Sea turtles are well-known for relying on magnetic signatures to navigate thousands of miles to their natal beaches. Now, researchers Dr. Timothy P. Murphy, Dr. Dean Grubbs and Dr. Bryan A. Keller from Florida State University; Dr. David S. Portnoy from Texas A&M University and Dr. Nathan F. Putman from LGL Limited, have some of the first solid evidence that sharks, too, rely on magnetic fields for their long-distance forays across the sea, as reported in the journal Current Biology on May 6.
“It was previously unknown how sharks navigated successfully to targeted locations during migration,” said Save Our Seas Foundation project leader Bryan Keller. “This research bolsters the theory that they navigate using the earth’s magnetic field; it’s nature’s GPS.”
Researchers were aware that certain species of sharks travel great distances year after year to reach very specific locations. Additionally, they were aware that sharks are susceptible to electromagnetic fields. As a result, scientists had long hypothesised that sharks navigated using magnetic fields. However, the difficulty was determining how to test this in sharks.
“To be completely candid, I’m surprised it worked,” Keller admitted. “The reason this question has endured for 50 years is that sharks are notoriously difficult to study.”
Keller recognized that the necessary research would be easier to conduct on smaller sharks. Additionally, they required a species that was known for returning to specific locations each year. He and his associates agreed on bonnetheads (Sphyrna tiburo).
“Each year, the bonnethead returns to the same estuaries,” Keller explained. “This demonstrates that sharks are aware of their ‘home’ and are capable of navigating back to it from a remote location.”
The question at the time was whether bonnetheads made their return journeys using a magnetic map. The researchers determined this by conducting magnetic displacement experiments on twenty juvenile wild-caught bonnetheads. They exposed sharks to magnetic conditions that simulated locations hundreds of kilometres away from where the sharks were actually caught in their studies. Such studies enable simple predictions about how the sharks should orient themselves in the future if they were indeed relying on magnetic cues.
When sharks derive geomagnetic information, the researchers predicted that they were moving to the north in the southern magnetic field and south in the northern magnetic field as they tried to compensate for their perceived displacement. They predicted that sharks would exhibit no orientation preference when exposed to a magnetic field corresponding to their capture site. And it turned out that when exposed to fields within their natural range, the sharks behaved as predicted.
According to the researchers, this ability to navigate magnetic fields may also contribute to shark population structure. The findings in bonnetheads also likely contribute to the explanation of other shark species’ impressive feats. For example, it has been documented that one great white shark migrates between South Africa and Australia, returning to the same location the following year.
“Isn’t it incredible that a shark can swim 20,000 kilometres in a three-dimensional ocean and return to the same location?” Keller enquired. “It truly is mind-boggling. In a world where almost everyone uses GPS to navigate, this capability is truly remarkable.”
Keller says he hopes to conduct future research on the effects of magnetic fields from anthropogenic sources such as submarine cables on sharks. They’d also like to investigate whether and how sharks use magnetic cues in their daily behaviour, not just during long-distance migration.
Bryan A. Keller, Nathan F. Putman, R. Dean Grubbs, David S. Portnoy, Timothy P. Murphy. Map-like use of Earth’s magnetic field in sharks. Current Biology, 2021; DOI: 10.1016/j.cub.2021.03.103
Main image credit: Loren Javier