only been proven for bonnetheads, Keller tells Treehugger it is likely other migratory species of shark have the same capability. 

“[I]t is unlikely that the bonnethead would have independently evolved this ability given similarities in their ecology to other species,” Keller says. 

However, there is still much that scientists don’t know about this ability, in bonnetheads and in other sharks. For one thing, they don’t know exactly what enables sharks to perceive the magnetic field. A 2017 study concluded that sharks likely had some magnetic-detection capability in their naso-olfactory capsules in addition to an electrosensory system.

Keller also said in the press release that he hoped to study how magnetic stimuli from human sources, such as submarine cables, might impact sharks. Further, he tells Treehugger he wants to explore how the earth’s magnetic field impacts sharks’ “spatial ecology” and how they might use the magnetic field for finescale navigation in addition to long distances.