Shark species have an eerie ability to find their way back to the same feeding areas each year – even areas thousands of miles away.
According to a study published on Thursday, this is because sharks have a superior navigation tool at their disposal: they can orient themselves using the earth’s magnetic field.
They are far from the only animals that do. Birds, whales and many other species use the same sixth sense to plan their migrations.
Bryan Keller, a biologist at Florida State University who co-authored the new study, compares this sense to “having an ‘internal GPS’.”
“This is, in my opinion, the best explanation for how migratory sharks navigate successfully during long-distance movements,”
‘Sharks collect map-like information from the magnetic field’
Nearly 2,000 miles below the earth’s surface, swirling iron conducts electricity in the planet’s outer core, generating a magnetic field. This field extends all the way from the interior of the planet to the space around the earth. This is what protects the world from deadly solar radiation.
But the direction in which 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 discovering these differences in field strength and flow. They then use this information to find out where they are and where they are going.
Scientists have long suspected that sharks could navigate using the field, as the animals can generally sense electromagnetic fields. But this hypothesis had been difficult to confirm until Keller’s study.
His team studied the bonnet shark, known as the Sphyrna tiburo, because the species is faithful – meaning it returns to the same estuary habitats each season.
“This means the sharks have the ability to remember a particular location and navigate back to it,” he said.
The team captured 20 bonnet heads off the coast of Florida in the Gulf of Mexico and then placed the sharks in a 10-to-10-foot tank. They generated a small magnetic field within an area of 3 square meters of that tank. (Bonnetheads reach only 4 feet in length, making them an ideal species to study in such a small pool, Keller said.)
The team then adjusted the localized magnetic field to mimic the electromagnetic conditions at various locations hundreds of miles away from where they had captured the sharks. If the animals really stood on magnetic field codes to navigate, the mindset went, then the bonnet hooves would try to rearrange 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 the Gulf Coast of Florida, the animals showed no preference in which direction they swam – suggesting that they assumed they were already in the right place.
“I’m not surprised that sharks gather card-like information from the magnetic field because it makes good sense,” Keller said.
Many animals use the magnetic field for navigation
Although the new study was conducted on bonnetheads, Keller said the results are likely to apply to other shark species as well.
How else could, for example, a great white migrate from South Africa to Australia – a distance of more than 12,400 miles – and then return to the exact same piece of sea nine months later?
“On the way to Australia, the animal exhibited an incredibly straight swimming course,” Keller said of large whites. “Given that the magnetic field may be the only constant and ubiquitous sign available to these migratory sharks, it makes sense that magnetic navigation is responsible for facilitating these incredible navigation successes.”
There are other navigation signals, including currents and tides, but Keller said the magnetic field “is probably more useful than these other aids because it remains relatively constant.”
Biologists are still not sure how sharks discover the field, but a 2017 study suggested that the animals’ magnetic receptors are likely located in their noses.
The ability to detect and orient using the magnetic field is quite common in the animal kingdom in general, according to Keller. Researchers have observed this type of behavior in bacteria, algae, mud snails, lobsters, eels, stingrays, honey bees, mule rats, salamanders, birds, fish such as tuna and salmon, dolphins and whales.
Sea turtles also rely on magnetic signals when they migrate thousands of miles to lay eggs on the same beaches where they hatched.
According to a study from June, dogs can meanwhile find their way home with the help of their impressive sense of smell and by orienting themselves using the magnetic field.
“The magnetic field can provide dogs with a ‘universal’ frame of reference that is essential for long-distance navigation,” the study said.