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Magnetic Fields Show the Way
11 October 2001 7:00 pm
From butterflies to newts, many creatures roam the neighborhood--or globe--and still find their way home. Now, two studies in the 12 October issue of Science reveal how sea turtles and mole rats tap a basic navigational tool: Earth's magnetic field.
Earth's churning liquid core casts a magnetic field across the planet's surface. Birds, fish, crustaceans, and a host of other kinds of animals appear to use regional variations in the magnetic field, along with sensory cues such as sight and sound, to navigate. The finer details of these strategies, however, remain largely a mystery.
Earlier work by Kenneth Lohmann of the University of North Carolina (UNC), Chapel Hill, and his colleagues had found that eastern Florida's loggerhead sea turtles sense magnetic fields. Now the team decided to examine whether the turtles used regional fields during migration. The researchers placed 79 hatchlings in a lab tank and fitted each one with a tiny bathing suit that tracked their movements. With a grid of electric coils, they simulated magnetic fields found at three critical points in the turtles' migratory routes. Each time, the turtles swam preferentially in the direction that would keep them on course.
In the second study, a team led by neuroanatomist Pavel Nemec of Charles University in Prague focused on the Zambian mole rat. These animals dig underground tunnels at least 200 meters long and build a south-facing nest at the end, according to the magnetic field. Nemec's team had 16 mole rats build nests in one of three conditions: a field with shifting polarity, a weak field, and the natural geomagnetic field. Then the scientists assayed the animals' brains for the c-Fos transcription factor, a marker of active neurons. They found low levels of c-Fos among mole rats within the weak field and among control animals. But mole rats that built nests within changing magnetic fields showed strong activity in a brain region called the superior colliculus, which collects spatial cues and directs orienting behavior.
Both studies win applause. "We're on the edge of a coherent story, from detector cell to behavioral response," says biologist Michael Walker of the University of Auckland in New Zealand. "If there is a general magnetic sense for vertebrates, we should be able to see common mechanisms." With such diverse species as turtles and mole rats offering insights, he adds, the nature of navigation may finally be within reach.