At first glance, the monarch butterfly wouldn’t appear to be the brightest animal on the planet. After all, they have tiny heads. Still, millions of them manage to make an annual migration of over 2,000 miles across the U.S. to very specific evergreen groves in New Mexico. How does a species of seemingly dumb insects manage such precision? According to researchers from UMass Medical School and Worcester Polytechnic Institute, it’s because they have tiny, built-in magnetic compasses.

Co-author Robert Gegear, PhD, assistant professor of biology and biotechnology at WPI, explained, “Our study shows that monarchs 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.”

The “compass” is part of an extremely complicated system. The scientists found that the butterflies use what’s known as a “light-dependent inclination magnetic compass” to help point them southward. During the day, mechanisms in the monarchs’ antennae use the sun for orientation. Somehow, they fly at night as well. The theory is that geomagnetic cues, then, also play a role through an ultraviolet-sensing component that can orient based on cloud-penetrating UV light. The magnetic compass was found to be dependent on exposure to light wavelengths (380nm to 420nm) found in the ultraviolet A/blue light spectral range.

“For migratory monarchs, the inclination compass may serve as an important back up system when daylight cues are unavailable,” Guerra said. “It may also augment hand-in-hand with the time-compensated sun compass to provide orientation and directionality throughout the migration process,” said Patrick Guerra, PhD, a postdoctoral fellow in the Reppert lab who examined the butterflies.

This is the first known instance of such a sophisticated navigational tool in a migratory insect.

“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. 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,” said senior study author Steven Reppert, MD, the Higgins Family Professor of Neuroscience and distinguished professor of neurobiology at UMMS.