Monarch butterflies have a keen sense of direction, even on cloudy days. This is because they have a magnetic compass to direct their migration in addition to navigating by the position of the sun, researchers report today in a Nature Communications study.
Scientists have long known monarch butterflies to navigate by a sun compass, meaning they use the position of the sun to determine which way is south. In the fall, millions of monarchs from all along the northeastern corridor of the U.S. to Canada migrate to the Michoacán mountain range in central Mexico, crowding together so densely that the air is filled with butterflies, said monarch butterfly expert and University of Minnesota professor, Karen Oberhauser.
Scientists initially believed monarch butterflies only used the sun to navigate to this mountain range, but found that the butterflies could still migrate on cloudy days. Steven Reppert, a neurobiologist and professor of at University of Massachusetts Medical School, led the research on monarch butterflies' migratory patterns to show that the butterflies use a magnetic compass to direct their flight toward the equator.
Reppert's lab found that the magnetic compass was ultraviolet light-sensitive and an essential orientation mechanism to aid migration when directional daylight cues are unavailable.
"It adds another dimension to this remarkable migration and navigation capabilities that these butterflies have," Reppert said.
Reppert used a flight simulator surrounded by a magnetic coil system to test the monarch's internal magnetic compass, which they found is located in the butterflies' antennas. They share this magnetic compass with vertebrates like birds and sea turtles.
Sea turtles have also been shown to have an internal map, Reppert said, and this study may indicate that monarch butterflies have one too. They could possibly have a map sense based on the magnetic field of the the Earth, Reppert said.
Reppert said that scientists still don't know how animals can sense the magnetic field, but they have an idea that there is a molecule that senses the magnetic field and somehow processes it and communicates it to the brain.
"I think the monarch (butterfly) will be the fundamental tool to find what is happening on a molecular level," Reppert said.
However, the study could also indicate some potential negative consequences for monarch butterflies' migratory abilities, Reppert said. Recent research shows that birds who navigate with a magnetic compass can have their guidance disrupted by electronic interference from most types of electronic gadgets.
"(This study) may show another species that may be vulnerable to human interference," Reppert said.
Further research will be needed to determine whether monarch butterflies are susceptible to electromagnetic interference, Reppert said, but the magnetic interference that was shown to disrupt birds was of such low intensity that it probably would affect the monarch butterflies.
Oberhauser said she hopes the results will spark more interest in monarch butterflies' migratory abilities.
"Because it's such an elegant study, I think people are really interested in this migration," Oberhauser said. "While it's not completely unexpected, it's just a really elegant piece of work that helps us understand this phenomenon."