New research has found how loggerhead sea turtles migrate across the open ocean.
“One of the great mysteries of animal behavior is how migratory animals can navigate in the open ocean, where there are no visual landmarks,” said Kenneth Lohmann of the University of North Carolina at Chapel Hill. The research was published online on February 24 in Current Biology.
“The most difficult part of open-sea navigation is determining longitude or east-west position. It took human navigators centuries to figure out how to determine longitude on their long-distance voyages,” said Nathan Putman, a graduate student in Lohmann’s lab and lead author of the study. “This study shows, for the first time, how an animal does this.”
They found that the turtles use magnetic signatures that vary across Earth’s surface in order to determine their position and steer themselves in the right direction. Several species, including sea turtles, are known to rely on magnetic cues to determine latitude, but it was unpromising as a method to determine east-west position, which these loggerheads can do. They rely on a combination of two features of the magnetic field: the angle at which the magnetic field lines intersect Earth (inclination) and the strength of the field.
Putman and Lohmann explained that near the equator, the field lines are roughly parallel to Earth’s surface. As one travels north, the field lines grow progressively steeper until they reach the poles, where they are directed straight down into Earth. The magnetic field also varies in intensity. It is usually strongest near the poles and weakest near the equator.
“Although it is true that an animal capable of detecting only inclination or only intensity would have a hard time determining longitude, loggerhead sea turtles detect both magnetic parameters,” Putman said. “This means that they can extract more information from the Earth’s field than is initially apparent.”
What had previously been overlooked is that the inclination and intensity varies across Earth’s surface so certain ocean regions have distinct magnetic signatures.
The discovery was made by subjecting the hatchlings to magnetic fields mimicking two locations along the migratory route but at opposite ends of the Atlantic Ocean. Each location had the same latitude but different longitude. The turtles were placed in a circular water-filled arena surrounded by a computerized coil system used to control the magnetic field and tethered to an electronic tracking unit that relayed their swimming direction. Turtles exposed to a field like one existing on the west side of the Atlantic near Puerto Rico swam to the northeast. Those exposed to a field like that on the east side of the Atlantic near the Cape Verde Islands swam to the southwest.
“This work not only solves a long-standing mystery of animal behavior but may also be useful in sea turtle conservation,” Lohmann said. “Understanding the sensory cues that turtles rely on to guide their migrations is an important part of safeguarding their environment.”
Copyright © 2011 by Marine Science Today, a publication of OceanLines LLC