The Incredible Journey of the Butterflies

PBS Airdate: January 27, 2009
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NARRATOR: It's one of the most profound mysteries in the natural world, an amazing transcontinental odyssey: the migration, each year, of millions of Monarch butterflies from Canada, across America, to Mexico.

CHIP TAYLOR (University of Kansas): You got a butterfly that's originating in Toronto, or it's originating in Detroit, Michigan, or it's coming down from St. Paul, maybe even Winnipeg, and it's moving south, and, somehow, it finds its way to Mexico. Could you do that?

NARRATOR: Starting from a tiny caterpillar, blossoming into a beautiful butterfly, these delicate creatures will fly thousands of miles in a feat of endurance and navigation unlike anything else in nature.

LINCOLN BROWER (Sweet Briar College): They've never taken a long flight in their lives, and they're on their way to an area that they've never seen before. Somehow they're recognizing landmarks, or following streams or following the Sun. They're following something.

NARRATOR: They're on their way to a remote area, high in the Mexican mountains. And they get there every year at exactly the same time.

BILL CALVERT (Zoologist): Butterflies have dazzled humans for millennia. It's a beautiful little creature, and on top of that, it migrates 2,000 miles, and this just staggers the mind.

LINCOLN BROWER: I think the Monarch butterfly is one of the most magnificent animals in the world. And it's unique in terms of the entire animal kingdom. There's nothing like it.

NARRATOR: Next on NOVA, The Incredible Journey of the Butterflies.

Major funding for NOVA is provided by the following:

One of the biggest challenges for geoscientists is trying to find the oil and gas that's hidden below the surface of the Earth. Lately, our researchers have developed a new technology called R3M. You can't see it and you can't feel it, but the Earth has electromagnetic waves, and if you build the right kinds of listening devices, you can make sense of those waves. So using those tools, we can supply more of the energy that we need, with less of an impact on the environment.

And David H. Koch. And...

Discover new knowledge: HHMI.

And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

NARRATOR: On a late August morning, just north of Lake Huron, in Canada, a miracle of nature is about to unfold. This tiny caterpillar is destined to become a Monarch butterfly. In one of the most amazing transformations in the animal world, the caterpillar will outgrow and shed its skin four times. The fifth time, the caterpillar disappears. It's transformed into a chrysalis, a delicate case within which a completely new being takes form.

After about 10 days in the chrysalis, the new creature is complete. All traces of the caterpillar are gone, and in its place is a butterfly with four delicate wings.

But the newly developed Monarch butterfly must wait a few hours for its wings to harden, and then, finally, it can fly.

This particular generation of Monarch butterflies is special. Every year, about a hundred million of them begin an astonishing migration. Coming from southern Canada and the northeastern United States, each butterfly, starting on its own, flies about 2,000 miles, arriving two months later in Mexico.

Their trip is part of a carefully timed cycle that began three generations back, when a group of Monarchs left Mexico at the end of the winter. They flew as far north as the Gulf States, mated, and died.

The second generation flew to the northern United States. There, they, too, mated and died, living only about a month. Their offspring, the third generation, completed the last leg of the journey to Canada, also surviving only about a month.

But the fourth generation will live almost nine months. And they'll fly all the way back to Mexico in one epic trip. It's an amazing natural cycle that so far eludes explanation.

The mystery starts at the very beginning of the trip, because no one knows exactly what triggers the exodus from Canada.

LINCOLN BROWER: Well, when the Monarchs leave Canada, they have a 2,000 mile trek ahead of them, at least. They're freshly hatched butterflies. They've never taken a long flight in their lives, and they're on their way to an area that they've never seen before. Somehow they're recognizing landmarks, or following streams or following the Sun. They're following something. We just don't know exactly how they do it. It's really an incredible journey.

NARRATOR: A Monarch's wingspan is just under four inches, and they weigh less than one fifth of an ounce. So how they survive their marathon migration is another mystery.

They only fly when conditions are perfect. If it's too cold, they get sluggish and can't flap their wings. If it's too hot, they stop flying so they don't get overheated. They must also stop often for nectar and water. But every time they land, there can be enemies lurking. Bad weather is also the Monarch's enemy. A rainstorm can be deadly.

If it survives enemy attacks and bad weather, a Monarch that started in Canada has to fly at least 50 miles a day to get to Mexico. The physical effort this requires is remarkable for a creature so small, with such fragile wings.

DAVID GIBO (University of Toronto): Butterflies are the worst possible body form for trying to make a long distance migration. They're simply a bad design. Every time they flap their wings they're using energy at least 20 times the rate than when they're not flapping it, so they're just burning their fuel up at a great rate, much like, say, a helicopter might. And so they have to compensate for their inadequacies by soaring.

Soaring is gliding in rising air, much like I'm doing right now. The sun heats the ground, the ground heats the air above it. As the air heats, it expands and becomes lighter and begins to rise, and pretty soon you have a column of rising air. That's a thermal. Under good conditions you can maintain the altitude you're at or even gain altitude. A more helpful maneuver is to circle in it. And you see hawks doing this and vultures doing this all the time, circling the thermal, staying within it. And this seems like a wonderful free ride, and it is. Soaring is the key to them getting to Mexico.

NARRATOR: On the shores of the Great Lakes, just days into their journey, the Monarchs face their first geographic hurdle: miles of open water and constantly shifting winds.

LINCOLN BROWER: As the Monarchs are migrating out of Canada, they hit the Great Lakes, which are a barrier. They can't see across them.

NARRATOR: With no land in sight, Monarchs use their finely tuned sense of the direction of the wind to carry them across the water. If wind from the south, a headwind, threatens to blow them off course, they stop and wait. When they sense that the wind has shifted in their favor, they fly on.

NARRATOR: The ultimate destination of their incredible journey is a tiny area, about 60 square miles and 10,000 feet high, in the mountains of Mexico.

The local people, called the Mazahua, have lived here for hundreds of years. They believe Monarchs represent the spirits of their ancestors, and the arrival of the butterflies each year begins a celebration called the Day of the Dead.

ALICIA GARCIA: It's a very beautiful time when the butterflies arrive. The butterflies would come down, surround us, coming down to give the final touch to the tradition of the Day of the Dead. For those who live here, it's our belief. From when I was a child, we would say they were the souls of our departed loved ones. Every year I make an altar. We put these things here because when our ancestors were alive, this is what they liked. That's why one waits for their arrival, to give them this offering.

HOMERO ARIDJIS: The legends of the people that live near the ocean and the mountains are important to them. For us ,there is a sense of space, the freedom to fly, to fly with the imagination, to fly just like a butterfly.

NARRATOR: Homero Aridjis is one of Mexico's best-loved writers. He grew up in these hills and has fought to preserve them for Monarchs.

Every year Lincoln Brower comes here to continue his study of the Monarch migration.

LINCOLN BROWER: When you were a young boy, Homero, you used to go up to see the butterflies?

HOMERO ARIDJIS: Yes. Every year, we came with the schoolchildren. And for us it was one of the most fantastic spectacles of the year, to go to the Plain of the Mule to see the butterflies. Butterflies also came to town. They were across the street.

LINCOLN BROWER: They flew through the town?

HOMERO ARIDJIS: Exactly. They were looking for water. Sometimes they was in your house. But there were millions of butterflies, and for us, it was a spontaneous

miracle to see butterflies here in the Cerro del Campanario. But we didn't know that they were coming from Canada, across the United States. And the Canadians and Americans didn't know they were coming to these places.

NARRATOR: It was not until 1975, that scientists discovered the full extent of the North American migration, when butterflies that had been tagged in Canada were found spending the winter here.

These Monarchs return each year to 12 specific sites in these mountains. This is their only destination in the world. It's a perfect environment for the butterflies because of the unique climate.

LINCOLN BROWER: We're talking constantly about this micro-climactic envelope: about 3,100 meters, usually on southwest-facing slopes. If you imagine the forest as a blanket that protects the butterflies by keeping the heat in, and also think of it as an umbrella that keeps the rain out. And the tree is like a hot water bottle; it's radiating heat out through the bodies of the butterflies. So when the temperature drops down really low, you'll see millions of Monarchs just festooning these beautiful trunk clusters. If you think about it, the bigger the tree, the more heat it holds. So this is an argument for maintaining the forest in its native state, to let the trees get as big as they can, and the butterflies will be protected during those cold periods.

NARRATOR: Monarchs live in other parts of the world, in warm climates. But only Canadian and North American Monarchs migrate such an incredible distance to avoid the certain death of a cold winter. And exactly how they navigate from Canada to Mexico is another unsolved mystery.

Scientists only have a few clues. One theory is that the butterflies navigate by following a specific angle of the Sun in relation to the Earth.

Another theory proposes that the Earth's magnetic field may provide a subtle orientation guide. And recently, biologists discovered specific cells in the butterfly's brain that regulate their internal clock and help keep them on course.

At the University of Kansas, Chip Taylor studies the forces at work in the Monarch migration.

CHIP TAYLOR: You got a butterfly that's originating in Toronto, or it's originating in Point Filé, or it's originating in Detroit, Michigan, or it's coming down from St. Paul or maybe even Winnipeg, and its moving south, and, somehow, it finds its way to Mexico. Could you do that?

NARRATOR: In 1992, Taylor started a project called Monarch Watch. Schoolchildren and teachers tag butterflies from all over the northeastern United States. The tags don't hurt the butterflies, and don't affect their ability to fly. But when tagged butterflies are recovered at various stops along the way to Mexico, tracing back the information on the tags helps reveal their flight path, and their traveling speed.

And one of Taylor's tagging experiments had a surprising outcome.

CHIP TAYLOR: We ran some experiments a few years ago. So we took butterflies, and we transferred them to Washington, D.C. And initially, when we released them in Washington, D.C., they behaved as though they were still in Kansas.

NARRATOR: The butterflies who'd been moved to Washington started out flying in the same direction they would have taken to Mexico from their original home in Kansas, almost directly south. But starting from Washington, that flight path would never get them to Mexico.

Amazingly, after a few days, the displaced Monarchs somehow reoriented themselves and changed course to a strong southwest heading. That meant that, even starting from an unfamiliar location, they still ended up in the right place in Mexico.

CHIP TAYLOR: Now, this is really exciting stuff, because what this says is that, somehow, this butterfly is acquiring celestial information, perhaps magnetic information, and it's integrating those and remodeling the physiology of the system to have a different vector, to have a different direction from where it came from. Now, that's pretty cool.

NARRATOR: By late September, about a month into the migration, the Monarchs are gathering into huge flocks. By this time, they've traveled more than halfway across America, over the industrial belt, through small Midwestern towns, across the Great Plains, and finally, approaching the Southwest.

No one knows how many Monarchs die along the way, but if they make it to Mexico, there's another threat. Their destination in the Mexican mountains, the forests that will keep them alive over the winter, is in danger.

HOMERO ARIDJIS: This, like...you see all these trees, Lincoln? Before, there were hundreds of thousands, and now you can count them.

NARRATOR: In 1986, the Mexican government protected some sections of these mountains as official sanctuaries for the butterflies for the winter months.

But that meant some parts of the forest local people had depended on for income, through legal logging operations, were suddenly off limits.

The result was an unexpected new threat to the Monarchs: illegal logging.

BALTAZAR GUTIERREZ: We all have needs, but those that cannot meet their needs, they are the ones doing the clandestine logging.

WOMAN: They come at two or three in the morning. They go down in the night to sell the wood.

NARRATOR: Mexican police patrol the forest but have not been able to stop illegal logging.

The World Wildlife Fund pays villagers to try to stop the destruction, but they are no match for the dangerous forces at work.

EDUARDO SALINAS (World Wildlife Fund): Logging is clandestine and involves dangerous people. So you cannot go around telling the world about it. Sometimes you find yourself alone, and even with the police, you can be left alone. They will follow you to kill you. It's not that easy.

INDEPENDENT LOGGER: Who would allow their children to die of hunger? We know that it's important to preserve the forest for the butterflies, but, because of our need, we have not been able to do it.

LINCOLN BROWER: We're talking about hundreds of hectares of forest being leveled and then burned. I have been told the reason they burn them after they log them is to destroy the evidence that they cut them, which sort of eludes my thinking completely. Even this small-scale logging operation is destroying the capacity of the Monarchs to use those sites, there are so few trees left. And, even if they did sit on the ones that were left, they'd freeze to death.

NARRATOR: With the sanctuaries shrinking, an unusually cold winter in Mexico can be a disaster for the butterflies. During one storm, 80 percent of them died in a single sanctuary. If a harsh winter is followed by more bad weather in the spring, then no one knows how many butterflies will be able to breed new generations for future migrations.

LINCOLN BROWER: If the numbers are reduced to the point where the migration starts to unravel.... We don't know what the critical low number is, but I'm worried that we might just get close to it.

NARRATOR: It's the middle of October. The butterflies are almost to the Mexican border. They started the migration scattered across thousands of miles of the northeastern U.S. and Canada. But at this point, they're flying together in a huge flock, only 50 miles wide, for the final leg south.

BILL CALVERT: I just saw the shadow of it.

NARRATOR: For over 30 years, Texas zoologist Bill Calvert has conducted extensive field studies of the migration. But this year he's worried; the butterflies are late.

BILL CALVERT: Well, this is perfect, except for one thing, no butterflies here. An endangered phenomena would not be the same as an endangered species. In the case of an endangered species, of course, we worry about all the members disappearing. In the case of an endangered phenomena, we're worried that the migration would be reduced to such a state that it would be unnoticeable or maybe even the migration itself would disappear.

I mean, the predictions are that this is going to be the lowest population ever.

NARRATOR: So far, he's only seen a single Monarch.

BILL CALVERT: Well, it's in pretty good shape. It's got a couple pieces missing out of a wing over here, but otherwise it's in pretty good shape.

There he goes, off to Mexico.

NARRATOR: At the end of the day, Calvert decides to take one more look in a secluded corner of the woods.

BILL CALVERT: Let's see what we've got in there. Oh, wow. Look at them up there! My god! It's just fantastic! Wow, there are hundreds of thousands passing us right now.

Butterflies have dazzled humans for millennia. It's a beautiful little creature. And on top of that, it migrates 2,000 miles, and this just staggers the mind.

NARRATOR: The butterflies have been traveling for six weeks from Canada. But they still face the most treacherous part of the journey. They must fly over hundreds of miles of scorching desert and navigate the towering Sierra Madre Mountains.

BILL CALVERT: Something has to focus them. I think the Sierra Madre Mountains serve that purpose. The mountains stick up pretty high. The butterflies encounter them, and they turn and they follow the mountains. And they can follow the mountains for 900 miles.

NARRATOR: Late October, in Mexico: The butterflies are expected soon, and the Mazahua people prepare to welcome them.

JUAQUIN SANTANA (Sanctuary Guide): It's a privilege that god has sent us this insect. We take advantage of the months that the butterflies are here, to earn our living, because the truth is that we have a community that is quite poor. In this season, we earn enough to make a living. It's not a lot of money, but you can rely on it.

NARRATOR: As they wait for the butterflies, the Mazahua pray for their safe arrival, along with the spirits of their loved ones.

It's now the first week in November.

CHILDREN: Three, four, five, six, seven, eight. There's tons of them.

NARRATOR: After two months and thousands of miles of flight, the butterflies have finally reached safety. Millions of them arrive over the next few days, and the people rejoice.

NARRATOR: Now, with their long journey finally behind them, the Monarchs rest.

They huddle together in huge clusters and cling to the trees for warmth. They'll leave the trees occasionally, to feed on nectar and water, but they return to these clusters and stay here, for almost five months.

When spring arrives, the butterflies bloom again. They open their wings to the Sun, warming up for flight. Most of these Monarchs will travel back to Texas. There they will stop to mate. Each female will lay 300 to 400 fertilized eggs. After the eggs are laid, the parents will die.

When the new generation hatches, it will keep flying north, mating along the way. A third generation will do the same. And almost a full year since the migration began, that special fourth generation of Monarchs will be born in Canada, and the miraculous migration will begin again.

LINCOLN BROWER: I'm frequently asked, "Well, what difference would it make if we lost the Monarch migration?" And I say, "What difference would it make if we lost the Mona Lisa or if we lost Mozart's music?" It's part of our culture.

I think the Monarch butterfly is one of the most magnificent animals in the world. It will absolutely floor anybody the first time they see it, as it did me the first time I saw it. It's one of the wonderful planetary cycles on this Earth. And it's unique in terms of the entire animal kingdom. There's nothing like it. It's really an incredible journey.

NARRATOR: On NOVA's The Incredible Journey of the Butterflies Web site, watch any part of this program, go behind the scenes and more. Find it on PBS.org.

Major funding for NOVA is provided by the following:

One of the biggest challenges for geoscientists is trying to find the oil and gas that's hidden below the surface of the Earth. Lately, our researchers have developed a new technology called R3M. You can't see it and you can't feel it, but the Earth has electromagnetic waves, and if you build the right kinds of listening devices, you can make sense of those waves. So, using those tools, we can supply more of the energy that we need, with less of an impact on the environment.

And David H. Koch. And...

Discover new knowledge: HHMI.

And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

This NOVA program is available on DVD. To order, visit shoppbs.org or call us at 1-800-PLAYPBS.

NOVA is a production of WGBH Boston.