The Migratory Behavior of the Monarch Butterfly

Karen Hanson
Nicki Nguyen
Hien To

I. Introduction:

The awesome sight of hundreds of monarch butterflies flying across a vast expanse of land inspires a feeling of wonder in all who are lucky enough to see such a beautiful sight. However many do not know the ordeal that these creatures must undergo during their life span. The migration cycle of the monarch presents numerous obstacles in which many lose their lives. No one truly understands why these creatures make such a dangerous journey, but there are many hypotheses as to the reason why.

II. Life/Reproduction Cycle:

In a monarch butterfly's, it goes through a complete metamorphosis involving four stages: egg, larva (or caterpillar), pupa, and adult. In addition, each individual monarch contributes to a larger population life cycle, involving many generations. The fall migrants are usually 3 or more generations removed from the monarchs that overwintered in Mexico during the previous winter. In other words, each fall the last generation of monarchs must navigate to a location, perhaps 2000 miles away, which they've never visited.

The majority of monarchs who make the fall journey are in reproductive dormancy. The goal of this initial population is to survive the trip to the overwintering sites in Mexico. After their season in the migratory site, the female monarch reproductive organs become fully developed and mating takes place.

As they migrate north in the spring, they lay eggs on milkweed along the way. These larvae appear in the southern return path in March and early April. This generation will also migrate North following their parents. The reproductive cycle continues and by August to early September, three to four generations will have evolved. So losses which have occured througout the migration cycle will be replenished by this population buildup.

It would be nearly impossible for an individual monarch butterfly to complete this entire migratory cycle. Because of this, their rapid system of reproduction is of great importance to the survival of the species and the completion of the migratory cycle from year to year.

III. Migrational Pattern/ Behavior:

The migration of the monarch butterfly begins in Canada and the northernmost parts of the United States. The fall migration begins in late August ending in the months of November and December. The destination of the butterflies lies in Central Mexico, in the Oyamel forests. Traveling in a southwesterly direction, the monarchs fly east of the Great Lakes and south-southwest in areas west of the Great Lakes. Those that reach the gulf of Mexico follow the coastline in a continuous stream. They continue in a southwest direction eventually reaching the overwintering site in the Transvolcanic Plateau of Mexico. As many as 300 million spend the winter there.

During the migration, monarchs encounter many dangers. These dangers include such things as storms, predators, humans (more accurately, their cars), and simple fatigue. Many butterflies are the casualties of storms and are eaten by birds. Hundreds are crushed by cars crossing the highways, and still many more can be seen limply trying to keep aflight, ready to collapse at any moment. Even after the monarchs arrive at their winter retreats, the danger of storms is still a major factor on the survival. The danger is greater, particularly in Mexico, where temperatures, strong winds, and snow kill thousands.

As mentioned before, this migration takes up to three generations to complete! The exact migratory path is still being plotted today. Scientists are tagging the butterflies, and recording their locations during the months of the fall migration.

During the migration, the monarchs feed extensively on flowers to gain carbohydrates from nectars which fuel daily activities and contribute to the build up of the fat body in the abdomen. This fat supply gives energy to the monarchs on their long journey. Monarchs travel distances as great as 3,100 miles during their migration, traveling roughly 50 miles per day. Monarch flight speeds have been measured at 12 miles per hour. Once they have reached their roosting site, they cluster in large numbers in the branches and trunks of the oyamel trees. While clustering they remain quiescent (they stay relatively sill and maintain low metabolic rates). In mid-February, the monarchs at the roost sites become more active and mating behavior begins. By the end of February, some of the monarchs begin moving northward, by mid-March the roost is usually depleted (Urquhart1987).

This initiates the start of the spring migration. The spring migration starts out with only about half of the original roosting population. Forty to sixty percent of the monarchs die during their stay in Mexico. During the spring migration, the monarch butterflies return to their homes in Canada and the northern most parts of the United States. Along the way, they roost and reproduce, giving rise to new butterflies that will continue the spring flight back.

IV. Migration Mysteries:

Now that the details about the actual migration process of the monarch butterfly has been covered, a common question that most people wonder about is why the monarch butterflies migrate in the first place? Unfortunately, there is no one simple answer to this question. Researchers in this field are collecting field data related to the monarch's biological migratory behavior to try to uncover this mystery. Although there are no definite answers to the question of why monarchs migrate, several hypotheses have been formulated.

The first and most simple explanation is that like migratory birds, monarch butterflies migrate to warmer climates to escape from the upcoming cold weather and the food shortage that will result from the temperature fall. But how do the butterflies "know " that the winters are cold? They don't. What the monarch butterflies sense is the changing amount of light present and the variability of day and nighttime temperatures. With the change of seasons from fall to winter comes the inevitable shortening of the days, longer nights, and also colder nighttime temperatures. Once these characteristics show up, the monarchs leave for their overwintering sites.

The question now is: Why do the monarchs follow such a particular flight pattern and destination? In his book the Monarch Butterfly: International Traveler, entomologist Fred A. Urquhart addresses this issue, and developed a hypothesis based on his studies of monarch butterflies. Mr. Urquhart states that migration is related to three principle factors: (1) monarch larvae feed exclusively on species of milkweed; (2) the migratory pattern is from northeast to southwest; and (3) there is a long history, extending over eons of time, of the distribution of the milkweed species of the genus Asclepias (p.119).

The conclusion is that, " the northwesterly-southwesterly migrations are correlated with the changing distribution of the species Asclepias resulting from changes in the North American land mass over millions of years." (Urquhart 1987) In plain terms, the migrational pattern presently observed originated in the distant past when the monarchs were following the milkweed species which were spreading westward. This east-west movement was eventually incorporated into the monarch's genetic code to produce a cyclical migration related to some as yet unknown response to seasonal changes on the planet.

V. Conservation:

"No other animal is more typical of a healthy environment, nor more susceptible to change, than a butterfly" (Feltwell 1986). It is easy to see why this statement rings true. Monarchs have no control over what happens to their environment, they can only respond to what changes occur, which usually means either surviving or dying. Humans are the ones who have the most control over what will happen to the monarch butterfly population and the biggest problem that the monarchs face is the loss of habitat.

Monarch populations are particularly vulnerable in their overwintering sites in the high-altitude fir forests of the Transvolcanic Range of Mexico; only two of the eleven known roosting sites are well protected from logging (Brower and Malcolm 1991). The oyamel trees on which the monarchs cluster are valuable lumber sources, and local people need additional sources of income (Snook1993). If the roost sites are destroyed, monarch populations are likely to decline precipitously. Protection of the roost sites will be difficult since preservation of the sites and the monarch butterfly, will conflict with the increasing needs and changing priorities of a growing Mexican population.

Milkweed, the host plant of the monarch, is also a concern. In Canada, milkweed has been declared a noxious weed. This means that the plant is considered illegal and cannot be allowed to grow on private or public lands in Canada. Although not labeled noxious in the states, farmers consider the plant a nuisance to crops and often use herbicides to control it along with other weeds. More and more roadsides are being planted in grass instead of being allowed to overgrow with wildflowers and weeds. The result is that butterflies have fewer places in the wild to find nectar and lay eggs.

So what can be done to help preserve the monarch population? The most important issue is to stop the destruction of the monarch's habitat. And the best way of doing that is to set up butterfly reserves, especially in Mexico where monarchs overwinter. Montes Azules is a good example of a butterfly reserve that has been established. This reserve's purpose is to not only preserve butterfly habitat, but to provide the human residents with their economic needs. The motto of Montes Azules is, "Conservation Through Management."

In following Mexico's lead, Canada has taken steps to protect the summer homes of the monarchs. Hundreds of thousands of monarch butterflies spend their summers along the shores of Lake Ontario and Lake Erie and in southern Quebec. Presently, three sights along Ontario's great lakes have been designated as butterfly reserves.

VI. Conclusion:

Monarch butterflies are incredibly fascinating creatures. Only recently have their life and migratory cycles been studied and recorded. Little was known of their enormous struggle for survival against such forces as nature and humans. We hope that this glimpse into the life of the monarch butterfly has helped you to appreciate more than just their breathtaking appearance.

Works Cited

1. Brower, L.P. and S.B. Malcolm. 1991. Animal migrations: endangered phenomenon.
Amer. Zool. 31:265-276.
2. Feltwell, J. 1986. The Natural History of Butterflies. Facts on File Inc. p.105.
3. Snook, L.C. 1993. Conservation of the butterfly reserves in Mexico: Focus on the forest. In Biology and Conservation of the Monarch Butterfly. eds. S. B. Malcolm and M.P. Zalucki. Los Angeles County Museum. No.38 pp.363-376.
4. Urquhart, F. 1987. The Monarch Butterfly: International Traveler. Nelson Hall. pp. 92,119,147,161-163.