Monarch butterflies use magnetic compasses.

Built-in magnetic compasses may orient monarch butterflies during their mind-boggling, southwestern migration, according to new experiments in Kansas.

The butterflies flutter up to 2,500 miles in autumn from breeding grounds in the eastern United States and Canada to a winter haven in Mexico. These millions of migrants are going to a destination not one of them has ever seen.

The generation taking to the air in the fall represents the great-great grandchildren, or even more distant descendants, of the monarchs that left Mexico the previous spring. Yet the new generation returns to the same dozen or so roosting areas that their ancestors used.

Previous studies showed that monarchs can orient by the sun. The new work provides the first direct evidence that monarchs can also sense directions from the magnetic field, according to Orley R. Taylor of the University of Kansas in Lawrence. He, Jason A. Etheredge of Kansas, and their colleagues describe the results in the Nov. 23 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

"This is not the last word," Taylor says. "What the butterflies are doing is very complicated."

The researchers caught migrating monarchs in fall and released them into a tube, in which they crawled upward to a circular tabletop arena. In a room with no shielding from Earth's magnetic field, most of the butterflies took flight toward the southwest. In a shielded room, however, they flew in random directions.

Next, researchers created a magnetic field oriented in the opposite direction from Earth's. Butterflies then flew northeast, as if their compasses had reversed.

The team studied fall migrants because the same generation flying north in spring gets hard to handle, Taylor says. "They're completing a reproductive death run, like salmon," as he puts it. Females lay eggs daily even though their route covers 1,000 miles. By the end, "they have practically no wings left; they're crawling from plant to plant to lay eggs--it's awesome," Taylor says.

Charles Walcott of Cornell University calls the monarch paper "terrific" and notes that other researchers have been stumped by the challenge of devising a way to test butterflies for magnetic orientation.

For animal compasses, "we used to think there was a single secret," he says, sounding wistful. Studies now show that animals combine methods using "a Chinese-menu approach," Walcott explains. Night-migrating birds, for example, can orient by the sunset glow and star patterns as well as by the magnetic field.

Kenneth P. Able of the State University of New York at Albany says he suspects that magnetic orientation "is a very widespread ability." Honeybees, some wasps, some fish, sea turtles, and even a species of mole rat can take bearings magnetically. Also, "it looks like every migratory bird you test, if you do it right, has a magnetic compass," Able says. However, he points out, discovering a compass takes the scientists just a small step toward explaining how monarchs navigate.

"I'm not sure anybody is ever going to answer that," says Karen S. Oberhauser, who studies monarchs at the University of Minnesota. "Insects sense the world in very different ways from humans. They use things we can't perceive, maybe even things we can't conceive."