Abstract
Each year, millions of monarchs migrate south to high altitude mountains
to overwinter in Central Mexico. Migrating monarch butterflies have been
members of the endangered species list since the early 1980s. Before human
impacts began affecting monarch survival, the only stresses that monarchs
faced during migration were freezing temperatures and predation. Many studies
have been performed to gain a better understanding of exactly how monarchs
tolerate freezing temperatures. Discoveries have been made about monarch
lipid concentration in reproductive and migratory butterflies and how these
concentrations affect their cold tolerance. It was found that an increased
lipid concentration increased the internal freezing temperature. Monarch
butterflies have been captured and manipulated in the laboratory to perform
experiments with temperature reduction and how this leads to increased
ability to withstand freezing temperatures in the monarchs overwintering
sites. It was discovered that moisture levels both in and outside the body
affect the ability of the monarch to survive temperatures below freezing.
Monarchs with excessive moisture content outside of their exoskeleton have
an increased chance of being injured or killed at below freezing temperatures.
The supercooling point was raised from -8.2 to -4.7 degrees when external
moisture was present. No matter what the physiological make-up of the monarch
is, the species cannot tolerate internal freezing. Thus, they are considered
to be freeze intolerant. Observations in the protected overwintering sites
in Mexico have also been made to learn about the effects of bird predation.
Environmental impacts largely affect monarch migration, but these animals
have adapted special techniques to insure their survival from these stresses.
Now, the main threat to monarch survival during migration is humans. It
has recently been discovered that the forest canopy in the monarchs overwintering
sites greatly influence their survival rates. A loss of protection from
the forest canopy leads to an increase of external moisture and also increases
predation. The forest canopy which is an essential part of the monarchs
survival is being threatened by logging industries in Mexico. This research
hopes to explain the details of the phenomenon of monarch migration as
well as cover the reasons why it is crucial to protect their overwintering
sites in Central Mexico.
Introduction
The monarch butterfly, Danaus plexippus, is a member of the family Nymphalidae. The objective of the monarch, as in any other organism, is to adapt mechanisms that will ensure their survival and reproduction. The more sexually producing offspring that survive, the greater the success of the species. The monarch butterfly has adapted a successful migratory pattern that takes over 100 million of them up to more than 3600 km per year in approximately 75 days (Brower, 1996). Migrating monarch butterflies have been members of the endangered species list since the early 1980s (Garcia and Zamora, 1997).
Monarchs are found all over the world and several populations exist in the United States. Monarchs have spanned great distances and have known to migrate throughout America to any place they can find milkweed, their primary source of food. Monarchs live on both sides of the Rocky Mountains and have different migratory destinations depending on the region they come from (Brower, 1996). The monarchs that spend the summers west of the Rocky Mountains spend their winters on the coast of California. The monarchs from east of the Rocky Mountains are the individuals who make the long journey to the high altitude (3500m) forests of Central Mexico.
Throughout the summer approximately 3 to 4 generations of monarchs are born. It is only the last generation that develops in August that makes the journey south. The life expectancy for the first summer generations of monarchs is only a few weeks. The last generation that survives the migration may live more than 135 days, the length of the overwintering period (Alfonso-Mejia et al., 1998).
The monarchs' overwintering sites in Mexico were discovered in 1975
by Urquhart. For many years it was known that monarchs migrated southward
during the winter but a destination as far as the trans-volcanic mountain
range in Mexico was unheard of. The monarchs were tagged by Urquhart and
other researchers and were recaptured or returned to them when found. They
had many returned from the southwestern United States, but none from Mexico
until 1975 (Wassenaar and Hobson, 1998). After the overwintering sites
in Mexico were finally discovered many questions that had puzzled scientists
for years began to be answered.
How migrating to Mexico benefits the monarch butterfly
Why do monarchs choose to migrate for over 3600 km every year? Monarchs rely on milkweed for energy to live and reproduce. In the fall, especially in northern monarch habitats such as Canada, the temperature begins to drop and milkweed begins to disappear. Monarchs choose to migrate to avoid freezing winter temperatures and starvation (Larson and Lee, 1994). The butterflies begin to feed aggressively to increase their lipid reserves as much as possible before making the long trip to their overwintering sites. The lipid reserves accumulated by the earlier generations are used for breeding and reproduction. This generation is relying on these reserves to get them through an extensive migration and a cold, long, winter. The monarchs must make sure they have enough lipid content after overwintering to head back north far enough to reproduce and find suitable milkweed plants for the summers first generation of offspring (Masters et al., 1988).
Monarchs use environmental cues to determine when it is time to begin heading south (Larson and Lee, 1993). A decrease in host plant availability, temperature, and day light, triggers their migration. The overwintering sites in the Oyamel mountains in Mexico are suitable overwintering sites because the cool temperature promotes the conservation of lipids, reduces their metabolism, and decreases activity from mid-November to mid-March (Larson and Lee, 1994). The conservation of these lipids is essential to the monarch's survival and are found to be the major reason why monarchs choose to overwinter in these cool, high altitude mountains in Central Mexico (Masters et al., 1988). Although the overwintering sites remain cool, during the winter enough sunshine is available to allow sufficient energy for short flights that may be needed for the monarchs to avoid predators or to take advantage of the plentiful and crucial water sources (Anderson and Brower, 1996).
It has also been discovered that the monarchs that occupy these areas
are from many different regions of North America and mate with one another
at random (Wassenaar and Hobson, 1998). This may be another benefit of
migrating to a very condensed region by allowing more genetic variation
to occur within the species.
How do monarch butterflies find these distant, secluded mountain regions without ever visiting them before?
Many hypotheses have been proposed about the orientation of monarch butterflies. How do these small, fragile insects make it over 3600 km to a specific section of fir forest year after year? The monarchs that arrive in the Oyamel mountains are 3 to 5 generations older than the last butterflies that overwintered in these areas (Brower, 1996). How do they know where to go?
Some scientists propose that monarchs use a sun compass to orient themselves in the right direction (Perez et al., 1997). The monarch's internal clock mechanism was manipulated in the laboratory to try to reorient the butterfly's flight direction. Whether released from captivity or in their natural environment the butterflies had similar flight orientation patterns (Perez et al., 1997). This confirms that monarchs do use the Sun as a compass to assist them on their southward winter migrations. They orient their bodies depending on the time of day and the location of the Sun. However, other factors must influence monarch flight because even on cloudy or overcast days monarchs still orient themselves in the correct southward direction (Perez et al., 1997 and Brower, 1996). The strongest evidence supporting the use of magnetism in monarch orientation is the discovery of magnetic particles in the butterflies thorax, probably consisting of magnetite (Brower, 1996).
Not only is the orientation of the monarch's flight a mystery, but other observations have been made to determine how the monarchs migrate these vast distances without using up all of their lipid reserves (Gibo and Pallett, 1979). The answer has been determined to be the utilization of winds. Monarchs have adapted mechanisms to use the wind to their advantage whenever possible. During their southwardly migration, if the wind is blowing toward the north the monarchs fly closer to the ground to avoid being affected by the head winds. The monarch then flies well above the ground when tail winds are present. This allows them to move fairly quickly (up to 50 kmh-1) in their desired direction only using small amounts of their lipid reserves (Gibo and Pallett, 1979). By utilizing these tail winds the monarchs obtain velocities that are significantly higher than their normal cruising airspeeds. During late summer and early fall monarchs were found to weigh approximately 600mg, 23% (140 mg) of this was found to be lipid reserves (Gibo and Pallett, 1979). If the monarchs attempted to migrate to Mexico using only their own flight speeds without the help of tail winds, their lipid reserves may have the chance of decreasing too much to make it through the winter or even to their overwintering sites. If the monarchs utilize tail winds, make occasional stops for feeding, and set out for their journey with at least 140 grams of fat content, they should have no problem surviving their migration to Mexico (Gibo and Pallett, 1979).
The master of the subject of monarch migration, Lincoln Brower, has his own hypothesis about monarch flight orientation. It is called the rotational orientation hypothesis. He believes that monarchs change their orientation to the sun 1o per day in the clockwise direction (Brower, 1996). The spring migration of the monarchs is due north. If the monarchs continue reorienting themselves 1o clockwise every day, even during overwintering, the direction of migration for the different times of year can be determined. Convincing evidence exists to support this hypothesis. First, he is convinced that the ability to migrate to such a precise destination has to be an innate behavior. How could this phenomenon be learned when the butterflies migrate to an exact destination flawlessly without ever being there before? They concentrate in 800 square kilometer area 70 to 170km west of Mexico City every winter (Brower, 1996). Brower agrees with many of the findings of Gibo and Pallett (1979). The butterflies may stop during migration to feed on nectar sources and their stay may vary depending on wind conditions. If a tail wind is present they will quickly return to migrating to take advantage of the wind. On the other hand, if the wind is a head wind they may aggregate in temporary clusters until they can migrate more efficiently (Brower, 1996). Brower believes that the observation of the wings of the monarchs on their overwintering sites is sufficient evidence that monarchs greatly rely on soaring flight during migration. If the monarchs were rapidly flapping their wings for their entire 3600km journey south, their wings would not be in the beautiful shape that is observed when they arrive in Mexico. These findings, along with the ability of monarchs to adjust to wind drift (Srygley, 1996) and to navigate successfully in even overcast conditions has led Brower to propose his rotational orientation hypothesis.
Surviving the long winters in Central Mexico
Moving south usually means warmer temperatures for most organisms. The monarch butterfly chooses to migrate south for other reasons. Although much warmer in the winter than several of the North American habitats occupied by the monarch, the trans-volcanic mountain range still reaches sub-zeroo temperatures from December to March (Anderson and Brower, 1996). The monarch butterfly comes to these 5 isolated, major mountain ranges to survive through the winter and to retain enough lipid reserves to reproduce in the spring.
The mountain ranges the monarchs have chosen for their overwintering sites are ideal for many reasons. The Oyamel fir forests are extremely dense and this creates a canopy that insulates the habitat beneath it (Anderson and Brower, 1996). The monarchs are trying to stay cool enough to conserve their lipid reserves, remain fairly inactive, and to keep their reproductive cycle suspended (Anderson and Brower, 1996). For a monarch, a fine line exists between remaining cool and freezing. Monarchs are freeze intolerant insects (Troyer et al., 1995). The forest canopy benefits them by decreasing wetness and also protects them from exposure to the open sky (Anderson and Brower, 1996). The density of the forest also allows the monarchs to condensate in large clusters which has also been found to greatly reduce the level of predation (Alonso-Mejia et al., 1998).
A monarch can withstand much cooler temperatures if its integument remains dry (Anderson and Brower, 1996). A dew-covered monarch will freeze at -4.2oC and a 100% will die if the temperature drops to -7.7oC. A monarch that remains dry will not freeze until it reaches
-7.7oC and a 100% mortality rate will not occur until the temperature drops to -15oC. When winter storms hit Central Mexico, the canopy protects the monarchs from getting wet. If the monarch's haemolymph freezes, they cannot recover. The freezing and supercooling temperatures of monarchs have been found to vary depending on the population (Anderson and Brower, 1996).
Monarchs that migrate may have a lower crystallization temperature than monarchs that are reproductive (Troyer et al. 1995). Migratory monarchs that had higher lipid concentrations could also supercool to much lower temperature than monarchs that were concentrating on reproducing. It was found that the mechanisms of diapause and migration are the conditions that allow the monarchs to be able to withstand damage to freezing temperatures and that gradual temperature acclimation had little, if any importance (Troyer et al., 1995). In another study monarchs were tested to see if they could tolerate cooler temperatures while migrating or while in their overwintering sites in Mexico. Monarchs can with stand cooler temperatures, for longer periods of time in their overwintering sites. The monarch is the first Lepidoptera that has been known to experience both rapid and long-term cold-hardening when faced with sudden freezing temperatures during their migration (Larson and Lee, 1994).
The monarchs orient themselves in a sun-minimizing position on warmer
days and have adapted basking and shivering mechanisms to allow them to
maintain a temperature that is suitable enough to allow them to fly when
needed (Masters et al., 1987). Monarchs are the first butterflies that
are known to ecologically rely on a type of shivering mechanism. If the
temperature falls to -2oC, some monarchs begin to die. If the
temperature increase to over 20oC, this affects diapause and
may trigger premature courtship (Masters et al., 1987). It is a combination
of the monarchs selectiveness of their overwintering sites along with their
own adaptations, that has allowed them to be so successful during migration.
Preservation of the rain forest canopy is essential to the monarch's survival
Monarchs would not survive overwintering without a forest canopy for protection. Monarchs may have been so selective in finding their overwintering sites because the trans-volcanic mountains of Central Mexico provide everything the monarch needs to survive the winter: Protection from freezing, protection from predators, an ideal climate, water, and a massive congregation of reproductively active members of the same species in the spring.
The Oyamel fir forest provides a barrier for the monarchs from outside elements (Anderson and Brower, 1996). Exposed butterflies freeze at much higher temperature and have a greater risk of getting wet during severe weather conditions. Deforestation causes an increase in temperature and reduces precipitation and evaporation (Zeng and Neelin, 1998). Evidence has been found that it is absolutely crucial to avoid forest thinning if the monarchs are going to continue overwintering in Mexico. Areas that use to be inhabited by monarchs during the winter are now empty due to deforestation (Anderson and Brower). If the monarchs cannot find suitable conditions in a certain area to overwinter, they simply will not stay and will move to a more desirable area. If the forests in Central Mexico keep getting thinner the monarchs will not have a more desirable area to move to.
Bird predation is greatly increased on monarchs that are in exposed areas. A study was done to compare the levels of bird predation between exposed monarchs and monarchs that were protected by the forest canopy (Alonso-Mejia et al., 1996). It was found that monarchs in open areas were more susceptible to predation. Monarchs are desirable to predators during overwintering because of their high lipid concentration. Monarchs also have a reduced level of cardiac glycosides as they get older. Lacking in their main mechanism of defense the monarchs strongly rely on the forest canopy for protection (Alonso-Mejia et al., 1996).
Several steps are now being made to ensure that the phenomenon of monarch
migration to overwintering sites in Mexico will continue. In 1986 the Monarch
Butterfly Special Biosphere Reserve (MBSBR) was created to help protect
the monarch butterfly's overwintering sites in Central Mexico (Alonso-Mejia
et al., 1996). Although this was a step in the right direction, the reliance
on the logging industry in Mexico has caused the government to consider
issuing logging permits in the main areas of the reserve. Monarchs that
are unprotected by the forest canopy are exposed to more sunlight, experience
greater temperatures and become more active than protected monarchs. This
may cause them to use up their lipid stores much faster (Mejia et al.,
1996).
Conclusion
Discovering the overwintering sites was a crucial step for scientists studying the fascinating migration of the monarch butterfly. Not only was the discovery essential to the understanding of this phenomenon, but if the sites had gone on unnoticed for any longer, they may have been deforested beyond repair. The sites were discovered in time to make the proper conservation efforts to ensure this great journey will continue.
Monarchs spend half of the year in these overwintering sites and thanks
to the efforts of many devoted scientists some of the mysteries have now
been unraveled. Although much has been learned about monarch butterfly
migration in present years, these insects have most likely been following
these migration patterns since the Pleistocene (Brower, 1996). Scientists
have only known about these overwintering sites since 1975. The Pleistocene
epoch of the Quaternary period began more than 1.8 million years ago. These
enchanting creatures have made this migration undisturbed for thousands
of years. The enormous population growth of humans has once again threatened
a species that has been on the earth long before us. We have discovered
these overwintering sites in time to protect them. We must respect these
creatures and educate others about the importance of protecting these overwintering
sites so this phenomenon can be appreciated by future generations.
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