Birds and sea turtles do it. Now a new study published in Nature Communications and co-authored by Robert Gegear, PhD, assistant professor of biology and biotechnology at Worcester Polytechnic Institute (WPI), shows that monarch butterflies also use a magnetic compass to help guide them on their long migrations. The study, conducted at the University of Massachusetts Medical School by lead author Steven Reppert, MD, Higgins Family Professor of Neuroscience, Gegear, and UMass postdoctoral fellow Patrick Guerra, PhD, provides the first evidence for the use of a magnetic compass by a long-distance migratory insect.
Millions of monarchs make an annual 2,000-mile trek from breeding sites in the eastern United States to central Mexico, where they overwinter. In previous research, Reppert, working with Gegear and other researchers, has shown that the butterflies rely on a time-compensated sun compass located in the insect's antennae to orient themselves on this southward journey.
Since the butterflies have been observed to continue to travel in a southerly direction even when flying under dense cloud cover or in other conditions when the sun does not provide clear directional cues, scientists have long believed that the insects also use a magnetic compass. However, previous studies aimed at confirming this hypothesis have yielded conflicting or unconvincing results.
"This new study shows that monarchs do, in fact, use a sophisticated magnetic inclination compass system for navigation similar to that used by much larger-brained migratory vertebrates such as birds and sea turtles," Gegear said. "This is likely a back-up compass for the butterflies, so they can continue to fly in the right direction even on days when they can’t see the sun."
In the study, monarchs collected at three locations along their migratory route were allowed to fly, tethered, for at least five minutes in a "flight simulator" that was surrounded by an artificially generated magnetic field that could be altered in the horizontal and vertical planes and varied in intensity. The researchers found that like most other migratory animals, the butterflies can sense the inclination angle component of the Earth's magnetic field.
In the northern hemisphere, the magnetic field is inclined toward the North Pole, allowing migratory animals to orient themselves along a north-south line and travel either toward or away from the pole. When the butterflies were flown in a magnetic field that mimicked the Earth's, they flew toward the south. When the researchers reversed the inclination, they flew toward the north.
The researchers also discovered that, similar to the magnetic compass in migratory birds, the monarch compass is light sensitive; in particular, it responds to wavelengths between 380 and 420 nanometers—the ultraviolet A/blue region. To test the light-dependence of the magnetic compass, the team applied a series of wavelength blocking filters to the lights in the simulator. Monarchs exposed to light only in the wavelength range above 420 nanometers flew in circles. Those exposed to light above 380 nanometers showed clear signs of directional flight.
The authors note that some of the previous research aimed at detecting a magnetic compass in monarchs was conducted under lighting conditioned that excluded these wavelengths, which may explain the inconclusive results. Reppert, Gegear, and Guerra also determined that the magnetic compass, like the time-compensated sun compass, resides in the monarch's antennae.
The researchers note that their findings, which expand our understanding of the monarch butterflies’ remarkable migration, may also help assure that they continue long into the future. "Greater knowledge of the mechanisms underlying the fall migration may well aid in its preservation, currently threatened by climate change and by the continuing loss of milkweed and overwintering habitats," they note in the Nature Communications paper. "A new vulnerability to now consider is the potential disruption of the magnetic compass in the monarchs by human-induced electromagnetic noise, which can also affect geomagnetic orientation in migratory birds."
In addition to his collaborative research with the Reppert Lab at UMass, Gegear is currently conducting research on a different pollinating insect: the bumblebee. His work is exploring, broadly, the relationship between bee and flowering plants. In work that encompasses biology, evolutionary ecology, cognitive psychology, and behavioral neuroscience, he is investigating how the bumblebee's simple brain is capable of remarkably complex cognition, and how the mental activity of bees and the decisions they make as they gather food and pollinate plants ultimately have profound consequences for the ecosystem and the human food supply.
• Read the Nature Communications Paper.
• Watch a video about the bumblebee research.