Feb 102015

The North American yellow-and-black monarch butterfly, that migrates every winter from southern Canada to central Mexico in search of milkweed to lay its eggs, has been making news of late mainly because of its declining numbers.

How the insects manage to travel thousands of kilometres without getting lost has been the subject of much curiosity. While it was known that they use the position of the sun, supported by an internal body clock, to navigate, there was still the question of how they manage even when skies are overcast.

On Wednesday, scientists from the University of Massachusetts Medical School and Worcester Polytechnic Institute published the results of their study which seem to have confirmed that monarch butterflies, like many birds and sea turtles, are aided by a geomagnetic compass — a mechanism by which they are able to tell the direction based on the angle of Earth’s magnetic field.

Inclination angle

Earth’s magnetic field lines emerge from the South Pole, loop across the equator, and re-enter at the North Pole. At any location, the field lines meet the surface of the Earth at a particular angle called inclination angle. Monarch butterflies made to fly in an experimental set-up called a flight simulator were shown to orient themselves based on this inclination angle, especially in cloudy conditions.

The scientists were also able to find out that it was proteins called CRY in the butterflies’ antennae that activate this inclination compass when light of a particular wavelength — the ultra-violet/blue end of the spectrum — fell on it.

The scientists were able to narrow down to this wavelength range because of previous studies that had shown the ability of ultraviolet radiation to cause similar behaviour in fruitflies “The defined spectral requirement for the inclination compass discovered in our studies potentially explains why previous flight simulator studies were unable to identify an inclination magnetic compass,” the authors say in their study published in Nature Communications.

Several implications

This finding may have several implications for conservation efforts of monarch butterflies, which have been given ‘near threatened’ status by the World Wildlife Fund. Their declining numbers may have more reasons than loss of milkweed habitats and climate change.

“Another vulnerability to now consider is the potential disruption of the magnetic compass in monarchs by human-induced electromagnetic noise, which can apparently disrupt geomagnetic orientation in a migratory bird,” the authors warn.

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