It is thought that monarchs were originally tropical butterflies that underwent range expansion. Scientists are not sure how long the monarch’s spectacular annual migration to Mexico has been occurring; it may be as old as 10,000 years (when the glaciers last retreated from North America) or as young as a few centuries. The earliest reports of overwintering clusters of monarchs in the United States date back only to the 1860’s in California.
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 two to six weeks as adults, adults in the migratory generation can live for up to nine 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. From across the eastern U.S. and southern Canada, monarchs funnel toward Mexico. Along the way, they find refuge in stopover sights with abundant nectar sources and shelter from harsh weather.
Eastern and Central North America
Monarchs that spend the summer breeding season in eastern North America (including states and provinces east of the Rocky Mountains: central and eastern Canada, mid-western and eastern United States) migrate to the Transvolcanic mountains of central Mexico. Many millions of monarchs from these regions fly south to Mexico each fall. Their flight pattern is shaped like a cone as they come together and pass over the state of Texas on their way south. In massive butterfly clouds, they sweep up into the mountain ranges of central Mexico. In 1975 the scientific community finally tracked down the wintering sites of the monarchs in Mexico. Until then, the monarch butterflies’ winter hideouts had been a secret known only to local villagers and landowners.
In Mexico, monarchs roost in Oyamel fir forests, which occur in a very small area of mountain tops in central Mexico. Overwintering sites are about 3000 meters (almost 2 miles) above sea level and are on steep, southwest-facing slopes. Because monarchs need water for moisture, the fog and clouds in this mountainous region provide another important element for the winter survival of the monarchs. The butterflies choose spots that are close to, but not quite, freezing. They cluster together, covering whole tree trunks and branches, and cling to fir and pine needles. The tall trees make a thick canopy over their heads. Protective trees and bushes soften the wind and shield the butterflies from the occasional snow, rain, or hail. Each of the above elements is important to the butterflies, making up the monarch habitat – trees in which to roost, other trees and shrubs to protect them, the cool air, and the presence of water.
Monarchs do not reproduce until the spring, so they do not rely on milkweed during this time. They do seek out water and drink nectar from flowers in the overwintering sites. (documents are in Spanish): PLANTAS de la Reserva de la Biosfera Mariposa Monarca, /PLANTAS.
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Western North America
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, especially in California, Nevada, Idaho, and Oregon, states that appear to be important sources of western monarchs. In areas of the desert southwest, monarchs use nectar and milkweed plants throughout much of the year. For western monarch information and resources, visit the Western Monarchs category on the Monarch Joint Venture Downloads and Links page.
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How do monarchs find the 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.
Sun Compass: 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.
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. 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.
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 Schmidt-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 attempted additional tests of magnetic orientation, but were not able to determine whether monarchs use the Earth’s magnetic field to orient.
However, researchers from the Reppert Lab (2014) showed that migratory monarchs indeed possess a 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 (previous magnetic compass experiments failed to account for light at this range). With UV-light being allowed to enter the flight simulator, eastern migratory monarchs consistently oriented themselves south. The light-sensitive magneto sensors reside in the adult monarch’s antennae. While the expert consensus remains that the sun compass is the monarch’s primary compass for navigation, the authors suggest migratory monarchs use the magnetic compass to augment their sun compass.
Genetics: Upon dispersal, the Central and South American, Atlantic, and Pacific populations lost the ability to migrate. This prompted researchers to identify the gene regions in North American monarchs that appeared highly differentiated from non-migratory populations. Kronforst et al. (2014) identified 536 genes significantly associated with migration. One single genomic segment appeared to be divergent in the non-migrating populations and was extremely different from the North American population. One gene, collagen IV alpha-1, showed high divergence between migrating and non-migrating populations. Collagen IV alpha-1 is an important gene for muscle function, and divergence of this gene implicates selection for different flight muscles between migrating and non-migrating populations. Surprisingly, Collagen IV alpha-1 was down regulated in migratory monarchs, perhaps preparing them for lengthy flight. Furthermore, migrating monarchs had low metabolic rates compared to non-migrants as a consequence of flight muscle performance, lowering energy expenditure in migrating monarchs muscles. This evidence led researchers to conclude that changes in muscle function afforded migrating monarchs the ability to fly farther and use their energy more efficiently. Dr. Kronforst used the analogy of a marathon runner vs. a sprinter, "Migrating butterflies are essentially endurance athletes, while others are sprinters."
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Monarch Joint Venture collaborates with partners to deliver habitat conservation, education, and science across the United States. Their vision is thriving monarch populations that sustain the monarch migration into perpetuity and serve as a flagship for the conservation of other plants and animals. For more information, visit their website at: monarchjointventure.org.
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(Image courtesy of the Xerces Society)