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Migration

Monarch cluster, Mexico 2012-13 overwintering season (Photo: Wendy Caldwell)

Unlike most other insects in temperate climates, monarch butterflies cannot survive a long cold winter. Every fall, North American monarchs fly south to spend the winter at roosting sites. Monarchs are the only butterflies to make such a long, two-way migration, flying up to 3000 miles in the fall to reach their winter destination. Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. Their migration is more the type we expect from birds or whales than insects. However, unlike birds and whales, individuals only make the trip once. It is their children’s grandchildren that return south the following fall.

Some other species of Lepidoptera (butterflies and moths) travel long distances, but they generally go in one direction only, often following food. This one-way movement is properly called emigration. In tropical lands, butterflies do migrate back and forth as the seasons change. At the beginning of the dry season, the food plants shrivel and the butterflies leave to find a moister climate. When the rains arrive, the food plants grow back and the butterflies return.

How do monarchs know when to leave? 

Monarch butterflies have a complicated life cycle, in that monarchs emerging at different times of the year do different things. Monarchs that emerge in the spring and summer months become reproductive within a few days. Monarchs emerging in the fall are in reproductive diapause, which is a state of suspended development of the reproductive organs. Even though these butterflies look like summer adults, they won’t mate or lay eggs until the following spring. Monarchs have to know when to fly south, and also when to begin the journey back north.

Fall Migration

When the late summer and early fall monarchs emerge from their pupae, they are physically and behaviorally different from those emerging in the summer. The shorter days, cooler air, and milkweed senescence (aging) of late summer trigger changes. In the northern part of their range, this occurs around the end of August, when monarchs begin to emerge in reproductive diapause. Diapause is controlled by the nervous system and by hormones. Environmental factors signaling the onset of unfavorable conditions are involved in triggering this physiological response. These factors include day length, temperature, and host plant quality.

Day Length:  Decreasing daylength is one of the most important factors that cause monarchs to emerge in reproductive diapause. In a series of experiments, Liz Goehring (U of MN graduate student scientist) found that monarchs reared under constant short and long daylengths were mostly reproductive, while those reared under decreasing daylength were more likely to be in diapause. Therefore, she concluded that it is the change in daylength that is an important cue, rather than absolute day length.

Temperature:  Fluctuating temperatures also contribute to the onset of diapause in monarchs, although not as strongly as decreasing day length. Temperatures become cooler in the fall in northern states, but they also begin to fluctuate more. It might still be quite warm in the day, but nights are much cooler than they are in the summer. In the same series of experiments mentioned above, Liz Goehring found that diapause was twice as likely to occur in monarchs reared under a fluctuating temperature treatment where night temperatures were lower (21°C / 70°F) than day temperatures (27°C / 80°F) than those reared under a constant temperature (27°C / 80°F).

Host Plant Quality: Another cue that monarchs might use is host plant quality. As cold weather approaches, plants begin to senesce and their leaves become yellow and dry. Liz Goehring manipulated the quality of potted tropical milkweed plants (Asclepias curassavica) grown in a greenhouse so that some were young (good quality) and some were old (poor quality). She found that monarchs reared on old plants were more likely to emerge in diapause than ones reared on young plants. However, in another experiment in which she compared cuttings of wild and greenhouse grown common milkweed (A. syriaca), host plant characteristics had no effect. This could have been because cuttings may not convey plant quality cues as accurately as uncut plants. It is also possible that a host plant effect is expressed differently in A. curassavica and A. syriaca. However, the first experiment suggests that host plant quality can be an important cue in the onset of diapause in monarchs.

These cues (decreasing day length, fluctuating temperature, and poor host plant quality) act together to induce diapause in monarchs. However, diapause can occur in monarchs exposed to only one cue. Making use of more than one cue to assess the current and near future habitat suitability could be a better strategy in unpredictable environments.

Spring Migration

North American monarchs spend the winter roosting in trees at sites in Mexico and southern California. They cluster together, covering whole tree trunks and branches. As the winter ends and the days grow longer, the monarchs become more active and begin a 3-5 week period of intense mating activity. In Mexico, they begin to leave their roosts during the middle of March, flying north and east looking for milkweed plants on which to lay their eggs.

The timing of diapause completion seems to vary considerably across individuals within an overwintering colony. Overwintering populations are comprised of monarchs coming from a wide geographic area, subjected to a wide range of environmental conditions. Consequently, these monarchs are not all the same age and haven’t experienced the same environmental conditions. Interestingly, monarch diapause appears to last longer in females than in males.

There are several factors that may influence the progression of diapause in monarchs and trigger the development of the reproductive organs. The rate of diapause development in insects is often driven by temperature. Other factors that may influence diapause development include day length, moisture, food, mating, host plant availability, and stimulation by body damage. Once diapause is complete, the insect may continue to remain dormant until environmental conditions are suitable.

Availability of Milkweed: Monarchs overwinter in patches of forest, which typically contain few, if any, milkweed plants. Optimally, monarchs should not begin development of their reproductive organs unless they will soon have access to milkweed, as females cannot lay their eggs on any other type of plant. Liz Goehring (U of MN scientist) conducted a series of experiments on post-diapause reproductive development in monarchs. She found that most females without access to milkweed lacked mature oocytes while the majority with access to milkweed developed mature oocytes within 3-4 days. Therefore, access to milkweed stimulated post-diapause reproductive development. However, all females developed mature oocytes within 2 weeks of experiencing warm temperatures, indicating that milkweed is not required for diapause completion.

Monarchs at the Mexican overwintering sites become very active as the winter ends in late February and may begin mating. (Photo: Wendy Caldwell)

Mating: Females must mate before they can lay fertile eggs, so females may be more likely to complete diapause and become reproductively mature after they have mated. This has been found to be the case in monarchs; postdiapause females produced mature oocytes more rapidly if mated. However, it also is not required for oogenesis in monarchs. Females can complete diapause and become reproductively mature before they have mated.

Day Length: Because decreasing day length is very important in signaling monarchs to enter diapause, one might suspect that increasing day length might be important in signaling them to complete diapause. However, Liz Goehring found no evidence to support this hypothesis. In her series of experiments, increasing day length had no effect on monarch post-diapause ovarian development, although it may be important in triggering other changes related to diapause completion.

Body Condition: Body condition might be important in determining when monarchs complete diapause. The longer a monarch remains in diapause, the more energy it uses. One might think that monarchs in poor condition would complete diapause earlier, when they begin to deplete their energy reserves. Several researchers have found this to be true of males. They have found that males mating early in the season (who have completed diapause early) are thinner and more tattered than males still roosting early in the season (who are still in diapause). However, Liz Goehring found evidence to support the opposite hypothesis in female monarchs. She collected hundreds of females at an overwintering site in Mexico, and found that larger females were more likely to have initiated ovarian development, suggesting that larger females complete diapause earlier.

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 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 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 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, 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."

How do scientists study migration? 

The amazing phenomenon of monarch migration has fascinated scientists for decades. Many methods have been employed in the attempt to unravel the mystery of monarch migration, including tagging programs, monitoring programs, and more technical chemical methods. Many of these programs have involved citizens of Canada, the United States, and Mexico in a cooperative effort to learn more about this remarkable journey.

Tagged monarch (Photo: Wendy Caldwell)

Tagging: Dr. Fred Urquhart of the University of Toronto began a tagging program in the 1930’s. After thousands of tagged butterflies and several decades of work, the overwintering roosts in the mountains of central Mexico were finally discovered in 1975. Although local residents had known about the roosts for generations, no one from outside the area had reported them. This collaborative effort continues today as we attempt to learn more about this migratory phenomenon.

Dr. Chip Taylor of the University of Kansas has continued the study of monarch migration through a different tagging program called Monarch Watch. Started in 1991, Monarch Watch is a collaborative network of hundreds of thousands of students, teachers, volunteers and researchers dedicated to the study of the monarch butterfly. These participants tag tens of thousands of monarchs each year throughout Canada and the United States. Through the recovery of tagged monarchs, we have learned a great deal about the routes monarchs take and how fast they move.

Monitoring Programs: Other organizations have formed with the goal of monitoring monarch migration in a way that doesn’t require catching and tagging butterflies. Journey North is one such organization. It was established in 1991 to with two goals in mind: to improve science and math education and to study several species of migratory animals. Journey North involves school children from every state in the United States and 7 Canadian provinces. These students report their first sightings of monarch butterflies every spring. Through these reports, we can learn about when and where monarchs travel as they migrate north in the spring.

Texas Monarch Watch is another organization that enlists citizens to collect data on monarch migration. Dr. Bill Calvert, of Texas Parks and Wildlife, organized this program in an effort to understand the movement of monarchs through Texas during their fall migration to Mexico and their spring migration northward. Volunteers call in reports of monarch sightings, providing information about where, when, and how many monarchs they have seen. This information helps us learn about major flyways through Texas and, by comparing sightings over several years with weather patterns, we can learn about how weather influences monarch migration.

Other monitoring programs include the Monarch Monitoring Project run through the Cape May Bird Observatory Center for Research and Education and the Western Monarch Migration Project run by Dan Hillburn of the Oregon Department of Agriculture.

Stable Isotopes: Stable isotopes are different versions of regular atoms that have different masses. For example, the common isotope of hydrogen has one proton and one electron. The hydrogen isotope calleddeuterium also has a neutron, and is almost twice as heavy as the common hydrogen isotope. Scientists can use these differences between atoms of the same element to identify the "signature" of the breeding grounds from which a monarch originated. They can do this because different parts of the world have different amounts of the various isotopes of a particular element. Rainfall is the likely cause of the difference in hydrogen isotopes, but other weather patterns and geology can cause variation in hydrogen and other isotopes. When plants take up water, they obtain a isotope pattern that reflects that of their geographical region. When monarch larvae eat milkweed plants, they "inherit" this isotope pattern as well. Scientists can first identify the isotope "signature" of various geographical regions, then determine the isotope pattern of a monarch to roughly determine its origin. (For a more detailed description of how this works, visit the "Why Files" page on stable isotopes and monarch migration.)

Leonard Wassenaar and Keith Hobson of Environment Canada, Saskatoon, Canada, conducted a field study in which they collected monarch butterflies from the 13 known overwintering sites in Mexico and analyzed each monarch to determine its isotope pattern. They then matched these isotopic patterns with "signatures" they had identified previously. The found that about half of the 597 monarchs collected originated in the Midwestern corn and soybean belt.

Genetics: Genetic analysis is becoming an increasingly popular method to investigate the molecular-genetic basis of migration. Kronforst et al. 2014 used gene sequencing to compare 101 Danaus genomes from around the world. Comparative genomics analysis using Single Nucleotide Polymorphisms (SNPs) revealed variation in Danaus genomes that illustrated a monarch evolutionary tree. This tree revealed that the North American migratory population resided at the base of the tree signifying it as the most closely related species to the common ancestor of all monarchs. Their results suggest the monarch began in the southern USA or northern Mexico, making annual migrations as glaciers receded. These genetic analyses also allowed researchers to infer the distribution patterns of non-migratory monarch populations. Genetic analysis was also utilized to identifying genes involved in migration (see "How do monarchs find the overwintering sites?).

References

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