Monarch butterflies have a one-track migratory mind. Like a bumbling sit-com dad leading a family vacation to Mexico, the insects just head southwest for thousands of miles and don't stop for directions.
Neuroscientists recently documented that the monarch butterflies of eastern North America don't use fancy navigational strategies; they just fly with a general southwest orientation until they reach the forests in the mountains west of Mexico City. Luckily for the butterflies, the geography of North America funnels them into Mexico.
Thus, the insects complete their massive migration although no living butterfly ever completes the entire migration to Mexico and back. The instinct to fly to the southwest seems to be the main strategy the monarchs pass on to the next generation to enable their epic migration that starts as far away as New England and Canada for some monarchs.
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To test the monarch's migratory strategy, a team of scientists captured butterflies in Alberta, Canada and measured their flight trajectory. The same butterflies were then transported 2,500 kilometers west to Ontario. In both locations the butterflies tended to fly southwest. If the butterflies were using a complex navigational technique they would have adjusted course in Alberta and flown to the south.
The butterflies' orientation was measured by attaching tiny metal rods to their backs. The rods were tethered to a tracking system which used computer software to determine the butterflies' direction. The insects were places in a white circular corral which blocked their view of landscape features they could have used for navigation. The sun was the only means of navigation available to the butterflies.
The study, led by Henrik Mouritsen of the University of Oldenburg, Germany, also reviewed 50 years of butterfly capture and release tracking data from the United States east of the Rocky Mountains. The five decades of data showed that free roaming butterflies tended to head southwest as well.
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The simple migratory plan of the monarchs is compensated for by a few other rules of the road, wrote the researchers in the study published in the Proceedings of the National Academy of Science. If the monarchs encounter a mountain or a coast, they follow a course along that geographical feature that most agrees with their southwestern orientation inclination.
The geography of North America funnels the insects into Mexico. The Rocky Mountains stop the insects from going too far west, while the Gulf of Mexico stops them from going too far south.
IMAGE: Monarch butterflies in a sanctuary in Mexico (Samuel, Wikimedia Commons)