Shining Light on Monarchs’ Migration
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.
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.”
