Butterflies Guided By Body Clocks, Sun Scientists Shine Light on Monarchs' Pilgrimage
By Guy Gugliotta
Washington Post Staff Writer
Friday, May 23, 2003; Page A03
Scientists have figured out for the first time how the monarch butterfly uses an intricate interplay of its internal body clock and the sun to guide its extraordinary annual pilgrimage from eastern and midwestern North America to its tiny winter home in the pine woods of central Mexico.
Researchers said yesterday that the monarchs -- the orange-colored denizens of summer gardens all over the United States -- use their body clocks to orient themselves in a southwesterly direction as they fly toward Mexico, guided by the position of the sun as it moves across the autumn sky.
The researchers also found that the sun's ultraviolet light is important, perhaps crucial, in stimulating the butterflies to begin their trip south, but that it has no apparent affect on the functioning of the insects' body clocks.
The analysis breaks new ground in highlighting the possibly crucial role that biological rhythms play in the migratory process, but it still does not explain other important aspects of one of animal biology's most unusual phenomena.
University of Kansas entomologist Orley "Chip" Taylor explained that the monarchs are tropical insects that have evolved in such a way that they emigrate every spring from a 30-acre stand of mountain pines in the Mexican state of Michoacan, west of Mexico City, to populate back yards as far north as Winnipeg, Canada.
Three or four generations later, nearly 100 million of the migrants' descendants -- with no clue where their ancestors came from -- emerge from chrysalises in early September and begin a 21/2-month flight home: "Monarchs all over the country take different bearings," Taylor said. "They are capable of distinguishing longitude, and we don't know how."
"We've known for a long time that the monarchs use the sun compass to navigate," said researcher Steven M. Reppert of the University of Massachusetts Medical School, who led the team publishing the new findings. "What we wanted to do was highlight the importance of the circadian clock in this process and obtain more direct evidence of its role."
The circadian rhythm, also known as the biological or body clock, is a regulatory mechanism within virtually all animals -- including humans -- that allows them to adjust cycles of rest and activity to the solar day. The word circadian derives from the Latin phrase circa diem, which means "about a day."
The clock "runs" all the time but is periodically reset or "entrained" to keep it synchronized with the varying cycles of light and darkness. The bigger the disturbance in the cycle, the longer it takes to reset. Jet lag, for instance, requires a more serious adjustment than daylight savings time.
Reppert, leader of the University of Massachusetts team that reported its results in this week's edition of the journal Science, said the researchers decided to examine how circadian rhythms relate to the monarchs' ability to migrate to Mexico -- flying more than 2,000 miles in some cases.
To test what role circadian rhythms play in the migratory cycle, Reppert's team trapped adult monarchs and placed them for a week in a laboratory chamber that simulated an early September day in the eastern United States -- light from 7 a.m. to 7 p.m.
When the butterflies were subsequently taken outdoors, they immediately oriented themselves in a southwesterly direction -- pointing toward Mexico with the morning sun over their left "shoulders."
But when the team advanced the lab's light-dark cycle by six hours -- from 1 a.m. to 1 p.m., the butterflies oriented themselves in the morning sun as if it were late afternoon, putting the sun over their right "shoulders" and pointing in a southeasterly direction.
That largely replicated earlier experiments, Reppert said. "The important thing was to take animals from the two groups and put them in [the lab] in constant light."
When these butterflies were brought outside, "they flew directly at the sun," Reppert said, their circadian rhythm totally disrupted. "We broke the clock," he said. "The animals no longer sensed the change in the lighting cycles." To orient themselves in relation to the sun, a key to migration, they needed a well-defined circadian cycle.
To find how this disorientation affected the inner workings of the monarchs' clock, the team also examined a butterfly gene, the period, or per, gene, that toggles "on" and "off" to regulate the cycle in many animals. When the light was constant the gene "wasn't being expressed," Reppert said. "This told us that this molecular gear may offer an entry point to study the genetic machinery of the entire migratory process."
Susquehanna University cell biologist David Richard said the per gene is "turned on and off sequentially during the light-dark cycle," along with another gene, as the day passes. "One would presume," he said, that the way the genes are expressed at different times "would impinge on migration behavior."
Finally, in a separate experiment, the U-Mass. team allowed its butterflies to fly in full sunshine for a minute, then applied a filter that screened out ultraviolet rays. The butterflies immediately stopped flying. Reppert said the team also found, however, that the absence of ultraviolet light did nothing to disrupt circadian rhythms.
Richard, however, suggested that the experiment could have implications for the debate over the relative effects of the sun's rays and the Earth's magnetic field on the monarchs' navigation techniques.
Taylor described the U-Mass. experiments as "really admirable in the way they were able to relate the circadian cycle to the sun compass," but he cautioned that orientation was not the same as navigation.
"Navigation implies directed flight toward an unseen goal," Taylor said. "A monarch in Georgia will be flying 270 degrees [due west] to get to Mexico, while a monarch at the same latitude in Texas will fly 220 degrees [southwest]. You tell me how that happens."