The moment they emerge from their underground nests, tiny loggerhead sea turtles face an enormous journey. They scramble to the sea and begin navigating 8,000 miles across the open ocean and back home again.
How do they find their way? New studies suggest that like many migrating birds and honeybees, the thumb-sized turtles are guided by very slight differences in the Earth's magnetic field.
Taking subtle cues from the field, which is most intense at the poles, these turtles keep themselves within a circular swathe of warm ocean current known as the Atlantic gyre, which stretches from the eastern U.S. to the coasts of Spain and Africa. Swimming outside it, the turtles would face colder waters and certain death. After swimming in the gyre for years the turtles make their way home to Florida beaches.
It's unlikely the turtles find their way using visual cues since the open ocean offers few landmarks. Temperature is also an unlikely indicator since water temperatures vary due to factors like the Gulf stream even within the Atlantic gyre.
"People have wondered for years now how young turtles navigate during that first migration because it seems utterly impossible," said Kenneth Lohmann, associate professor of biology at the University of North Carolina in Chapel Hill. "We're finally on the verge of understanding how they do it."
Mini Ocean and Turtle Bathing Suits
To prove that the loggerhead turtles use magnetic fields for navigation, Lohmann and his team constructed a miniature ocean in a four-foot-wide fiberglass tank filled with seawater and surrounded it by a miniature magnetic field created by a carefully charged network of copper coils.
Then, one by one, they dressed 79 newborn turtles in tiny bathing suits and lowered them inside the tank.
Attached to the Velcro/Lycra bathing suits were harnesses rigged to a fishing line. The fishing line connected to a mechanical arm that measured every small turn the turtle in the tank made.
By manipulating the tank's magnetic field to mimic the magnetic angles and intensities that the turtles would experience at sea, Lohmann found the turtles immediately swam in a direction that would have kept them inside the warm current.
For example, when exposed to conditions like those found off the coast of northern Spain, the turtles turned in a southerly direction. When subjected to magnetism like that at the southern edge of the North Atlantic gyre between Africa and South America, the turtles headed northwest, a direction that would send them homeward to the U.S. coast.
After one test swim, each turtle was released into the ocean.
"The major finding is the turtles can distinguish between the magnetic fields they encounter in different locations along a migratory route and they use these fields as navigational markers to guide them," said Lohmann, who published his results in today's issue of Science.
Guiding Particles in the Brain?
Loggerhead turtles aren't the only animals that use the Earth's magnetic field for navigation. Scientists believe a magnetic sense guides animals ranging from honeybees and homing pigeons to trout and whales across vast distances. Studies have shown that during magnetic storms, homing pigeons take longer to reach their home destinations.
Robert Beason has studied the navigation secrets of migratory birds including the bobolink and quails. Rather than testing his birds in a tank, Beason has outfitted planetariums with magnetic fields to see how birds would respond.
"As you change the field, the bird immediately changes direction in flight," said Beason.
Beason has also looked into what mechanisms in birds might help them sense their location inside a magnetic field and has located a particular receptor nerve that may be responsible for cuing the birds' navigation system.
Lohmann says no parallel studies have been done in loggerhead turtles because they're an endangered species so "we can't cut open their brains to investigate." But one possibility, he says, is the turtles may host magnetic particles of an oxide of iron, called magnetite, in their heads. These particles could act like compass needles and cue specific navigating receptors in the turtles' brains. Another idea is that magnetic fields set off a complex set of chemical reactions in the turtles' eyes that affect how the turtles see and orient themselves.
One thing Lohmann is certain of is that these turtles inherit their keen magnetic sense of direction at birth since they start their long journeys soon after hatching. Furthermore, it seems that colonies of loggerhead turtles are born with magnetic maps that are specifically calibrated to their homeland. That means that colonies of loggerhead turtles originating from the coasts of Florida could not find their way if displaced to the coasts of Japan or Australia, where other colonies live.
Since populations of loggerheads are severely endangered in Japan, that could be valuable information for ecologists.
"If the Japanese loggerhead becomes extinct, we couldn't reestablish populations by bringing in other turtles from other regions," says Lohmann. "These turtles inherit their maps at birth and would be lost starting from any other place than home."