Migration of the Monarch

Resource for Grades 3-8

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Video

Running Time: 4m 39s
Size: 13.8 MB

Source: NOVA: "The Mystery of Animal Pathfinders"

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It is one of the most impressive displays of migratorial instinct and ability in the animal kingdom: Each year millions of monarch butterflies migrate from eastern Canada and the United States to Mexico -- a journey of more than two thousand miles for some. This video segment from NOVA: "The Mystery of Animal Pathfinders" explores why monarchs make such an epic journey and how they manage to do it.

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Background Essay

The mass migration of monarch butterflies from eastern Canada and the United States to isolated mountain ranges in central Mexico certainly rates as one of the most impressive natural phenomena in the world. Even among birds and mammals, such travel feats are uncommon. Yet, each fall these wispy insects embark on a journey thousands of miles long to a place none of them has ever been before. They fly with only their innate sense of timing and direction to guide them.

Although mass migrations of monarchs occur every year, only every third or fourth generation of monarchs makes the epic flight to Mexico. Between March and September, up to four generations of monarchs may be born, but the majority of these butterflies typically fly only short distances, moving northward during the spring and summer months as they follow the growth of milkweed, their primary food source.

What seems to decide for the monarchs whether they will be local butterflies or far-flung travelers is when during the spring/summer season they are born. First-generation monarchs, the offspring of the generation that spent the winter in Mexico, typically hatch from their eggs in early spring. After spending two weeks as caterpillars and two weeks in the chrysalis stage, they spend their month of adulthood mating, laying eggs, and moving northward. Second-generation and most third-generation monarchs, born in late spring and early summer, live out their two months of life much as their parents before them, mating, laying eggs, and following the milkweed.

Fourth-generation and some third-generation monarchs, however, hatch much later in the season -- July through September -- and are subject to different conditions. These conditions, including shorter days, cooler temperatures, and older and lower-quality milkweed, foreshadow the coming of winter and are probably some of the factors that cue the butterflies that it is time to migrate south.

The journey to Mexico from the eastern United States, experts estimate, takes the average monarch about two months. Like birds, the butterflies probably use a combination of the angle of the sun in the sky and the earth's magnetic fields to guide them southward. It remains a mystery, however, exactly how the monarchs find the relatively tiny patch of land in central Mexico that they will call home throughout the winter. Here the fourth-generation monarchs will stay for the next five or six months -- far longer than the entire life span of their parents. During this time, in a nonreproductive state called diapause, they will not mate or eat. At last, in early spring, the monarchs will come out of diapause, mate, and begin their egg-laying journey northward, where the entire life cycle of this intrepid insect will begin again.

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Discussion Questions

What characteristics of the Mexican environment make it a good place for the monarchs to overwinter?

What would happen to the monarchs if the milkweed supply diminished?

How do you think the caterpillars and adult monarchs survive when they are so brightly colored and can be seen easily by predators such as birds?

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Transcript

NARRATOR: Some animals must be born knowing how and when to go to far-off places they have never seen before. It's late summer in these New England hills, and one of the longest of all annual insect migrations is quietly taking shape.

The key to this journey is the milkweed plant, and a homing instinct carried by the brilliantly colored caterpillars of the monarch butterfly that are feeding on it. The caterpillars, the eggs, the pupae, even the adult butterflies of other species can survive the harsh northern winter. But the monarch cannot.

After weeks of feeding and fast growth, the three-inch caterpillar begins the miraculous transformation that will allow it to escape the certain death that winter would bring. In a few hours, the caterpillar has shed its skin and the hard, protective casing of a chrysalis has formed. Inside, an utterly different creature is taking shape. And with it, the instinct to make a great migration, out of all proportion to its apparent fragility.

Using a long, slender tongue, it will now stoke up on nectar. Monarchs usually breed and then die within a few weeks, but not this generation. The temperature and day length may keep them from breeding, and that gives them eight more months of life than their parents

The autumnal equinox seems to trigger the migration. Very little is known about how they navigate, but somehow they will find their way to the only known place hey can survive the winter.

One theory holds that monarchs may steer by the sun, correcting for its movement across the sky like honeybees. The monarchs that start in New England and eastern Canada have the longest trip. The monarchs west of the Rockies move to the coast of California, but the vast majority of North American monarchs, those east of the Rockies, move toward Mexico. When they cross the tropic of Cancer, leaving cold temperatures far behind, they might be expected to disperse, but they don't. What they do instead is the greatest enigma of all.

Having crossed half a continent, tens of millions of butterflies converge toward a pinpoint on the map of Mexico, a 30-by-50-mile area of high mountains and Spanish Colonial towns.

Except to the people who live here, the wintering colonies of the monarch butterfly were unknown until 1974. Above the hillside farms in the thin, cool air at 10,000 feet, there is a habitat that may be unique in Central America.

Called the Transvolcanic Range, it is a rocky wilderness of ravines and fir forests. These forests combine the seasonal stability of the Tropics with the coolness and the moisture of the mountains. It's not warm enough to breed and lay eggs, so the monarchs are in reproductive cold storage until next spring.

There are known to be about 15 separate colonies in the Transvolcanic Range. Precisely how the monarchs find them is still a mystery. From December until March, they eat nothing, living only on fat reserves. But they must drink water, and a stream is crucial to the location of each of the colonies.