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Science File

Shining Light on Monarchs' Migration

Some theorize the butterflies use the sun or the Earth's magnetic field. Two scientists, using a tethering system, think they know.


Every fall, eastern monarch butterflies start an epic expedition, flying up to 3,000 miles south to the mountains of Mexico. They will spend the winter in suspended animation and six months later start the journey north again.

This unusual migration, whose extreme length and duration is more characteristic of birds than insects, has been one of the tantalizing mysteries of entomology.

Researchers have long debated if monarchs use the sun, the Earth's magnetic field or some other unknown phenomena as their guide. The sun has been the primary candidate--an obvious but unreliable source of directional information. How can the sun, with its daily trek across the sky, be an efficient compass? And how can a sun compass function on rainy days, a common occurrence during the butterfly's fall migration?

Two scientists from Canada and Germany believe they have found the solution. Using a technique to tether butterflies in flight, the duo has determined that monarchs navigate with the sun using a complex system that compensates for time of day as the sun changes its position.

The study focused on the migration of monarch butterflies that begin their journey in Canada, but the finding should apply to most monarch populations. The method "captures the essence of migratory behavior," said Barrie Frost, principal scientist on the study and a professor of physiology at Queen's University in Canada.

Lincoln P. Brower, a leading monarch expert and professor at Sweet Briar College in Virginia, said Frost's technique opens a new realm of possibilities for studying butterfly behavior. Previously, scientists studied monarch navigation by capturing them and then observing their flight after release.

There are "so many natural variables that can affect their behavior," said Brower. "We tried everything under the sun--kept them in coolers, poked them to fly." But when monarchs are provoked to fly, they don't behave naturally, he said.

Brower called Frost's tethering system "an innovative method to get directionality of butterfly behavior under controlled conditions."

Monarchs begin their migration in the late summer and early fall, when shorter days and cooler temperatures signal that it is time to head south. During the journey, monarchs stop breeding and put on weight. They build up fat by drinking nectar from flowers during their journey.

Monarchs west of the Rocky Mountains have a relatively short migration, traveling to wintering sites along the California coast.

The eastern monarchs studied by Frost fly 1,000 to 3,000 miles, gathering at the same 14 high-mountain spots in the fir forests of central Mexico. They spend the winter in "cold storage," said Brower, waiting to mate and lay eggs the following spring.

The same monarchs fly partway back north the next March, stopping to lay eggs on new milkweeds, the plant that nourishes the young caterpillars after hatching. Each successive spring generation moves farther north, following the budding milkweed and living only for a month or two.

How does the sun help monarchs find their way back and forth across the continent? Frost says there are different strategies for using the sun as a compass.

If you always kept the sun in front of you, you would make a general arc, going east in the morning, then south, then west in the afternoon, Frost said. The average direction is south, but, as Frost points out, this is not a very efficient method.

A more economical approach, Frost said, would be to change direction relative to the sun and the time of day--a time-compensated sun compass.

To use this navigation strategy, the monarch must possess both an internal clock and map. The butterfly would know that in the morning, the sun is in the eastern part of the sky. To head south, the butterfly would face the sun and turn right, Frost said.

Frost and his collaborator, Henrik Mouritsen of the University of Oldenburg in Germany, designed a unique contraption to figure out which strategy the monarch is using.

A wire stalk was glued to the back of the butterfly, while a gentle stream of air was blown beneath it to encourage the insect to flap its wings and fly. The insect is free to steer its movement but is tethered in one location.

A small sensor attached to the butterfly's wire keeps track of the direction the insect is headed, allowing the scientists to re-create the path of the monarch as if it were flying free.

Frost and Mouritsen captured Canadian monarchs in the fall, during the pre-migration period called "diapause." After being tethered and placed outside, they immediately oriented toward the southwest--the same direction they would go on their journey from Canada to Mexico. The butterflies flew just 1 degree off from the course leading to their Mexican wintering grounds.

To test their time-compensated hypothesis, Frost and Mouritsen gave the butterflies jet lag. Monarchs were kept indoors and exposed to either an early schedule of light, 1 a.m. to 1 p.m., or a late schedule, 1 p.m. to 1 a.m.

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