Local Magnetic Fields Confuse Sea Turtles Heading Home

Loggerhead sea turtles perform one of nature’s most amazing navigational feats. After years swimming the oceans, they return to their birthplaces to breed, following magnetic lines thousands of miles home.

But sometimes they land on beaches that are radically different—except for the local magnetic fields. The result is some very unusual genetic patterns.

Most of the time, animals living in the same area will be the most genetically similar. In other cases, animals in living in separate but similar habitats will evolve some traits in common. But the best predictor of genetic similarities among loggerhead turtles is not their location or the nature of their habitat, but the magnetic field of the beach where they were born.

The best predictor of genetic similarities among loggerhead turtles is the magnetic field of the beach where they were born.

Scientists believe that sea turtles internalize an imprint of the magnetic field of their nesting area shortly before or after they hatch. Years later, this serves as the compass that allows females to find their way home and lay their eggs on the very beach where they were born, even if it means traveling incredible distances—sometimes thousands of miles—through the open ocean.

A team from the University of North Carolina at Chapel Hill tracked 834 nesting female loggerhead turtles across the United States’ southeastern coast, in a study led by graduate student J. Roger Brothers. Since turtles ordinarily return to their birthplaces to lay their eggs, it’s no surprise that their offspring will be genetically similar to others in that area.

The surprising part is that some turtles get confused and, rather than return to their birthplaces, they go to beaches with similar magnetic fields, even if they are farther away, or, for example, on opposite sides of the Florida peninsula. This seems to explain the unusual pattern of genetic similarities among turtles.

Here’s Merrit Kennedy, reporting for NPR.

That’s probably why Brothers’ research found that turtle populations are most likely to be genetically similar when they come from beaches with magnetic similarities.

“It’s through these navigational errors that we might see a population structure where, regardless of geographic distance or environmental similarities between the two nesting beaches,” he says, “the difference in the Earth’s magnetic field is a really strong predictor of how genetically similar or different the two nesting populations are.”

Knowing how these turtles choose where to nest can help conserve this threatened species, because man-made structures like seawalls, power lines, and large buildings—any construction laced with metal—can affect the local magnetic field. These small variations in the magnetic field can have an impact on how attractive certain beaches are for nesting turtles—and likely other migrating animals too.