How Do Monarchs Find the Overwintering Sites?
Knowing When To Leave
Where Do They Go?
Finding Overwintering Sites
Orientation is not well understood in insects. In monarchs, orientation is especially
mysterious. How do millions of monarchs start their southbound journey from all
over eastern and central North America and end up in a very small area in the mountains
of central Mexico? We know that they do not learn the route from their parents,
since only about every fifth generation of monarchs migrates. Therefore, it is certain
that monarchs rely on their instincts rather than learning to find overwintering
sites. What kind of instincts might they rely on? Other animals use celestial cues
(the sun, moon, or stars), the earth’s magnetic field, landmarks (mountain
ranges or bodies of water), polarized light, infra-red energy perception, or some
combination of these cues. Of these, the first two are considered to be the most
likely cues that monarchs use, and consequently have been studied the most.
Since monarchs migrate during the day, the sun is the celestial cue most likely
to be useful in pointing the way to the overwintering sites. This proposed mechanism
is called a sun compass. Monarchs may use the angle of the sun along the horizon
in combination with an internal body clock (like a circadian rhythm) to maintain
a southwesterly flight path. The way this would work is illustrated below. For example,
if a monarch’s internal clock reads 10:00 AM, then the monarch will fly to
the west of the sun to maintain a southern flight direction. When the monarch’s
internal clock reads noon (12:00 PM), the monarch’s instincts tell it to fly
straight toward the sun, while later in the day the monarch’s instincts tell
it to fly to the east of the sun.
However, this would have to be combined with the use of some other kind of cue.
If all the monarchs in eastern and central North America maintained a southwesterly
flight, they could never all end up in the same place. It has been proposed that
mountain ranges are important landmarks used by monarchs during their migration.
For example, when eastern monarchs encounter a mountain range, their instincts might
tell them to turn south and follow the mountain range. This kind of instinct would
serve to funnel monarchs from the entire eastern half of North America to a fairly
small region in the mountains of central Mexico.
Scientists have suggested that monarchs may use a magnetic compass
to orient, possibly in addition to a sun compass or as a “back-up” orientation
guide on cloudy days when they cannot see the sun. Studies of migratory birds have
indicated that they register the angle made by the earth’s magnetic field
and the surface of the earth. These angles point south in the Northern Hemisphere
and north in the Southern Hemisphere, as shown in this illustration. While it is
certainly possible that monarch use a magnetic compass to orient during
their migration, there have been conflicting scientific reports about whether monarchs
actually do use this mechanism.
James Kanz (1977) conducted experiments to test the orientation of migratory monarchs
held in cylindrical flight chambers. He reported that the monarchs flew in southwesterly
directions on sunny days, but flew in random directions on cloudy days. He concluded
that monarchs primarily use the sun to orient, and that magnetic orientation was
unlikely, since the monarchs did not appear to be able to orient when they could
not use the sun. However, Klaus Scmidt-Koenig (1985) reported conflicting evidence.
He recorded the vanishing bearings (the direction in which a monarch disappears
from sight) of wild, migratory monarchs, and found that even on cloudy days, most
monarchs still flew in a southwesterly direction. Scientists have since attempted
additional tests of magnetic orientation, but have not yet been able to determine
whether monarchs use the Earth’s magnetic field to orient.
Continue to: How do scientists study migration?