Every year, monarch butterflies undertake what seems like an impossible journey.
By the millions, they leave their summer breeding grounds in the United States and Canada to fly thousands of miles to a small area of alpine forest in central Mexico.
Ecologist Lincoln Brower has been studying monarchs for almost 60 years.
He was in St. Louis this week to accept the University of Missouri-St. Louis Harris Center Conservation Action Prize. It recognizes his lifetime of work devoted to the conservation of this remarkable insect.
I met Brower during his visit. We talked about the monarch's remarkable migration and the threats that may cause it to disappear. You can hear part of our conversation here:
Brower has been fascinated by butterflies ― and monarchs in particular ― for most of his 82 years. He calls their migration, "the most complex insect migration in the world."
"It's very bird-like," Brower said.
In the fall, as the days get cooler, and the days get shorter, it turns on the migratory instinct of the monarch. And instead of breeding, which they do over the spring and summer, they go into their migratory mode. And they know to go to this pin-point on a map in Mexico.
That "pin-point" is a mountainous region of oyamel fir and pine forest in central Mexico. The butterflies head high up into those mountains, above 10,000 feet, to spend the winter clustered on branches and tree trunks.
Then, after about five months, when spring warms up the forests, the monarchs start their voyage back to the U.S. "Those butterflies have flown almost 5,000 miles in the course of their lifetime," Brower said.
Each year, a new generation of monarchs makes that arduous trip. The young monarchs have never been to Mexico before. But they always end up in same patches of forest, on the same 12 mountain peaks.
"Nobody knows how they do it," Brower said. Still, part of the answer involves an internal solar clock.
"Butterflies and birds, both can tell time. And they have clocks in their brains which enable them to look at the angle of the sun. If they followed the sun they’d start going towards the east, and as the sun went over the sky they’d end up headed to the west. But they compensate. And the time clock in their brain enables them to keep a constant angle and head in the right direction."
The thing is, Mexico is a pretty big place. The great mystery, Brower says, is how the monarchs find their way with such precision on that last leg of the journey, to those same mountaintop forests that sheltered their grandparents the previous winter. Brower has a theory about that. You can listen to it here:
With the help of thousands of volunteers, scientists have tracked the monarch's migration through tagging programs. "People paste little labels on the butterflies," Brower said. Then, if somebody finds a dead monarch with a numbered tag on it, they report where and when they found it.
But the future of tracking animal migrations may be much more high tech. Satellite telemetry is already being used to follow the migration patterns of larger animals. Soon, a radio receiver on the International Space Station may allow the movements of something as small as a butterfly to be tracked in a similar way.
In their winter habitat, the monarch butterflies cover the branches and trunks of the oyamel fir trees that dominate the alpine forests of central Mexico.
Brower describes being surrounded by millions of monarchs as "a really strong spiritual experience." He still vividly remembers the first time he saw the monarchs clinging to the branches of a Mexican forest, in January, 1977. "I was overwhelmed," Brower said. "I walked down this beautiful mountain path, and suddenly the color of the forest changed from green to gray. And I realized I was looking at a wall of monarch butterflies."
Brower says there can be as many as 10,000 monarchs on a single small branch ― all completely silent. But, he says, sometimes they all take flight at once, their wings rubbing against each other. Then you can hear them: a "beautiful, susurrant sound."
But monarch butterflies are in trouble.
Brower says the high mountain forests that shelter them during the winter months have suffered large-scale deforestation. The images below, taken by a commercial satellite, show the effects of illegal logging in the Monarch Butterfly Biosphere Reserve in central Mexico.
The circles on the 2004 image show the approximate locations (not to scale) of a monarch colony in different years. Using these and other images, Brower and his colleagues estimated that 1,110 acres of the monarch's forest habitat had been clear cut or thinned by 2008.
Brower says the Mexican government has taken steps to get illegal logging under control, but it hasn't been entirely successful.
Brower blames more recent monarch population declines on a different source: Widespread herbicide use in U.S. agriculture over the past decade. He says crops like corn and soybeans have been genetically engineered to tolerate weed-killing chemicals like glyphosate (trade name, Roundup).
The herbicides don't kill the monarchs directly. Instead, they kill a key plant in the butterfly's lifecycle: milkweed. Monarchs lay their eggs on milkweed plants during the spring and summer, and their caterpillar larvae use them for food.
Brower says monarchs fell to their lowest numbers ever this past winter in Mexico.
"In a good year, there would be a billion," Brower said. "This year, we think it's gotten down to 33 million. It's a 90 percent decline."
Scientists can't count the monarchs directly, so they use the area they cover in Mexican forests as a surrogate. At their peak in the mid-90s, monarchs inhabited about 52 acres ― an area equivalent to about 39 football fields. This past year, they were found on less than two acres: about a football field and a half.
Brower says monarch butterflies are still widespread, ranging from Australia to South America and the Caribbean.
"The species will not go extinct," Brower said. "But if you think about this migration, it's a unique biological phenomenon. And that phenomenon could go extinct."
Follow Véronique LaCapra on Twitter: @KWMUScience