When we think of migrating animals, we often think of ducks and other birds that fly to places with warmer weather. However, birds are not the only animals that migrate; monarch butterflies also have unique migrating behavior. Each fall, North American monarchs will migrate almost 3,000 miles from their summer breeding grounds. There is a generation of monarchs that lives east of the Rocky Mountains that will overwinter in Central Mexico, and some western monarchs have a shorter migration to the southern coast of California. How does their migration work, and why are they practicing this behavior?
There are different variables that contribute to the monarch migration, such as temperature changes and aging monarch food sources like milkweed and nectar. This migrating generation of monarchs lives much longer than the summer generations of monarchs. Summer generations of monarchs typically live 2-6 weeks, whereas this migrating generation of monarchs can live up to 9 months. These migrating generations typically emerge in mid-August and go immediately into reproductive diapause, meaning they will not reproduce for some time. Instead, they go in search of a place to overwinter. This is where they begin their journey across the Eastern U.S. and funnel toward Mexico. Along the way, they will find stopover sites that will provide them with abundant nectar sources and sheltering sites. They will reach Mexico around November, and it is there that they will overwinter. Once there they will congregate on oyamel fir trees on south and south-west facing slopes. These trees provide them with all the resources they will need: cool temperatures, water and adequate shelter to protect them from predators. They will utilize these resources to maintain enough energy to carry them through the winter. It is not until March that they will begin their journey “home,” flying north through Texas. They will find resting and nectaring sites to breed and reproduce along the way just as they did on the flight south to Mexico. Once the butterflies are four or five generations removed from these migrators, the migration cycle starts over again.
As mentioned above, there is another group of monarchs that migrates to the Pacific coast of California. These monarchs reside on the western side of the Rocky Mountains. They choose to overwinter on eucalyptus, Monterey cypress, Monterey pine, and other tree species along the California Pacific coastline. Less is known about these generations of monarchs; however, the Pacific coastline provides the same suitable conditions for these monarchs to overwinter. Unlike the Mexico-migrating-monarchs (say that three times fast!), they will start to disperse and head back north sooner, closer to mid-February.
How do they do it? How do monarchs know to travel to the same location every year? We know for a fact that they do not learn from their parents since they are about the fifth generation from the original migrating monarchs. They must rely on their instincts just as other migrating insects do. Animals that migrate will use cues from the sun, moon, and stars, as well as the earth’s magnetic field and landmarks to give them the needed cues to migrate. However, it is the sun’s compass and the earth’s magnetic field that prove to be the most important to monarchs when it comes to migrating. The sun’s compass is the proposed mechanism that helps tell monarchs when to migrate. Monarchs rely on the angle of the sun and the horizons in combination with their own internal body clock to maintain a southwesterly flight path.
Another method that scientists believe monarchs use to help them migrate is their magnetic compass. This is thought to be used as their backup source of guidance on days when there is no sunlight. It works by using the angle that is naturally made by the earth’s magnetic field and the surface of the earth. It will allow them to orient themselves south towards their overwintering sites by using their internal magnetic compass. Finally, there is one last factor that scientists believe contributes to a monarch’s ability to migrate: genetics. This generation or population of monarchs have genes that allow them to exhibit traits, such as low metabolic rates, that are favorable to migration. This is important because it allows the monarchs to lower the energy expenditure in their muscles, thereby enabling them to fly further and use their energy more efficiently.
As an entomologist, I may be biased, but insect behavior is fascinating. Even though monarch butterflies are small, they are able to conquer large feats such as migrating across the United States – an impressive accomplishment for a small butterfly!
Kristen Stevens, BCE