Worthwhile reading below despite reporting that Monarchs winter in
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
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
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
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
"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
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
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
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
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
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 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
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