Clockwork in Orange and Black

Monarch butterflies navigate using an internal compass and a mammal-like clock.

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Monarch butterflies in flight.

Monarch butterflies have fascinated me since I visited them in their wintertime gathering place in Mexico five years ago. Each fall, millions of monarchs migrate from across the United States to a small cluster of overwintering sites in Mexico. In the spring, their descendants return to northern climes. A full round trip takes three generations, which means that no migrating butterfly knows from experience where it's going. I find that pretty impressive.

The southbound monarchs, moreover, have a small target: Most of the species's overwintering sites lie within just 70 square miles of forest that cover a few adjoining hillsides in central Mexico. (I've previously written about my visit to one of these sites, Sierra Chincua.) Compare the butterfly's navigational skill to that of Christopher Columbus, who set out for India and accidentally navigated to an island off the coast of the wrong continent.

To pull off their amazing feat of navigation, monarchs use an internal solar compass and a circadian clock that helps the compass adjust for the continuous movement of the sun. The circadian clock enables a butterfly to tweak its flight path to keep a constant bearing—even as its landmark, the sun, arcs overhead. Had early European navigators had such a "time-compensated sun compass," they might not have needed a special chronometer to calculate longitude at sea.

Now, researchers have learned a new secret about the monarch's magic. In a scientific paper that has just appeared in the free online journal PLoS Biology (and an accompanying paper in PLoS One), neurobiologist Steven Reppert of the University of Massachusetts Medical School in Worchester and his colleagues unveil a surprising discovery about the workings of the monarch's internal clock. It turns out that the butterfly has two so-called cytochrome proteins, not just one, as many other animals have.

Both butterfly proteins are sensitive to light and appear to play a timekeeping role in the monarch's brain. Surprisingly, the newfound monarch cytochrome is more similar to a mammal's version of the protein than to another insect's version. And in addition to its timekeeping job, the new protein appears to link the monarch's circadian clock to its sun compass. It has a "dual function," Reppert says, in a statement released by the Public Library of Science (PLoS).