Eastern North American monarch butterflies (Danaus plexippus) use a light-dependent, inclination magnetic compass to orient during migration, according to a new study reported in the journal Nature Communications.
Monarch butterflies cluster in Santa Cruz, California. Image credit: Brocken Inaglory / CC BY-SA 3.0.
The monarch butterfly belongs to the subfamily Danainae in the family Nymphalidae. The species ranges from North and South America and the Caribbean to Australia, New Zealand, the oceanic islands of the Pacific, Mauritius, the Canary Islands of the Atlantic, and Europe.
These butterflies are bright orange with black borders and black veins, with a wingspan of about 10 cm. The veins on the female are thicker than those of the male.
Monarch butterflies are poisonous to vertebrates; their poison comes from the milkweed they feed on.
The eastern North American monarch population is notable for its southward late summer-autumn migration from breeding sites across the United States and southern Canada to an overwintering habitat in specific groves of fir trees in central Mexico.
These insects use a sophisticated navigation system to transverse more than 3,000 km. In a new study, entomologists of Worcester Polytechnic Institute and the University of Massachusetts Medical School have discovered a new component of this complex system.
“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,” said study co-author Dr Robert Gegear of Worcester Polytechnic Institute.
Monarch butterflies use a time-compensated sun compass in their antenna to help them make their migratory journey to overwintering sites.
During the absence of daylight cues, such as under dense cloud cover, migrants have been, surprisingly, seen flying in the expected southerly direction.
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, the entomologists suspected that monarch butterflies possessed a light-dependent magnetic compass.
Using flight simulators equipped with artificial magnetic fields, the team examined monarch flight behavior under diffuse white light conditions.
The scientists found that tethered monarch butterflies in the simulators oriented themselves in a southerly direction.
Further 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.
To test the light-dependence of the monarch’s 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 nm exhibited a lack of direction by flying in circles. Monarchs exposed to light in the wavelength range above 380 nm showed clear signs of directional flight.
These tests showed that the monarch’s magnetic compass, and thus directional flight, was dependent on exposure to light wavelengths (380 – 420 nm) found in the ultraviolet A – blue light spectral range.
Together, these results provide the first demonstration that the monarch butterfly uses a light-dependent, inclination compass during its long journey. It is also the first evidence of such a navigational tool in a long-distance migratory insect.
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Patrick A Guerra et al. 2014. A magnetic compass aids monarch butterfly migration. Nature Communications 5, article number: 4164; doi: 10.1038/ncomms5164
