The Monarch Migration

Each autumn, millions of monarch butterflies in eastern North America fly south to spend the winter in the mountain forests of central Mexico. Some travel more than 4,000 kilometers. For many years, scientists did not know where the butterflies went, and the winter sites in Mexico were not identified by researchers until 1975.

What makes this journey especially remarkable is that no butterfly makes it twice. [A] Monarchs born in early summer live only a few weeks. [B] The butterflies that hatch in late summer, however, are different: they do not reproduce right away, and they can live for up to eight months. [C] These are the butterflies that fly to Mexico, even though they have never been there before. In spring, they begin the return trip, but it is their children and grandchildren that complete it. [D]

How do they find their way? Research suggests that monarchs use the sun as a compass. Because the sun moves across the sky during the day, the butterflies must also keep track of the time. Surprisingly, experiments have shown that part of this internal clock is located in their antennae. Monarchs whose antennae were covered with black paint flew in the wrong direction, while those whose antennae were covered with clear paint did not.