Sun Serves as Map for Monarch Butterflies - Study

[Paul Cherubini]

WASHINGTON (Reuters) - The monarch butterfly depends 
on the sun rather than the Earth's natural magnetic field to 
help it find its way from Canada to Mexico every year, 
researchers said on Monday. 

The study, published in the Proceedings of the National
Academy of Sciences ( news - web sites), found monarch
butterflies would naturally fly southwest toward their
winter home in Mexico under normal sunny conditions.
But if the butterflies' exposure to sunlight were altered,
they would travel in a different direction. 

The findings challenge earlier research that suggests the 
black and orange butterfly uses the Earth's magnetic field 
to help it travel 2,175 miles from eastern Canada to its 
winter home in the Transverse Neovolcanic Mountains 
of Central Mexico. 

"We think it's the beginning of a big scientific story that 
is going to evolve because now we have this nice method 
of studying in the lab their migratory behavior with great 
precision," Barrie Frost, researcher at Queen's University
 in Kingston, Ontario, said in a telephone interview. 

"We see this as the first of many, many studies" to
 understand the migratory patterns of monarch butterflies 
and other insects such as moths and dragonflies, Frost 
said. 

The migration of the monarch has long fascinated scientists, 
who did not discover until the mid-1970s many of the 
butterflies' final destinations. 

The monarch is struggling to recover from an intense, 
unexpected January freeze in Mexico that killed millions 
of the butterflies. 

Frost and co-author Henrik Mouritsen of the University 
of Oldenburg in Germany collected 59 monarch butterflies 
near Lake Ontario, Canada, between Sept. 9 and Oct. 2, 2001,
near their peak autumn migration. 

At a test site in Canada, the monarchs were tethered to the top
 of four plastic cylinders and monitored with a tiny camera 
as a steady stream of air was blown beneath their wings. 

CHECKING THE SUN 

To see if the sun played a role in the monarch's migration, 
the scientists divided the butterflies into three groups that 
were exposed to light at different 12-hour intervals. 

First, the butterflies lived indoors under these conditions for 
at least five days, then they were tested outside in natural 
sunlight. 

The scientists found the first group of monarchs, which 
were exposed to sunlight from 7 a.m. to 7 p.m., flew in 
a southwesterly direction toward Mexico. 

However, when the butterflies were tested between 1 a.m. 
and 1 p.m., and exposed to fewer hours of sunlight, the
monarchs flew toward the southeast. 

The third group of butterflies, tested between 1 p.m. and 
1 a.m., flew toward the northeast. 

The researchers said the butterflies, which check where 
the sun is first thing in the morning, migrated off course
when their "day" started six hours later than usual. 

In a separate series of experiments to test whether monarchs 
use the Earth's magnetic field to navigate, the butterflies 
were placed in a wooden boat house under simulated 
overcast skies. 

During the first 20 minutes, the flying patterns of the 18 
monarchs tested under simulated cloud cover were different
from each other and "all over the map," Frost said. Eleven
of 18 butterflies continued to fly in the same direction 10
minutes after scientists simulated a change in the Earth's 
magnetic field nearby. 

Frost said there is evidence that butterflies and other insects 
can still migrate on cloudy days. "I suspect those are not 
favorable conditions to migrate," Frost said. "But if they do 
fly, there is some evidence ... that there are enough 
polarized rays of light that tell them where the sun is 
even though they can't see it."

 
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