For centuries, determining longitude was an extremely difficult task for sailors, so difficult that it's been thought improbable – if not impossible – for animals to do it.

But migratory sea turtles have now proved capable of sensing longitude, using almost imperceptible gradients in Earth's magnetic field.

"We have known for about six years now that the magnetic map of turtles, at a minimum, allows turtles to … detect latitude magnetically," said biologist Ken Lohmann of the University of North Carolina, who describes the turtle's power Feb. 24 in Current Biology. "Up until now, that was where the story ended."

Lohmann specializes in animal navigation, and work from his laboratory and others have exhaustively demonstrated how sea turtles – along with many birds, fish and crustaceans – use gradients in Earth's magnetic field to steer.

__Magnetic Reception Found in Pigeon Ears__It's not just sea turtles showing off geomagnetic tricks. Birds, known to use geomagnetic location through magnetically sensitive particles in their eyes and beak, also appear to sense magnetism with their ears.

In another Current Biology study published Feb. 24, Washington University neurobiologists Le-Qing Wu and David Dickman follow up on earlier observations of magnetically sensitive compounds in birds' vestibular lagena, an inner-ear structure.

Wu and Dickman held 23 homing pigeons in total darkness for 72 hours within a rotating magnetic field. Aftewards they killed the birds and searched their brains for activation in regions linked to orientation, spatial memory and navigation.

The researchers then repeated the study with five birds whose lagenae were surgically disabled. The brain navigation patterns were altered, suggesting a navigational role for the lagena.

According to Wu and Dickman, cell receptors in the lagena, which are known to respond to head tilt in relation to gravity, likely interact with those magnetically sensitive particles. The results may encode a "geomagnetic vector" that links motion, direction and gravity.

Fish, amphibians and reptiles also possess the same ear structure, raising the possibility of the mechanism being widespread in the animal kingdom.

Those differences, however, are far greater by latitude than by longitude. Travel north or south from Earth's magnetic poles, and their pull weakens noticeably. Travel straight east or west, and the pull doesn't change. Instead the pull's angle changes, and only to an infinitesimally slight degree.

That turtles and other migratory animals could detect such a small change was considered unrealistic, but experiments on animals released in out-of-the-way locations repeatedly described them finding home with unerring accuracy and efficiency, explicable only as a product of both longitudinal and latitudinal awareness.

Several nonmagnetic explanations were proposed, foremost among them a "dual clock" mechanism analogous to human methods of calculating longitude, which sailors perform by comparing precise differences between the time locally and at an arbitrary longitudinal line, such as the Greenwich Meridian. No such mechanism has been found, however, and longitudinal differences in local airborne or waterborne chemicals don't seem to explain animals' uncanny long-distance steering.

"A skeptic could reasonably believe that the latitudinal cue is magnetic, but that determining east-west position depends on magic," wrote James L. Gould, a Princeton University evolutionary biologist, in a 2008 Current Biology commentary on animal navigation.

In the new study, researchers led by Lohmann and graduate student Nathan Putnam, also a UNC biologist, placed hatchling loggerhead sea turtles from Florida inside pools of water surrounded by computer-controlled magnetic coil systems.

By varying the currents, Lohmann and Putnam could precisely reproduce the geomagnetic characteristics of two points at identical latitude, but on opposite sides of the Atlantic. Into each pool they placed the hatchlings, which in the wild would instinctively follow a migratory path from their home beach and into the currents that circle the Sargasso Sea and loop around the Atlantic.

In the first pool, programmed to the geomagnetic field in the western Atlantic near Puerto Rico, the turtles swam northeast, on the same trajectory as loggerheads in the wild at that locale. In the other pool, set to the geomagnetics of the eastern Atlantic near the Cape Verde islands, the turtles swam northwest.

No other cues could explain their directions. Against reasonable expectation, the turtles clearly sensed differences in geomagnetic angle.

Gould, who was not involved in the study, wrote an accompanying commentary. Whereas his earlier article was titled "Animal Navigation: The Longitude Problem," this was called "Animal Navigation: Longitude at Last." The findings are "the final piece of the puzzle," he wrote.

Lohmann now plans to study whether currents affect the turtles' longitudinal compass, and whether the turtles detect differences over short distances. He also suspects that other animals may have a similar longitudinal compass.

"The mechanism we've found in turtles might also exist in birds," he said.

Image: Upendra Kanda/Flickr.

See Also:

Citations: "Longitude Perception and Bicoordinate Magnetic Maps in Sea Turtles." By Nathan F. Putman, Courtney S. Endres, Catherine M.F. Lohmann, and Kenneth J. Lohmann. Current Biology, Vol. 21 Issue 4, Feb. 24, 2011.

"Animal Navigation: Longitude at last." By James L. Gould. Current Biology, Vol. 21 Issue 4, Feb. 24, 2011.

"Magnetoreception in an Avian Brain in Part Mediated by Inner Ear Lagena." By Le-Qing Wu and J. David Dickman. Current Biology, Vol. 21 Issue 4, Feb. 24, 2011.