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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.


The duo hypothesized that when taken outdoors, the jet-lagged monarchs would orient themselves in the opposite direction. Butterflies on an early schedule headed northwest, just as expected for an insect that mistook the afternoon sun for early morning. Butterflies on the late schedule did just the opposite, flying southeast.

Frost said the beauty of their experiment is that it allowed them to track the flight of the monarchs over a long period of time, simulating an enormous distance.

"Some have covered 30 miles. A few flew for four hours, covering the equivalent of 100 miles," he said. "This is a much longer distance than you could track outside, and you have a detailed record of every turn."

Monarch expert Brower hopes to use the contraption to test one of his own theories that monarchs are in migratory mode throughout the year.

Brower points out that monarchs fly south every winter and then six months later turn around and go north--a 180-degree change in direction over 180 days. So he proposes that there is a "directional clock" in the butterfly that advances the migratory direction 1 degree clockwise per day.

"The clock ticks through every day during every stage; caterpillar, pupae and butterfly," Brower said.

Butterflies leaving Mexico in March would have a clock oriented north. By late April, they arrive in Texas to breed. The directional clock would tick over the next month so that the next generation would leave in May headed northeast. By June the butterflies would be oriented toward the east. By September, the clock would have ticked 360 degrees, pointing the butterflies south once again.

"There is currently no direct evidence for the theory, but it fits what we know about migration," Brower said. He hopes to test his hypothesis using the tethering apparatus on butterflies captured at different stages in their migration.

The butterfly tether has helped scientists understand another mystery of the monarch's epic journey south: If the monarch uses a sun compass, how can it find its way on cloudy days?

Some scientists thought the butterflies used the Earth's magnetic field to guide them. Frost and Mouritsen's findings dispel this idea: Monarchs placed into an artificial magnetic field did not orient in any particular direction. Nor did they change direction as the magnetic field was turned, Frost said.

Frost thinks that insects can tell where the sun is by the pattern of polarization in sunlight.

The sun hits the atmosphere at different angles during the day, resulting in characteristic patterns of scattered light. Unlike humans, insects can see these polarization patterns, Frost said. They are still visible on overcast days, so monarchs may use them for navigation.

To test the idea, Frost and colleagues put a polarizing filter over their butterfly contraption. The monarchs still knew where to go.

Frost and Mouritsen have shown us how monarchs can make their way to Mexico, but one big question remains.

Karen Oberhauser, a professor at the University of Minnesota who also studies monarch migration, said that monarchs go to the same over-wintering spot--within about 328 feet--year after year.

"We still don't understand the fine details of how they know where they are and how they get to the same spot year after year. Monarchs must be using some other cues because they need more than a compass and a clock to get to them."

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