Good Beach for a Turtle
April 29, 2015
There are beaches, and then there are beaches.
Among surfers, the latter has big waves. For kids, beaches might have an abundance of shells to collect and hermit crabs to catch. Sunbathers’ beaches may just need soft sand, a light breeze and close proximity to the parking lot.
No matter what type of beachgoer, some beaches are just better than others. The same is true, it turns out, for sea turtles, who lay their eggs on the beach. They like soft sand that’s easy to dig in, few predators and easy access to the water.
The trouble for turtles, though, is they do not have TripAdvisor to tell them which strips of shoreline have what they are looking for. But a study from UNC Chapel Hill shows that sea turtles can tell beaches from beaches by their magnetism.
Sea turtles, much like salmon, return to the places they were born to lay their eggs. Scientists have known about this behavior for decades, but how these animals that travel thousands of miles in the open ocean find their way home has been a mystery.
UNC biologist Kenneth Lohmann proposed these animals are able to navigate based on the Earth’s magnetic field, like we do with a compass. But according to Lohmann’s hypothesis the animal compass is far more sophisticated. While our compasses always point North, these animals are able to remember the unique magnetic signatures of the places they were born, according to Lohmann. Then they can just follow that signature home.
As anyone who has ever used a compass knows, Earth generates its own magnetic field. As the Earth spins, it swirls the liquid iron in its core, building electrical current and a magnetic field that spans the globe and reaches into space. The picture on the right shows the magnetic “field lines” circling the swirling core.
As you can see in the picture, there are places where the field lines move in different directions, places where they are packed in close or spread further apart. Those differences represent unique changes in the strength and orientation of Earth’s magnetic field — and on the Earth’s surface, the field varies wherever you go.
Lohmann’s theory is while humans cannot pick up on the difference, turtles and salmon have special organs in their brain that notice extremely specific aspects of the field, and the specific strength of field they were born imprints on the animals. So in theory, all they would need to do is find the coast and swim along until they find a place where the field is the right strength.
So the question became how to test this hypothesis. As it would be very difficult to track the turtles year round to see exactly how they move with relation to magnetic field, Lohmann and graduate student J. Roger Brothers decided to analyze specifically where the turtles nested. Scientists have kept track of loggerhead turtle nests in Florida for the past 19 years, and as this area boasts the most turtle nests in North America, Lohmann and Brothers had plenty of data to work with.
As expected, the turtles returned to their native beaches within a few miles, changing places slightly every year. More compelling evidence, however, came from shifts in Earth’s magnetic field.
Earth’s magnetic field is created by swirling iron in the core, but if the iron swirls faster or makes a bigger or smaller swirl, that can change how the field lines look on the surface. As a result, sometimes a greater range of field strengths get packed into a smaller space and sometimes they widen out.
When Lohmann and Brothers compared sea turtle nests to changes in magnetic fields from year to year, the data correlated strongly. When a range of field strengths was packed into a smaller space, the turtles nested closer together. When those strengths spread out, so did the turtles.
So why do sea turtles bother with all this? The answer to that question brings us back to beaches and beaches. Sea turtles know they need the soft sand, low population of predators and an easy run to the ocean for the babies — and according to Brothers, when it comes to making nests, they are not as adventurous as Finding Nemo would make them seem. Brothers says the turtles most likely figure, “If that beach worked for me, it will work for my kids too.”
Brothers and Lohmann will continue researching the concept of magnetic imprinting in marine animals. Future research may include different species or trying to find and analyze the magnetic sense organs that Lohmann hypothesizes are in these animals. The findings for this research appeared in the journal Current Biology.
— Daniel Lane
Daniel Lane covers science, engineering, medicine and the environment in North Carolina.