Each fall, millions of monarch butterflies migrate thousands of kilometers south to the same wintering grounds their ancestors left the previous spring. How do they figure out where they're going when the sun's not out? Researchers have found that they, like migratory fish and birds, have a built-in "compass" that enables them to orient to Earth's magnetic fields.
In the winter, monarch butterflies hang out at a dozen sites in the mountains of central Mexico. Come spring, they head up to Texas, where they reproduce and die, leaving their offspring to fan out to points north. A couple of generations later, as winter looms, the butterflies head for Mexico.
Researchers suspected that the critters relied on magnetism for navigation, because a monarch zapped with a brief magnetic pulse will become disoriented. So entomologist Orley Taylor of the University of Kansas, Lawrence, and colleagues captured some southbound monarchs and put them into a 1-meter-diameter column in which the magnetic field could be manipulated. The 40 butterflies that flitted through Earth's standard magnetic field tended to head southwest. When the field was reversed, another group went northeast. And when the field was eliminated, a third contingent flapped around in all directions, the researchers report in the 23 November 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," says entomologist Karen Oberhauser of the University of Minnesota, Minneapolis. But how they home in repeatedly on the same tiny wintering grounds is still a mystery.
