Magnetic
Cues?
"Scientists have suggested that monarchs may use a magnetic compass
to orient, as has been demonstrated in some migratory birds. However,
Mouritsen and Frost (2002) showed that migratory monarchs . . . did
not respond to magnetic field shifts, suggesting that monarchs do not
use the earth’s magnetic field to orient during migration."
— Ecology Online Sweden
Still
a Puzzle
“It is annoying that the press implies the problem has been solved.
“The findings do not suggest a navigational mechanism by which monarchs
from the eastern two thirds of North America can find 13 or so small areas
in the Transvolcanic Belt of Michoacan. Using a sun
compass can lead (orient) monarchs in a general direction with respect
to the sun. But this mechanism alone will not get them to Michoacan. Since
the sun is constantly changing its position in the sky, monarchs must
and apparently can compensate for the changing position of the sun.
— Dr. Bill Calvert, monarch biologist
Sun
Compass
"Monarchs may use the angle of the sun along the horizon in combination
with an internal body clock to maintain a southwesterly flight path. 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.
[But] 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."
— Dr. Karen Oberhauser, Monarch Lab
More
Mystery
“If monarchs just used the sun to orient south, few would make
it to Mexico since most would end up in the Gulf of Mexico. Although
the papers on sun compass orientation are valuable contributions toward
our understanding of the monarch migration, how monarchs set their bearing
remains a mystery and the one that needs to be solved before we fully
understand how they navigate during the migration."
— Dr. Chip Taylor, Monarch Watch
New
Research: Brains and Eyes Combine (August 2005)
Light in general is essential to the functioning of the "biological
clock" in the butterfly brain – governing . . . its "signal"
to migrate. But the researchers discovered that it is specifically the
ultraviolet (UV) band of light [which humans can't
detect] that is crucial to the creature's orientation. The butterflies
have special photoreceptors for UV light in their eyes.
They proved
that this ultraviolet "navigation" is crucial by placing butterflies
in a "flight" simulator. When a UV light filter was used in
the simulator, the butterflies lost their orientation.
Further
probing revealed a key wiring connection between the light-detecting
navigation sensors in the butterfly's eye and its brain clock. Thus,
it was shown that input from two interconnected systems – UV light
detection in the eye and the biological clock in the brain
– together guide the butterflies "straight and true"
to their destination.
— From press release, The Hebrew University of Jerusalem
Geographic
Cues
"I personally think that the Mexican mountains focus the migration
once the butterflies enter Mexico."
— Dr. Bill Calvert, monarch biologist
(More of Bill's comments on Monarch
Orientation Mechanisms)