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Migrating monarch butterflies

Monarch Butterfly Navigation During Migration

Imagine this — You are asked to fly 3,000 miles to a destination that you have never visited, you don't have a compass or GPS and you don't even know that this place exists. Imagine all that and remember that you weigh only 1 gram!

Map of monarch butterfly fall migration
Map of monarch butterfly fall migration

Monarch butterflies migrate thousands of miles in the fall from the northern United States and southern Canada to spend the winter in more temperate climates in southern Mexico. Then a different generation of Monarch butterflies return in the spring.

Scientists have puzzled over how this information was integrated and turned into action inside the butterfly's brain.

Orientation

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 5th 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 such as the sun, moon, or stars, the earth’s magnetic field, landmarks such as 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.

Monarch butterflies appear to use celestial cues and the earth's magnetic field as their primary means of navigation during migration.

Navigation Using The Sun

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.

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.

Map of monarch butterfly fall migration
Monach navigation using 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. 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.

Navigation Using A Magnetic Compass

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.

Their magnetic compass that aids in orienting migrants south towards their overwintering grounds during fall migration. Remarkably, the use of the magnetic compass requires short wave UV-light. The light-sensitive magnetosensors reside in the adult monarch’s antennae. While the expert consensus remains that the sun compass is the monarch’s primary compass for navigation, its suggested that migratory monarchs use the magnetic compass to augment their sun compass.

Eastern Monarch Migration

Decreasing day length and temperatures, along with aging milkweed and nectar sources trigger a change in monarchs; this change signifies the beginning of the migratory generation. Unlike summer generations that live for 2 to 6 weeks as adults, adults in the migratory generation can live for up to 9 months.

Most monarch butterflies that emerge after about mid-August in the eastern U.S. enter reproductive diapause (do not reproduce) and begin to migrate south in search of the overwintering grounds where they have never been before.

Western Monarch Migration

Western monarchs gather to roost in eucalyptus, Monterey cypress, Monterey pine, and other trees in groves along the Pacific coastline of California, arriving beginning in late October. The climate of these locations is very similar to that of the Mexico overwintering locations.

The colonies generally break up slightly earlier than those in Mexico, with dispersal generally beginning in mid-February. Less is known about the timing and location of breeding and migratory movement in the western US, but milkweed and nectar plant availability throughout the spring, summer and fall will benefit western monarchs.

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