Magnetic Migration.

One of the most spectacular migrations in the insect world is the trip tens of millions of monarch butterflies take as they traverse the two thousand miles from southern Canada and the eastern United States to Mexico. Every August these black and orange lepidopterans head south to spend the winter in the fir forests of central Mexico's Transvolcanica Range. Covering the trees with a population as dense as 4 million butterflies per acre, they roost for over five months.

More amazing than the masses of migrating monarchs is that, unlike migratory birds, the butterflies are going to a place they have never been before. The same butterflies that arrive in Mexico are not the same butterflies that left Mexico to migrate north, but are instead the great-great grandchildren of the original group. So how do they know how to get there? Researchers working on this question believe monarch migration is accomplished by a complex system of orientational and navigational tools involving the sun, the earth's magnetic field, and physical land forms. They have theorized that once the monarchs begin migrating they use the sun as a compass for orientation during their flight.

Sandra Perez and Orley R. "Chip" Taylor, of the University of Kansas, have explored this theory by performing time-shift studies on monarchs. Taylor is the director of Monarch Watch, an outreach program focused on science education, butterfly conservation, and monarch migration. Perez and Taylor kept the butterflies in the dark long enough to make the insects' internal clocks think it was seven in the morning, but then released them at one in the afternoon.

"If they [monarchs] are using the sun as a proximate orientation mechanism and you shift them six hours, you should shift them ninety degrees in their orientation," Taylor explains, "and that's what happened in the experiment. We shifted them and got the response we expected. Instead of flying, as the control group did, at 210 degrees, the time-shift butterflies flew off at about 280 to 290 degrees."

To complement their orientation skills, monarchs have developed a complex navigational system. Evidence suggests that, like birds and sea turtles, monarchs use the earth's magnetic field to navigate. Because researchers have found magnetite--a small iron crystal that is sensitive to the earth's magnetic field--in the insect's body, they hypothesize that monarchs use that chemical to detect the earth's magnetic field. The next task is to figure out how the butterflies are using the magnetic field. "We know at this point from our unpublished data, that they do respond to electromagnetic pulses," Taylor says. "That is really the first step in establishing whether or not they use the electromagnetic field."

What that theory does not explain is why along the migration route about the time the butterflies enter Mexico they change their course dramatically. "Most of the data is consistent with the idea that they are using the magnetic field in some way," Taylor explains. "However when they reach the mountains of Mexico, they move along the mountains in a way that is totally inconsistent with using the magnetic field." Basically, the butterflies veer off course and make a left turn. Researchers believe that at that point the butterflies switch navigational tools from the earth's magnetic field to land forms. If they were to continue on their original course, they would end up in the Pacific Ocean, where there is no food source. By following the mountains, they take a course with plenty of food that leads them to the same roosting sites their great-great grandparents left six months before.