Every year, thousands of great white sharks’ (Carcharodon carcharias) travel from South Africa to Australia, following an almost perfect straight line across the ocean. They don’t have any signs to guide them and no stable landmarks by which they can set their course -- yet they carry on in the same way, year after year. Researchers now know how they do it.
Many have speculated that sharks could be using the Earth’s magnetic field as an atlas, just as several other animals do. But this was hard to prove as sharks are notoriously difficult to study. It’s tricky to keep them in captivity and to design an experiment big enough to test them in a laboratory -- but in a new study, researchers were able to pull it off.
A team from Florida State University found that sharks have an internal navigation system which allows them to use Earth’s magnetic forces to travel long distances with accuracy. They subjected 20 bonnethead sharks (Sphyrna tiburo) to “magnetic displacement” exercises that replicated geographical locations from where they were captured.
“We’ve known for some time they have the ability to detect the magnetic field, this is the first time that’s tested successfully that they use those abilities to infer their location or if they’re garnering map-like information from the magnetic field,” Bryan Keller, lead author, told The Guardian. “We expect these abilities are also observed in other species.”
For the study, the researchers built a wooden cube with a large tank at the center and then coiled copper wiring around the cube at precise intervals.
When connected to power, the copper conducted electrical current and created a magnetic field. The team could create a stronger or weaker field by adjusting the power to mimic the conditions the sharks encounter in the oceans.
They tested the sharks in three artificially generated magnetic fields. One was like the field they’d naturally encounter in Florida, where they were captured. Another one was like the field at a point 600 kilometers south along their migratory route, and another was like a point 600 kilometers north in Tennessee, where the sharks had never been to.
The animals didn’t produce a response to the field from their home area or to the one mimicking the northern location. But when they were exposed to the magnetic field like the one they would find south in their migratory route, they consistently oriented themselves with their heads pointing to the north. This means sharks use this information to decide which direction to travel.
“The question has always been, even if sharks are sensitive to magnetic orientation, do they use this sense to navigate in the oceans, and how?” Robert Hueter, a retired senior scientist at Mote marine laboratory on Florida’s west coast, told the Associated Press. “These authors have made some progress at chipping away at this question.”
While very useful, the finding doesn’t explain how sharks’ sense magnetic fields. Still, there are a few theories. One argues that animals have magnetite crystals, which sense true north, embedded somewhere in their brains or nervous systems, while another one believes that magnetic fields affect receptors in their visual systems.
Sharks also have pores in their snouts filled with receptors which detect electrical currents in the water, known as the "ampullae of Lorenzini". They find prey by electrically sensing their heartbeats and researchers suspect these receptors double as magnetic fields, or pick up on them indirectly by noticing how they interact with electrical currents.
But nothing is certain yet.
The study was published in the journal Current Biology.