Turtles' map holds if Earth's magnetic field drifts

EARTH'S magnetic field is an invaluable navigational tool for humans and other animals, but the way it gradually changes with time is liable to confuse migrants. Evolution, it seems, is one step ahead of the problem: turtle hatchlings don't all respond to a given magnetic field in the same way, suggesting some will arrive at the correct destination even if the field shifts.

The motion of Earth's fluid outer core, which contains iron and nickel, helps to generate a planet-wide magnetic field. Some animals can detect this field, which reveals the direction of magnetic north, and use it to navigate (see "Whales and terns turn together").

One such migrant is the loggerhead turtle (Caretta caretta). Hatchlings emerge from their nests on the Florida coast and head for the open ocean. Here they spend six to 12 years in a circular current called the North Atlantic Subtropical Gyre before returning to the North American coast to breed.

To find out how the turtles remain within the gyre, Ken Lohmann of the University of North Carolina at Chapel Hill and colleagues made hatchlings swim in the presence of magnetic fields that simulated locations off Portugal, by which the gyre passes. They were in for a surprise. Near the simulated north coast of Portugal, most turtles swam south, which would keep them within the gyre. But near the southern coast, most turtles swam south-west, even though swimming south would have kept them in the gyre.

Lohmann thinks that the turtles are moving south-west to escape the predator-rich shallow waters of northern Africa, through which the gyre passes. This indicates that the turtles' ability to navigate using Earth's magnetic field is very finely tuned, he says.

But such precision could be a hindrance when the magnetic field varies, as it does over timescales of a year or more. It can also reverse in polarity over hundreds of thousands of years.

Lohmann's experiments suggest a solution to the problem. His team noticed that for any simulated location within the gyre, a few hatchlings consistently swam in directions that would take them out of the gyre and into dangerous waters.

These wayward hatchlings may be evolution's insurance policy against changes to the magnetic field, Lohmann says. After such a change to the field, some of these hatchlings will inadvertently read the magnetic field in the correct way to remain in the gyre. He presented his findings at a meeting of the American Geophysical Union in San Francisco last month.

Joseph Kirschvink, a geobiologist at the California Institute of Technology in Pasadena, agrees. "Turtles lay a lot of eggs during their lives," he says. "Having a large genetic control over the swimming direction versus magnetic cues may be an evolutionarily robust way of preserving the populations over a long stretch of geological time."

Whales and Terns Turn Together

Buller's albatrosses, Arctic terns, humpback whales and leatherback turtles may all read Earth's magnetic field in the same way.

Travis Horton at the University of Canterbury, New Zealand, plotted the migratory tracks from each species over a detailed magnetic map of Earth, which shows how the magnetic field's inclination and declination - the angular difference between the field lines and true north - vary from point to point.

The magnetic field lines originate in the Earth's core and emerge near the poles, where their angle of dip - or inclination - relative to Earth's surface is almost 90 degrees. Approaching the equator, inclination is roughly 0 degrees. Meanwhile, fluctuations in local geology alter both the inclination and the declination.

Horton found that important locations on each species' migration - where they stopped or changed direction, for instance - were characterised by ratios of declination to inclination that were "dyadic" fractions of the value at the start of the migration route. Dyadic fractions include one-half, one-quarter and one-eighth but not one-third or one-fifth.

Horton told last month's meeting of the American Geophysical Union in San Francisco that this suggests the animals are somehow sensing this ratio, perhaps relying on the symmetry of the bio-magnetite crystals they are thought to use to detect the magnetic field. "You rotate the crystal by half, quarter, one-eighth - it still looks the same."

Joseph Kirschvink, an expert on bio-magnetite at the California Institute of Technology in Pasadena, is not convinced. He points out that bio-magnetite comes in forms that are not so symmetric - pigeons, for instance, have elongated crystals.

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