Every fall, monarch butterflies from southern Canada and the northern United States head south, some of them flying up to 3,000 miles. They end up in a handful of high mountain forests in central Mexico, clustered on fir trees by the millions.
The amazing part is that none of these butterflies has ever made the trip before. The short answer to how they do it is a built-in navigation kit. Monarchs use the sun as a compass, with an internal clock that corrects for the sun's movement across the sky, and they also have a magnetic compass. Scientists still haven't fully worked out how they all zero in on the same few spots.
A Compass That Knows What Time It Is
Monarchs travel during the day and steer by the position of the sun. But the sun moves from east to west as the hours pass, so a butterfly flying by the sun alone would drift off course. To stay pointed south, monarchs use their internal daily clock to adjust for the time of day. Scientists call this a time-compensated sun compass.
Researchers tested it in an elegant way. They raised monarchs on artificial light schedules and placed them in flight simulators, where tethered butterflies could fly in any direction. Normally the migrants chose south. But when the scientists shifted the butterflies' clocks by six hours, the monarchs picked the wrong direction, because they were reading the sun at what their bodies thought was a different time.
The Clock Is in Their Antennae
Here's the twist almost nobody expects. The brain combines the sun's position with patterns of polarized light in the sky, picked up by a special region of the eye. But the clock that keeps the compass on time sits mostly in the butterfly's antennae.
Monarchs with their antennae removed still flew in steady lines, but as a group they no longer agreed on a direction. Even antennae studied in a lab dish kept their own daily rhythm. One working antenna is enough to stay on course, yet two antennae with conflicting clocks send the butterfly the wrong way.
A Relay Race Across Generations
The monarchs that fly south in fall are special. They belong to a generation that lives about eight times longer than their parents and grandparents. Butterflies born in early summer usually live less than two months, but the fall migrants hold off on breeding, store extra fat, and wait out the winter.
In spring, they head north and lay eggs along the way. It takes two to three more generations to fill the breeding grounds in the United States and Canada again. That means the butterflies arriving in Mexico each November are several generations removed from the ones that left Mexico the spring before.
A Discovery That Came Late
People living near the Mexican forests knew about the butterflies for centuries. For the local Mazahua people, the monarchs' arrival is tied to the Day of the Dead celebrations. Scientists, however, didn't locate the winter sites until 1975.
Researcher Fred Urquhart, who started mapping the butterflies' route in 1937, had spent decades tracking monarchs with tags. He advertised in the Mexican press for help, and Catalina Trail and Kenneth Brugger answered. In January 1975, they led him to one of the major roosting sites, high on volcanic mountains where the butterflies cover oyamel fir trees.
So the next time a monarch drifts past your garden in September, give it a little credit. It's carrying a sun compass, a magnetic sense, and a clock in its antennae, and it's headed for a forest it has never seen. Scientists are still trying to figure out the rest.
