Each year, as winter frosts and frigid air begin to envelop the northeastern U.S. and eastern Canada, monarch butterflies summering in these regions set out on a 3,500-kilometer journey to the mountains of central Mexico. Precisely how the insects find their way to this refuge has long eluded biologists. But new findings may help unravel the mystery. According to a report published online this week in the Proceedings of the National Academy of Sciences, the creatures appear to navigate using the sun.

Previous work hinted that the sun and the earths magnetic field might guide the monarchs migration. But few of those studies examined the butterflys ability to navigate in a controlled setting, relying instead on observations of the animals in the wild. In the new work, Henrik Mouritsen and Barrie Frost of Queen's University in Kingston, Ontario, Canada, started by creating a butterfly flight simulator. They tethered the insects in clear Plexiglas cylinders, blowing light air from the tube's bottom to stimulate flight. Then, to assess whether the monarch triangulates the sun's position using its internal clock to determine which direction is southwest--the way to Mexico-- the researchers reset some of the butterflies circadian rhythms by 6 hours in either direction of the actual time. Butterflies whose rhythms had not been changed consistently flew in a southwesterly direction when put outside on sunny days, even though the researchers often forced them to change course throughout the experiment. Insects with altered cycles, however, headed either southeast or northwest, depending on their perceived time and where the sun would be at that hourthat is, because their clocks were shifted, these monarchs miscalculated the actual position of the sun and flew in the wrong direction. Additionally, when the researchers simulated overcast conditions in the lab, the monarchs flew about randomly. Even after introducing strong magnetic fields, the butterflies did not head in any particular direction.

"The clear and predicted directional shifts produced by clock-shifting the butterflies provide strong evidence that migratory monarchs use a time-compensated sun compass," the authors conclude. "These experiments do not provide any evidence that monarch butterflies use a magnetic compass during migration."