Butterfly secret revealed

The monarch may rely on its biological clock to navigate its migration to Mexico.

Somewhere over the Pacific Northwest this morning, a monarch butterfly is skipping uncertainly across the landscape. A few months ago, its grandparent left this misty stand of eucalyptus and cypress on the California coast some 700 miles away. By mid-autumn, its grandchild will begin the trip back, perhaps even to the same branch, to complete one of the most remarkable migrations in the animal kingdom.

For years, few answers have emerged as to how a creature that weighs less than a dime and has a brain smaller than a pin head could navigate its annual circuit with GPS precision.

Today, however, scientists say they have found their first big clue - evidence that monarchs rely on their biological clock to precisely chart the motion of the sun and calibrate their course.

More work remains, scientists say, before they can comprehend the migration entirely. But they hope this discovery will lead them to conclusions about an enigma that has long captured the imagination of backyard Darwins across North America.

"This is a really big step ahead in figuring out how monarchs find their way to [California and] Mexico and understanding such a complex process," says Lincoln Brower, a biologist at Sweet Briar College in Virginia.

In this week's issue of the journal Science, the authors of a new report suggest that monarch's circadian clock - the biological clock that sets sleep patterns in humans - is crucial to its ability to know where it is going. In other words, the monarch's clock inherently knows how to compensate for the hourly and daily movement of the sun across the sky, allowing the butterfly to fly in the exact direction it desires.

The study brings the way the circadian clock "tells an animal how to orient itself in space," says author Steven Reppert of the University of Massachusetts Medical School in Worcester. "It's like a four-dimensional clock."

What it does not answer is how monarchs know which direction they want to fly in the first place, and how they know when they have arrived at the exact wintering grounds in Mexico and coastal California that only their great-great grandparents saw.

It is a mystery that has given the monarch a unique niche in American culture, from nationwide counts of butterflies to migration maps pinned to cork boards in countless elementary schools.

Yet, here in Pacific Grove, the oldest known wintering site for monarchs in the United States, the creature's mystique has shaped the town's very identity. Street signs at the entrances to this wood-shingled Cape Cod of the California coast welcome drivers to Butterfly Town, USA.

Kindergartners outfitted with orange-and-black wings have trooped down Lighthouse Avenue in the Butterfly Parade every year since 1939. Harming a butterfly has been a crime punishable by a $1,000 fine since that same year. And when the city considered razing a butterfly sanctuary in 1990, 69 percent of citizens voted to raise taxes by $1.2 million so they could save it.

Today's study will not explain why the monarchs return year after year to this inauspicious grove amid mildewed houses and a pink motel, where palm fronds rattle like castanets in the moist Pacific wind.

Noting that monarchs spend almost exactly 180 days in their winter roosts in the Mexican highlands, Dr. Brower wonders if monarchs' circadian clock spins 1 degree each day, sending them due south in autumn and due north in spring.

Others suggest that the clock might govern huge portions of monarch behavior. The monarchs born in early autumn, when days are short, for instance, live for eight months so they can migrate all the way south. Every other generation lives only about four to eight weeks.

"The study explains the underlying mechanism," says Orley "Chip" Taylor, an ecologist at the University of Kansas in Lawrence. "Now, we begin to examine it."

Moreover, Dr. Reppert's study represents the inaugural use of a new "flight simulator" that tethers butterflies so that they fly in place inside a drum. For the first time, it allows scientists to observe how butterflies react when they change the light source and location. In today's report, Reppert says he and his colleagues discovered that the butterflies became disoriented when exposed to constant light.

The success of the contraption, others add, will give scientists a new tool to the migration.

"Now we will be able to really nail down these things experimentally," says Brower.