Sharks are among a group of animals with a 6th sense that humans don't have - they use Earth's magnetic field to navigate

• Some shark species swim thousands of miles back to the same feeding grounds every year.
• They most likely use Earth's magnetic field to orient themselves, a new study shows.
• Other species, including dogs, whales, and sea turtles, also seem to use the magnetic field to navigate.

Shark species have an uncanny ability to find their way back to the same feeding grounds every year - even areas thousands of miles away.

According to a study published Thursday, that's because sharks have a superior navigational tool at their disposal: They can orient themselves using Earth's magnetic field.

They're far from the only animals to do so. Birds, whales, and many other species use the same sixth sense to plot their migrations.

Bryan Keller, a biologist at Florida State University who co-authored the new study, likens this sense to "having an 'internal GPS.'"

"This is, in my opinion, the best explanation for how migratory sharks successfully navigate during long-distance movements," Keller told Insider.

'Sharks garner map-like information from the magnetic field'

Nearly 2,000 miles below Earth's surface, swirling iron in the planet's outer core conducts electricity that generates a magnetic field. This field stretches all the way from the planet's interior to the space surrounding the Earth. It's what protects the world from deadly solar radiation.

But the direction that the electromagnetic energy flows, as well as the strength of the resulting protective sheath, depends on where on the planet's surface you are. So animals that use the magnetic field to orient themselves do so by detecting these differences in field strength and flow. They then use that information to figure out where they are and where to go.

Scientists long suspected sharks could navigate using the field, since the animals can sense electromagnetic fields in general. But that hypothesis had been difficult to confirm until Keller's study.

His team examined the bonnethead shark, known as Sphyrna tiburo, because the species exhibits site fidelity - meaning it returns to the same estuary habitats each season.

"This means the sharks have the capability to remember a specific location and to navigate back to it," he said.

The team captured 20 bonnetheads off the coast of Florida in the Gulf of Mexico, then placed the sharks in a 10-by-10-foot tank. They generated a tiny magnetic field within a 3-square-foot area of that tank. (Bonnetheads only reach 4 feet in length, which made them an ideal species to study in such a small pool, Keller said.)

The team then tweaked that localized magnetic field to mimic the electromagnetic conditions of various locations hundreds of miles away from where they'd caught the sharks. If the animals were truly relying on magnetic-field cues to navigate, the thinking went, then the bonnetheads would try to reorient themselves and start swimming in the direction they thought would lead to the Florida coast. That's exactly what happened.

When the researchers mimicked the conditions of the magnetic field on Florida's Gulf Coast, the animals exhibited no preference in which direction they were swimming - suggesting they assumed they were already in the right place.

"I'm not surprised that sharks garner map-like information from the magnetic field, because it makes perfect sense," Keller said.

Many animals use the magnetic field for navigation

Even though the new study was done on bonnetheads, Keller said the findings likely apply to other shark species as well.

How else could a great white, for example, migrate from South Africa to Australia - a distance of more than 12,400 miles - then return to the exact same chunk of ocean nine months later?

"En route to Australia, the animal exhibited an incredibly straight swimming trajectory," Keller said of great whites. "Given that the magnetic field is perhaps the only constant and ubiquitous cue available to these migratory sharks, it is sensible that magnetic-based navigation is responsible for facilitating these incredible navigational successes."

Other navigational cues do exist, including currents and tides, but Keller said the magnetic field "is likely more useful than these other aids because it remains relatively constant."

Biologists still aren't sure how sharks detect the field, but a 2017 study suggested that the animals' magnetic receptors are probably located in their noses.

The ability to detect and orient using the magnetic field is fairly common in the animal kingdom overall, according to Keller. Scientists have observed that type of behavior in bacteria, algae, mud snails, lobsters, eels, stingrays, honey bees, mole rats, newts, birds, fish like tuna and salmon, dolphins, and whales.

Sea turtles, too, rely on magnetic cues when they migrate thousands of miles to lay eggs on the same beaches where they hatched.

Dogs, meanwhile, can find their way home both using their impressive sense of smell and by orienting themselves using the magnetic field, according to a June study.

"The magnetic field may provide dogs with a 'universal' reference frame, which is essential for long-distance navigation," that study said.