Monarchs fly a multigenerational circle around the eastern United States every year. In the fall the last generation heads for Mexico.
When we think of the great animal migrations, we tend to think of great animals--creatures like caribou or wildebeests. But one of the most spectacular movements of life is undertaken by the four-inch-wide monarch butterfly. Every fall tens of millions of monarchs disappear from the United States. Until the mid-1970s, no one knew where they went; it was only then that an amateur lepidopterist found their hiding place in a high mountain range in Mexico. Researchers now know that all monarchs east of the Rockies fly down to Mexico in vast swarms at speeds of up to 30 miles an hour and altitudes of up to a mile. (Western monarchs head for southern California.) After traveling as much as 2,200 miles, they settle down on 30 sites located in a 40-mile-long stretch of mountains just west of Mexico City. They winter there, weighing down the trees in densities of up to 4 million butterflies per acre.
Why should a monarch fly so far? Monarchs belong to a subfamily of butterflies that live almost exclusively in the tropics, where they lay their eggs only on milkweed plants. Yet milkweed also blossoms across the United States. Over the last 10 million years or so, researchers suspect, monarchs evolved a way of life that let them come north without succumbing to the cold winters up here. Several generations of monarchs are born and die in the United States while milkweed is in season, but the generation born in late summer put their sexual development on hold, fatten themselves up on nectar, and make the long trip down to Mexico. There they avoid freezing. Sadly, the forests they winter in have been degraded by logging and cattle ranching for decades. Unless the trees are preserved, a winter may come when the butterflies discover they have no place to rest.
Finding the secret winter home of the monarchs answered one question about this great migration, but others remained open. The most obvious one is how the butterflies manage to find their destination-- particularly since the individuals that make the great journey are born in the United States and have never been to Mexico before. To Lincoln Brower, a leading monarch expert at the University of Florida and with Wildlife Conservation International, this question is really part of a much bigger one: How do monarchs navigate all year round?
For a long time researchers interested in tracking monarchs have had to rely either on sightings reported by butterfly lovers or on tagging experiments--a somewhat thankless method, since the chances of ever catching a tagged butterfly again are not that great. In the mid-1970s, though, Brower invented a powerful new tool for studying the distribution of monarchs. Some of the compounds in milkweed plants vary from species to species and from site to site, and these compounds are retained in the monarch’s body as it grows from a larva into an adult. Brower has measured the composition of milkweed around the United States with enough precision that he can say whether a monarch has just come from northern or southern parts of the country. With this technique, Brower was the first to establish that the monarchs that fly south to Mexico in the late summer and fall are the same ones that fly back north in the spring. Yet the movements of monarchs in the United States remained a confusing blur.
Last year the organizers of a conference on animal navigation invited Brower to give a talk. He began combing through the massive surveys he has made over the last decade of monarchs in different parts of the eastern United States at different times of the year, this time looking for an overall pattern. I thought, ‘What the hell, it’s probably a lot simpler than anybody thought it was.’
He turned out to be right. By reading the milkweed fingerprints in the monarch butterflies’ bodies, Brower discovered that he could track successive generations of them as they move in a relatively steady path around the eastern half of the country over the course of a year. Most of the monarchs coming back from Mexico make their way north to Texas and Louisiana, where they lay their eggs and die. The first-generation butterflies that hatch from those eggs reach the Great Lakes region, where they too reproduce and die. Then their progeny head east to the Appalachians and the East Coast; from there the next generation, the third, head south toward the Gulf Coast and veer west, finally reaching their winter home. Sometimes it takes two generations to reach the Great Lakes, so it ends up being the fourth that heads for Mexico. But the geographic pattern stays the same.
Brower can’t explain how this simple pattern could have gone unnoticed for so long. Hindsight is always twenty-twenty, he says. Clusters of monarchs do get blown off course from time to time, which would tend to confuse matters. But Brower has a tidy hypothesis to explain how monarchs in general manage to follow the giant circle he has charted.
There is evidence, he points out, that monarchs, like birds and sea turtles, may use magnetic fields to navigate--they carry crystals of magnetic minerals in their bodies. Moreover, some magnet-navigating birds are known to change the direction of their migration over the course of the year, which suggests they can shift their internal compass seasonally-- probably using the changing length of daylight as a cue. Brower thinks that monarchs do something similar but more dramatic: they spin their internal compass in a full circle each year.
When the monarchs come out of their winter torpor in March, the compass points north, but with each subsequent day it turns about one degree clockwise. Thus by the time they stop to lay their eggs in Texas and Louisiana, their compass is pointing northeast. Their offspring, which mature with spring sunlight, start flying with a northeast-pointing compass, which guides them to the Great Lakes. By the beginning of summer, the next generation of young are ready to fly away, and they head due east for the Appalachians. When this third generation sense the dwindling days and cooling nights of late summer, they prepare to migrate. Their initial heading is set for southeast, but as they make their long migration, their compass headings shift south and then west, enabling them to reach their Mexican hideaway. There, as they wait out the winter, their compass turns back to north, making a full circle over the year.
Aside from the fact that it must involve those magnetic crystals, Brower has no idea what the physiological basis for such a remarkably subtle compass might be. But when he presented his hypothesis at the animal navigation conference, he says, there were just these beatific smiles on people’s faces, and nobody since has challenged the idea. Brower plans to test it on lab-reared monarchs by trying to reset their hypothetical compass. If the changing length of the day is what sets the compass heading in nature, he should be able to adjust it to whatever heading he wants by raising butterfly larvae under different patterns of light and dark. When they emerge from their chrysalises, he’ll bring them outside before dawn. As the sun warms their bodies, the monarchs should immediately fly off in their different, predetermined directions.
After 40 years of research on the monarch, Brower is still looking forward to his most interesting work. Scientifically, the monarch butterfly is like a magic bottle of wine, he says. You pour out a draft, and you look back, and the bottle’s full again.