Loggerhead turtles can do from birth what humans struggled to master for centuries – tell longitudinal, or east-west, direction to navigate thousands of miles of ocean with no visual landmarks. They do so using magnetic cues, according to a study by researchers from the University of North Carolina – Chapel Hill published in the journal Current Biology.
The turtles set off on a grand migration around the Atlantic immediately after they hatch and enter the sea. They swim from Florida to the circular currents swirling around the Sargasso Sea, called the North Atlantic subtropical gyre. They stay in the gyre and slowly migrate around the Atlantic, before returning home.
Such travels need the ability to tell both east-west and north-south direction. It is well known that many animals can tell latitudes, or north-south direction, using cues from the Earth’s magnetic field. But a few, such as the loggerhead turtle, also seem to be capable of deciphering east-west direction using a magnetic map. Before this paper, no one has actually shown this is true.
Telling east-west direction is quite a feat because while the Earth’s magnetic field varies predictably with latitude, less information is available about the field with longitude.
To see whether the turtles somehow use magnetic information to decipher east-west direction, researchers recreated ocean-like conditions. They placed hatchlings inside a circular, water filled arena. The space was surrounded by coils to generate a magnetic field. The turtles were tethered to a tracking device that monitored their swimming direction.
When the turtles were exposed to a field like that which exists on the southwest side of the Atlantic, near Puerto Rico, the animals swam northeast, which would take them back toward North America and their normal migration route.
When they were exposed to fields as exist on the northeast side of the Atlantic, near the Cape Verde Islands, they swam southwest. This would again take them to the North American coast.
Since the hatchlings had never been in the ocean, the ability to recognize east-west direction seems to be inherent. “Turtles exploit at least two different geomagnetic features that vary in different directions across the Atlantic,” states the report.
How they do so remains a mystery. The scientists postulate that they are able to tell longitude by keeping track of the inclination angle at which the magnetic fields intersect the earth surface, and intensity of the field. These change with longitude and latitude and form a “magnetic signature.”
“Viewed in this way, turtles might have a bi-coordinate magnetic map based on inclination and intensity, one that does not encode latitude and longitude per se but that nonetheless provides turtles with both east-west and north-south positional information along the migratory pathway,” they say.
Image: photo by coda via Flickr under Creative Commons.