Every couple years or so, female sea turtles migrate thousands of kilometers across entire ocean basins to nest on the same stretch of coastline that their mothers had picked out before them. After all, successful nesting requires a hard-to-find combination of just the right features: soft sand, appropriate temperatures, few predators, and easy access to the beach. But how they find their way back to where they were born has been a mystery for decades. According to a study published in Current Biology this week, adult turtles do this successfully by seeking out geomagnetic signatures along the coast. Their natal homing devices work like an internal GPS.
Previous studies have shown that sea turtles use Earth’s magnetic field to guide them at sea, and until now, researchers weren’t sure if these magnetic features help adult turtles recognize the beaches where they hatched—the idea is difficult to test in the open ocean. So J. Roger Brothers and Kenneth Lohmann University of North Carolina, Chapel Hill, examined changes in nesting behavior over time.
"We reasoned that if turtles use the magnetic field to find their natal beaches, then naturally occurring changes in the Earth's field might influence where turtles nest," Brothers says in a news release.
After analyzing 19 years’ worth of data on loggerhead sea turtle (Caretta caretta) nesting along the eastern coast of Florida, the duo found a strong association between the location of nests and the subtle shifts in Earth’s magnetic field. "Our results provide evidence that turtles imprint on the unique magnetic field of their natal beach as hatchlings and then use this information to return as adults,” Brothers explains.
When Earth's field shifted, the magnetic signatures of adjacent beachfront sites converged closer together in some places. When that happened, turtles packed themselves closer together along a shorter stretch of coastline: The researchers found an average increase of 35 percent in nesting density, Los Angeles Times reports. On the other hand, when the magnetic signatures diverged, the turtles spread out, laying eggs in nests that were fewer and farther between: Nesting density in these areas decreased by an average of 6 percent.
"The only way a female turtle can be sure that she is nesting in a place favorable for egg development is to nest on the same beach where she hatched," Brothers says. "The logic of sea turtles seems to be that 'if it worked for me, it should work for my offspring.'" The turtles likely have tiny magnetic particles in their brains that respond to the planet’s field, giving them a magnetic sense—though no one knows for sure yet.
Images: J. Roger Brothers