Study in Nature Communications shows monarch butterflies use a UVA-dependent inclination magnetic compass during migration
Every autumn, millions of monarch butterflies (Danaus plexippus) undertake a remarkable migration, traveling roughly 2,000 miles from breeding areas across the eastern United States to overwintering groves of fir in central Mexico. Researchers at UMass Medical School and Worcester Polytechnic Institute have identified an important new element of the navigational toolkit monarchs use for this journey. Their study, published in Nature Communications, demonstrates that monarchs rely on a light-dependent inclination magnetic compass—sensitive to ultraviolet A/blue wavelengths—to help orient and maintain a southerly course during migration.
“Taken together, our findings reveal another fascinating component of monarch migratory behavior,” said senior author Steven Reppert, MD, Higgins Family Professor of Neuroscience and distinguished professor of neurobiology at UMass Medical School. He emphasized that understanding the biological mechanisms behind fall migration can inform conservation efforts. Habitat loss, diminishing milkweed, changing climates and human-generated electromagnetic noise are all threats that may interfere with the sensory systems monarchs depend on.
Co-author Robert Gegear, PhD, assistant professor of biology and biotechnology at Worcester Polytechnic Institute, added, “Monarchs employ a sophisticated inclination compass similar in principle to systems used by much larger-brained migratory vertebrates, such as birds and sea turtles.”
Monarchs are known to use a time-compensated sun compass located in their antennae to orient during migration. Yet observers have long reported that monarchs can maintain southerly flight even when the sun is obscured by clouds. This observation prompted scientists to investigate whether the butterflies also rely on geomagnetic cues when daylight orientation signals are weak or unavailable.
Previous research attempting to demonstrate an internal magnetic inclination compass in monarchs produced mixed or inconclusive results. The new study considered an additional factor: the magnetic response may depend on ultraviolet light, which can penetrate cloud cover and reach the butterflies’ photoreceptive molecules.
The team focused on cryptochromes (CRYs), a family of light-sensitive proteins found in many animals, including monarchs. In other insects, CRY proteins are known to respond to ultraviolet-A and blue light and can mediate light-dependent magnetic sensing. On this basis, the researchers hypothesized that monarchs possess a similar light-dependent magnetic compass.
Using flight simulators capable of generating controlled artificial magnetic fields, postdoctoral researcher Patrick Guerra, PhD, tested tethered migratory monarchs under diffuse white light. The butterflies consistently oriented toward the south. Additional experiments showed that monarchs responded to the inclination angle of Earth’s magnetic field—the angle between field lines and the horizontal plane—rather than to field polarity. When the simulated inclination was reversed, the butterflies reversed their orientation and flew north instead of south.
To determine whether the magnetic compass required specific wavelengths of light, the researchers introduced a range of wavelength-blocking filters. Monarchs exposed only to light above 420 nm (longer blue to red wavelengths) lost directional orientation and flew in circles. By contrast, butterflies receiving light that included wavelengths above 380 nm displayed clear, consistent directional flight. These results indicate the magnetic inclination compass in monarchs depends on exposure to ultraviolet-A/blue light in the 380–420 nm range.
Collectively, these experiments provide the first clear demonstration that monarch butterflies use a light-dependent, inclination magnetic compass during migration. They also represent the first evidence of this kind of navigational tool in a long-distance migratory insect. The inclination compass likely serves as an important backup when sunlight cues are unavailable and may operate together with the time-compensated sun compass to maintain orientation throughout the journey.
“For migratory monarchs, the inclination compass may act as a crucial support system when daylight cues are obscured,” Guerra said. “It likely works in concert with the sun compass to provide reliable orientation and directionality over the course of migration.”
Future work by Reppert and colleagues will examine the molecular and genetic basis of magnetoreception in monarchs and the specific role cryptochrome proteins play in mediating the light-dependent magnetic sense.
Contact: Jim Fessenden – University of Massachusetts Medical School
Source: University of Massachusetts Medical School press release
Image Source: Image credited to JamesDeMers (public domain)
Original Research: “A magnetic compass aids monarch butterfly migration” by Patrick A. Guerra, Robert J. Gegear and Steven M. Reppert. Published in Nature Communications, June 24, 2014. DOI: 10.1038/ncomms5164
A magnetic compass aids monarch butterfly migration
Flight simulator experiments demonstrate that migratory monarch butterflies possess an inclination magnetic compass that helps direct their flight equatorward during fall migration. This inclination compass is light-dependent, operating with ultraviolet-A/blue wavelengths between 380 and 420 nm.