Monarch butterflies use the Earth's magnetic field and a 'sun compass' in their antenna as navigational tools for their long-distance migration, scientists say.
Monarchs use a time-compensated sun compass in their antenna to help them make their long-distance migratory journey to overwintering sites, researchers found.
Each year millions of monarch butterflies use a sophisticated navigation system to transverse 3,218 km from breeding sites across the eastern US to an overwintering habitat in specific groves of fir trees in central Mexico.
Scientists at the University of Massachusetts Medical School and Worcester Polytechnic Institute found that monarchs use a light-dependent inclination magnetic compass to help them orient southward during migration.
"Taken as a whole, our study reveals another fascinating aspect of the monarch butterfly migratory behaviour," said senior study author Steven Reppert, the Higgins Family Professor of Neuroscience and distinguished professor of neurobiology at UMMS.
"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," Reppert said.
During the absence of daylight cues, such as under dense cloud cover, migrants have been, surprisingly, seen flying in the expected southerly direction.
It's been hypothesised that monarchs use geomagnetic cues to help navigate when day light cues are unavailable to them.
Given the ability of monarch cryptochromes (CRY), a class of proteins that are sensitive to ultraviolet A/blue light, to restore a light-dependent magnetic response in CRY-deficient Drosophila, Reppert and colleagues suspected that monarchs also possessed a light-dependent magnetic compass.
Using flight simulators equipped with artificial magnetic fields, Patrick Guerra, a postdoctoral fellow in the Reppert lab, examined monarch flight behaviour under diffuse white light conditions.
He found that tethered monarchs in the simulators oriented themselves in a southerly direction. Tests in the simulator revealed that the butterflies used the inclination angle of Earth's magnetic field to guide their movement.
Reversing the direction of the inclination caused the monarchs to orient in the opposite direction, to the north instead of the south.
Researchers also tested the light-dependence of the monarch's magnetic compass.
Tests showed that the monarch's magnetic compass, and thus directional flight, was dependent on exposure to light wavelengths (380nm to 420nm) found in the ultraviolet A/blue light spectral range.
