New research suggests monarch butterflies navigate their epic migrations using magnetic compasses.
Every year monarchs from across the United States make the journey to their overwintering habitat, which is located in specific groves of firs in central Mexico, a University of Massachusetts news release reported. Researchers have revealed a new component of the butterflies' complex navigation system; a "light-dependent inclination magnetic compass."
"Taken as a whole, our study reveals another fascinating aspect of the monarch butterfly migratory behavior," senior study author Steven Reppert, MD, the Higgins Family Professor of Neuroscience and distinguished professor of neurobiology at UMMS, said in the news release. "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."
"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," Co-author Robert Gegear, PhD, assistant professor of biology and biotechnology at WPI, said in the news release.
When daylight is present a time-compensated sun compass, which is located in the antennae, helps the butterflies make the 2,000-mile migration southward. Researchers have been puzzled to still see monarchs flying in the right direction when the sun is blocked by dense cloud cover; it has been hypothesized that in these situations the butterflies use geomagnetic cues to navigate.
Monarch cryptochromes (CRY) are a class of proteins that are sensitive to ultraviolet light and have the ability to restore a light-dependent magnetic response in CRY-deficient Drosophila, the team suspected monarchs also have a light-dependent magnetic compass.
A research team used flight simulators coupled with artificial magnetic fields to examine monarch flight behavior under diffuse white light conditions. They found that tethered monarchs in simulators used the inclination angle of the Earth's magnetic field to guide their movements; reversing the inclination caused the butterflies to also reverse their direction.
The team also applied a series of wavelength-blocking filters to the lights in order to test the light dependency of the butterflies' compass. They found that monarchs exposed only to light in wavelengths above 420nm exhibited an ability to detect direction. These butterflies flew in circles. The experiment showed that the monarch's magnetic compass is dependent on exposure to wavelengths found in the ultraviolet light spectra.
"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."
