When monarch butterflies wing their way south to central Mexico each fall, they use the sun to ensure that they stay on course. But how they head in the right direction on cloudy days has been a mystery—until now.

It turns out they use Earth's magnetic field as a kind of backup navigational system.

It's not unusual for animals engaged in long-distance migrations, including sea turtles and birds, to use an internal magnetic compass to get to where they're going. But whether monarch butterflies have a similar ability had previously been unclear: Some studies had found weak evidence for a magnetic compass, while others found none at all. (Read about other great animal migrations.)

A paper published today in the journal