Contributed by Dr. Bill Calvert
by which monarch navigate from almost continent-wide breeding grounds
to a dozen or so tiny areas of about one acre in extent in Mexico's Transvolcanic
Belt has mystified monarch researchers for over half a century and remains
one of the most intriguing questions of monarch biology.
One group of scientists from the University of Kansas* has shown that
a sun compass is involved in guiding the monarchs to Mexico. This means
that the monarchs are using the sun as a celestial cue.
Here's how this would work: Since the sun changes its "azimuth"
(its direction with respect to north) during the day, unless monarchs
always fly directly towards the sun (or at some constant angle to it),
they must be able to compensate for the changing azimuth of the sun during
the day. (Imagine for a minute: Where do you think the monarchs would
end up if they simply followed the sun?)
To do this they must know what time it is -- not hours and minutes, but
whether it's early morning, late morning, mid day etc. In short they must
possess what researchers call a biological clock.
To show that monarchs have a sun compass the Kansas researchers clock-shifted
a group of monarchs and compared the direction of their migration with
two other control groups that had not been clock-shifted.
One control group was subjected to the same conditions that the clock-shifted
group had been subjected to-- a chamber where daylength was controlled
by artificial lighting. (The daylength was the same as natural daylength
for the season. The beginning and end were the same as natural sunrise
The second control group was wild migrants captured and maintained on
a natural daylight regime.
The scientists released the butterflies, and compared the flight direction
of the three groups of monarchs. They carefully recorded the vanishing
azimuths of the three groups and also noted the monarchs' body orientation.
What is body orientation and why do you suppose that they needed this
measurement? Monarchs are light creatures weighting during migration about
600 milligrams. They have relatively huge wings. Even light winds blow
them about. They will not travel during high winds that oppose their migration.
When light winds oppose their flight, they may cease flying or fly just
above the ground. How do you think flying near the ground would help them
against opposing winds?
When the winds have a northerly component they are up and off, flying
to the southwest. Although the winds have a northerly component, they
are seldom blowing perfectly to as to carry the monarchs directly to the
southwest. Monarchs compensate for imperfections by facing into the winds
and "crabbing" in the desired direction. The direction they
are facing may be a better index of the direction they want to go than
the wind direction. This is why researchers pay attention to the direction
the monarchs are faced.
The clock-shifted monarchs were faced nearly 90 ° differently than
either of the control groups. If for example, all groups were released
at 10:00 AM, the controls groups were faced at azimuths that were large
angles with respect to the sun as would be appropriate for butterflies
flying southwest in the morning when the sun was in the east. The clock-shifted
group were faced at azimuths much closer to the azimuth of the sun as
would be appropriate for butterflies flying six hours later at 4 PM when
the sun was in the west. Clock-shifting experimental animals is a much
used method especially in bird research to demonstrate that the animals
are using the sun to orient.
It has been proposed that monarchs also use the earth's magnetic field
to orient as many birds do. But so far no one has produced consistent
evidence to demonstrate this
* The authors were S.M. Perez, O.R. Taylor and R. Janders. 1997. A sun
compass for monarch butterflies. Nature 387:29.
This! Measure the Azimuth of the Sun
- Go outside
on a sunny day at several different times. Measure the azimuth of the
sun (its direction with respect to north). Record the angle and time
for each measurement.
- As Dr.
Calvert suggested--Imagine for a minute: Where do you think the monarchs
would end up if they simply followed the sun?