To navigate across the world for their migrations, sharks use the Earth's magnetic field like a map to orient themselves toward home, according to a new study from biologists at Florida State University.
While sharks are known to be sensitive to magnetic fields, no conclusive evidence has been found to connect this sensitivity to their ability to navigate long distances. The study, published Thursday in Current Biology, solves this mystery, and could also have implications for conservation, which is important for maintaining the valuable ecosystem services sharks provide in the face of climate change and overfishing.
"This has been a question in the shark field for 50 years," said first author Bryan Keller, who completed the work as part of his Ph.D. in biological oceanography at FSU. "Do sharks use the Earth's magnetic fields [to] sort of derive map-like information?
Keller's study provides an answer in the affirmative.
"When sharks are exposed to magnetic conditions, like far south of their home, they use the magnetic information available to them," he said. "Basically, they're using the magnetic field to create a map that allows them to successfully navigate or orient back towards the target location."
While not all sharks migrate, some are among the most impressive migrants in the ocean. In the Pacific, great white sharks migrate as far as 2,500 miles each year, relying on fat stores in the liver to sustain them on the long journey. White sharks also migrate through the whole of the North Atlantic and even from the southern tip of Africa to Australia. And no matter how far they travel, sharks always manage to find their way home.
"An animal can migrate thousands of kilometers in the ocean, or somewhere on land, or in the sky, and find a really precise location, and they don't have a map, they don't have a phone, anything like that," Keller said.
However, the precise mechanism of sharks' navigation has remained unproven. Some scientists hypothesize that sharks follow their noses, using their sense of smell to chart a course through the water. But the prevailing hypothesis is that sharks detect the Earth's magnetic field, like an internal compass.
To put this to the test, the researchers faced the challenge of finding the right shark. While large sharks such as the great white are impressive migrators, they are so large that they're unsuitable for the type of laboratory study Keller and his colleagues undertook.
They settled on bonnetheads, Sphyrna tiburo. At just two to three feet long, these sharks are small enough to be useful in lab studies, easy to catch off the coast of Florida, and are also excellent navigators.
"The bonnethead returns to the same estuaries each year," Keller said. "This demonstrates that the sharks know where 'home' is and can navigate back to it from a distant location."
The researchers collected 20 of these sharks from the wild and put them into a bizarre experimental setup, known casually to the team as the "Magneto Cube." The setup, which can be seen in an accompanying video abstract, consists of a large tank surrounded by electromagnets that could be adjusted to different levels of power to approximate the effect of the Earth's magnetic field at different locations.
"It's kind of crazy to think about putting a shark in a huge tank, surrounded by a magnet, like a magnetic box, and actually eliciting a behavioral response," Keller noted.
But that is exactly what happened. When the ambient magnetism matched the southern magnetic field, the sharks oriented themselves northward, and vice versa. There was no orientation preference when the magnetic field was adjusted to match their capture site. These results strongly suggest that sharks use magnetic forces to determine which direction to swim.
While Keller and his colleagues were not especially surprised by the findings because this had been suspected by many researchers in the past, they were "happy and somewhat surprised" that this strange new experimental setup was a success.
The researchers are next interested in exploring the effects of human-generated magnetic fields on shark navigation, which could be of interest to the fishes' conservation.
"There are plenty of submarine cables that route energy from the shore, or from the coast to the shore. And when those cables transport the energy, they produce an electromagnetic field," said Keller. "It will be interesting to see how that sort of magnetic anomaly would affect them."
The researchers also suggest that the ability to navigate by magnetism may contribute to the way shark populations are structured in different parts of the world, which could offer insight into the more heroic migrations of other marine species such as the great white.
But for now, after years of fascination with animal migrations, Keller and the team are just thrilled to have begun unraveling the mystery of shark navigation.
"How cool is it that a shark can swim 20,000 kilometers round-trip in a three-dimensional ocean and get back to the same site?" Keller asked. "It really is mind-blowing. In a world where people use GPS to navigate almost everywhere, this ability is truly remarkable."
The study, "Map-like use of Earth's magnetic field in sharks," published May 6 in Current Biology, was authored by Bryan A. Keller, R. Dean Grubbs and Timothy P. Murphy, Florida State University; Nathan F. Putnam, LGL Ecological Research Associates; and David S. Portnoy, Texas A&M University.