A case of magnetic attraction: sea turtles use magnetic coordinates to navigate to their nesting sites

What do salmon, sea turtles and songbirds have in common? They are all excellent navigators capable of finding their way home after a long journey. These animals use a process called natal homing to make their way back to their birthplace to reproduce. Every two or three years, female sea turtles return to nest on the same beach where they first emerged as hatchlings. New research has shown that these turtles use the Earth’s magnetic field to guide them back home.

On a map of the world, you can pinpoint the exact location of any place using its unique latitude and longitude coordinates. Replace those latitude and longitude lines with intensity and inclination and you’ll have a new map of the world based on the Earth’s magnetic field. Inclination, the angle at which the magnetic field hits the Earth’s surface, and intensity are two important descriptors of the Earth’s magnetic field. Together, they can be used as a magnetic coordinate system, giving each place a unique magnetic signature.

Since the 1990s, biologists have known that sea turtles use the Earth’s magnetic field as a navigation system. One hypothesis in the field is that baby turtles imprint on the unique magnetic coordinates of their “home beach” as a way of guiding them back when they, themselves, are ready to nest. For the first time, researchers at the University of North Carolina, Chapel Hill have found evidence to support this idea of geomagnetic imprinting in sea turtles.

Having imprinted on the unique magnetic signature of their birth beach, newly hatched baby sea turtles head to the ocean. (Source)
Having imprinted on the unique magnetic signature of their birth beach, newly hatched baby sea turtles head to the ocean. (Source)

In a study published in the journal Current Biology, Roger Brothers and Kenneth Lohmann used data from Florida’s Statewide Nesting Beach Survey to map out loggerhead sea turtle nesting sites along the Florida coastline. These data allowed them to see changes in nesting site locations over a 19-year period from 1993 to 2011. The researchers compared changes in the Earth’s magnetic field during that time to changes in the nesting locations and found a strong correlation between the two.

Unlike latitude and longitude lines, the magnetic map of the Earth is slowly, but constantly shifting with time. These shifts mean that at different points in time, the same magnetic coordinates could lead you to different physical locations.  If female sea turtles imprinted on the unique magnetic fingerprint of their home beach as hatchlings and used those coordinates to guide them to their nesting site as adults, changes in the magnetic map would lead to corresponding changes in their nesting locations.

When shifts in the magnetic field brought inclination lines closer together, magnetic coordinates became more clustered and nesting sites were found closer together . Similarly, shifts that drew inclination lines further apart increased the spread of magnetic coordinates and correlated with nesting sites found further apart. These results support the idea that sea turtles imprint on a specific set of magnetic coordinates to help guide them back to their nesting sites many years down the road. Two recent studies also demonstrated that salmon use geomagnetic imprinting to migrate back to their home rivers to spawn. In the future, it will be interesting to see if other animals that use natal homing also rely on geomagnetic imprinting to help them navigate.

The researchers are careful to point out that sea turtles do not  choose nesting sites based solely on magnetic cues. Once they arrive at their home beach, other factors like sand quality, predation, and sensory cues come into play. Ultimately, the turtles want to choose a site that will give their hatchlings the greatest chance of survival.

For sea turtles and other animals that rely on natal homing, the strategy of returning to their birthplace seems to be a successful one. After all, if it was good enough for mom and dad, it must be good enough for the kids, right?

Reference:
Brothers JR, & Lohmann KJ (2015). Evidence for Geomagnetic Imprinting and Magnetic Navigation in the Natal Homing of Sea Turtles. Current biology : CB PMID: 25601546

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