Monarch Magnetism

Each fall, monarch butterflies migrate thousands of kilometers and--mysteriously--manage to find the same wintering grounds their ancestors left the previous spring. One piece of navigation equipment crucial for other migratory species on cloudy days is a magnetic compass; now researchers have shown that monarchs too orient themselves to magnetic fields as they embark upon their long journey.

Monarch butterflies winter in the Transvolcanic Mountains of central Mexico, clustering in a dozen or so sites spread out over 50 kilometers. In the spring, butterflies head north and get about as far as Texas, where they stop, reproduce, and die. Their offspring fan out across the eastern United States and southern Canada. There, another generation or two lives and dies before the monarchs pick up and return to Mexico in early fall.

Not much is known about how monarchs in Mexico find their way north, but research on the southbound migration has shown that the butterflies can orient by the position of the sun. On cloudy days, however, they must navigate with other cues. Migratory fish and birds rely on Earth's magnetic field to get their bearings; when researchers zapped captive monarchs with brief magnetic pulses, the butterflies became disoriented--suggesting that they too possess some kind of magnetic compass.

Probing this further, entomologist Orley Taylor of the University of Kansas, Lawrence, and colleagues captured monarchs in Kansas as the butterflies were heading south. They allowed the monarchs to climb through a tube into a round, 1-meter-diameter chamber with opaque walls in which the magnetic field could be manipulated. The 40 butterflies that flitted through Earth's standard magnetic field tended to head southwest. Another 40 deprived of a magnetic field headed every which way. When the researchers imposed a magnetic field that was the reverse of Earth's field, a third group of butterflies headed northeast, they report in tomorrow's Proceedings of the National Academy of Sciences.

"I'm convinced by the Kansas research that monarchs use magnetic fields to know which way is south--it makes sense," says entomologist Karen Oberhauser of the University of Minnesota, Minneapolis. She notes, however, that butterflies elsewhere in the United States would have to use different compass orientations to find their way back to the wintering grounds. And how they home in on tiny wintering grounds is still a mystery, she says.