Around this time every year, monarch butterflies in Canada and the eastern United States begin their journey southward to spend the winter in Mexico, where they will arrive in November. Monarchs on the west side of Canada and the United States migrate to California. Monarchs are the only butterflies known to make a north-south migration.
A beautiful monarch butterfly. Photo by Sid Modsell, Creative Commons
Monarch migration is a remarkable feat for many reasons.
Every year, there are four generations of monarchs. Non-migrating adult monarchs only live for about two to six weeks. But the last generation of the year—the generation that migrates to Mexico, spends the winter, and starts heading north again—will have been alive for about eight months!
So, how do the monarchs alive now know they are supposed to migrate, and not other monarchs born earlier in the year? In the monarchs alive now, genes get expressed by a combination of triggers (not one main trigger): an internal clock in the monarch, Earth’s magnetic field, and the position of the Sun. As a result of the expressed genes, the migrating generation of monarchs needs less oxygen. Further, the monarchs stop mating until it’s time to head back north in the late winter/early spring. At that time, they start mating and laying eggs in milkweed plants on their way back. The new monarchs will continue to head north, and the cycle continues.
How can they fly around 5,000 kilometres? They’re so small—they weigh less than a gram. A comparison of monarchs migrating from different areas, as well as monarchs that don’t migrate at all, showed that over time, the butterflies have adapted to typical conditions: the monarchs that do the long-distance migration from Canada to Mexico have larger bodies than the non-migrating monarchs that live in Puerto Rico, Hawaii, and other tropical regions. The migrating monarchs also have larger bodies and more angular forewings.
And the monarchs that live in western North America have bodies and wings more suited to gliding.
Another interesting feature of migrating monarchs is their path: the monarchs heading to Mexico follow a few defined, separate paths, but the paths all unite near southern/central Texas. And when the monarchs head north again, they follow the same route: one path until southern/central Texas, then they diverge and follow the paths that took them south in the fall. The generations of monarchs heading north didn’t migrate south, yet they know which path to take.
When the monarchs arrive in Mexico, they return to the same dozen or so spots in oyamel forests on the hillsides of mountains that are about 2 to 4 kilometres above sea level, in Michoacan. The coolish temperature there (0 to 15 degrees C) and the humidity allow them to survive without losing too much energy. They need to conserve energy because they don’t feed a lot while in Mexico; they live on their stored energy. So migrating to a spot that allows them to conserve energy is ideal, because they need the energy to begin their northward migration.
You can take part in monarch migration by visiting Journey North, at https://journeynorth.org/. Help track their migration by reporting your monarch sightings.