March of the Monarchs: The Migration Habits and Mechanics of the Monarch Butterfly Bryan Quarker Colorado State University, Fort Collins, CO. 80521 aking2rule@yahoo.com Abstract Monarch Butterflies undertake a 2,000 mile journey from the Eastern United States and Canada to the mountains of Central Mexico every year. The entire journey encompasses the life spans of 4 generations of Monarch Butterflies in a continuous cycle. Using the sun and a timecompensated circadian clock that is located in their antennae, they navigate to a small area of Oyamel Fir trees in the Pelon Mountains in what is known as the Transverse Neovolcanic Belt. It has been found that the absence of Juvenile Hormone gives the migrating monarchs the longevity that they need to make the long journey. When the hormone is present, the butterflies only live a couple months, but when it is absent the butterflies can live for 8-9 months. Increased forest destruction and invasive plant species have played a major role in decreasing the number of monarch butterflies. The Swallow-Wart (of the genus Vincetoxicum) is an invasive plant species that looks like the milkweed plants (belonging to the family Asclepiadaceae) that female Monarch butterflies normally lay their eggs on. This confusion leads to a higher rate of larvae mortality and the Swallow-Wort is also displacing the milkweed plants that monarchs need.
Introduction Insects are some of the most successful and fascinating organisms on the planet. They have been around for hundreds of millions of years and have filled just about every environmental niche on this planet. There are many amazing feats that are done by insects like the mass awakening of the periodical cicada in which millions awaken after a 17 year slumber and the slaughter march of army ants as they clear an entire forest of other insects and small animals; but arguably the most amazing of all these is the mass migration of millions of monarch butterflies on a nearly 2,000 mile journey from Canada to Mexico. Each spring a monarch butterfly flies thousands of miles to mate, lay eggs on milkweed plants, and then die (Helen & Gore, 2008). Using the sun and polarized light, the Monarch butterfly can process this information in its brain and use it to navigate. Also, there are circadian clocks which are located in the antennae which help tell the Butterfly when to start its migration (Reppert, 2010). The Monarch butterflies orient their bodies in a Southwest direction and fly thousands of miles until they land in the fir tree forests of Central Mexico. The area that they stop in is relatively small, about 800 square kilometers of fir groves in Central Mexico (Kyriacou, 2009). Only a handful of fir groves in Central Mexico are suitable for monarch butterflies to roost, and these are being threatened. There are literally millions of butterflies concentrated in just a few parts of Central Mexico specifically the Chincua, Chivati-Huacal, Herrada, Mil Cumbres, Palomas, and Pelon regions (Slayback, 2007). Illegal logging is rapidly destroying the roosting trees that the Monarchs winter in and this has caused the deaths of entire colonies of butterflies (Wilcove, 2008). This is a worry to scientists and more study has been done on the populations of the butterflies.
Monarch Butterfly Life Cycle The life cycle of the monarch butterfly starts when a female butterfly mates with a male butterfly during the spring and summer breeding season (Wilcove, 2008). The female than lays her eggs on a milkweed plant during the spring and summer (Wilcove, 2008). Monarch butterflies use nearly all of the roughly 100 species of milkweed plants belonging to the family Asclepiadaceae, as this is the only food that that their larva can eat (Oberhauser & Solensky, 2001). By laying her eggs on milkweed plants, the female butterfly guarantees her larvae a stable food supply. Female monarchs lay around 300-400 eggs in the wild and close to 700 eggs in captivity in a period of around 4 weeks of egg laying (Oberhauser & Solensky, 2001). The eggs hatch after 4 days and the larva first eat their egg shell and then they begin to eat milkweed leaves constantly. The deciding factors in the larva s rate of development are the temperature and availability of food in their environment, which causes them to develop faster or slower (Oberhauser & Solensky, 2001). The larval stage of the monarch butterfly (caterpillar) lasts for 10-14 days in which the caterpillar does most of its growing (Oberhauser & Solensky, 2001). As the caterpillar grows, it no longer is able to fit inside its skin, so it sheds this in a process called molting. After each molt, the caterpillar s exoskeleton is soft, but then it hardens and molds to the body of the caterpillar. The period of time between each molt is called an instar, and during this time the caterpillar grows and continues to eat insatiably (Oberhauser & Solensky, 2001). Monarch butterflies go through 5 instars before they move on to the next stage in their life cycle, known as the pupa. After 10-14 days of continuous eating and molting, the caterpillar then stops feeding and attaches itself to a leaf stem. While attached, it covers itself in a cocoon which serves as
protection while it goes through its metamorphosis into an adult butterfly. At this stage in its life, it is known as a pupa (chrysalis) ( www.monarchwatch.org). Monarch butterflies like most butterflies have camouflaged chrysalis which serves as their only form of defense during this stage of their lives. During this stage, the pupa does not eat or defecate and it will only move with quick jerks if disturbed (www.monarchwatch.org). This stage lasts for 10-14 days after which the cocoon opens and an adult butterfly emerges. The adult butterfly must than wait a few hours for its wings to fully dry, stretch out, and become rigid enough for them to be used for flight. A liquid called meconium fills the crumpled wings and causes them to unfold and adds support. The adult stage of the monarch butterfly exists primarily to reproduce. Adult generations born during the summer time live for 2-5 weeks (www.monarchwatch.org). The adults that are born during the late summer-early fall, are the final generation in the 4 generation cycle and these are the butterflies that undergo the 2,000 mile migration from Canada to Mexico. These adult butterflies make this journey and wait out the winter months and breed the following spring and summer. These generations of adult Monarch butterflies live for 8-9 months (www.monarchwatch.org). The Monarch Butterfly Migration The Monarch Butterfly migration is one that is comprised of millions of butterflies and there are two populations of migratory monarchs. One population lives west of the Rocky Mountains and breeds in the spring and summer months. When autumn comes, these monarch butterflies migrate in a southwest direction and overwinter at low altitudes along the Pacific coast of California (Brower, 1996). The second population consists of individuals that live east
of the Rocky Mountains. In autumn, these monarchs migrate nearly 2,000 miles in a southwestward direction and overwinter in the higher altitude mountains of Central Mexico. Both populations overwinter in the same areas year after year despite the generations that make the migration having never visited the area during their lives. This is especially remarkable in the eastern population of Monarch butterflies because the generation that makes this trip is 3-5 generations distant from their ancestors who made the trip; and yet they still know how to get to their overwintering grounds 2,000 miles away (Brower, 1996). I will elaborate on the mechanic of this astounding journey later. The population that is east of the Rocky Mountains is the population that makes the huge migration that Monarchs are famous for. The entire journey is done during the life-spans of 3-5 (usually 4) generations of butterflies (Brower, 1996). The first generation of Monarch butterflies in this cycle is the generation that makes the huge trip from the U.S. and Canada to Central Mexico. I will start with the second generation so that I can accurately show the cycle in its entirety. The second generation of Monarch butterflies begin the migration in early spring in Central Mexico (specifically on a mountain known as Sierra Pelon) in which they fly as far north as Texas and Oklahoma, after which they then mate, lay eggs, and die (Brower, 1996). The second generation is born in mid-late spring and these butterflies continue the migration north and then they mate, lay eggs, and then die. The third and fourth generations follow the same pattern as the first and second generations while continuing north until they reach their homes in the United States and Canada where they mate and die (Brower, 1996). Each of these generations has a lifespan of about 2 months after which they mate and die (www.monarchwatch.org).
The offspring of that fourth generation restarts the whole cycle and is now the first generation. This is the generation that undergoes the 2,000 mile journey which monarchs are famous for. This generation is born in late summer-early autumn and they enter a state of suspended reproduction and growth known as diapause (Kyriacou, 2009). During this state, the adult butterflies do not reproduce and can live up to 9 months (Brower, 2008). This enables them to conserve valuable resources and helps them survive until the following spring. The monarch butterflies then start their journey in August and by the end of October, they have completed their 2,000 mile journey and the overall cycle back to the mountains of Central Mexico (Howard, 2009). The migrating monarchs stop every night throughout this journey to rest and consume nectar (Howard, 2009). Locations of Monarch Overwintering Sites The two migratory populations of Monarch butterfly (west and east of the Rocky Mountains) have two very different overwintering sites. The populations of Monarch butterflies west of the Rocky Mtns. overwinter along the Pacific coast of California specifically in the Santa Cruz area of California (Brower, 1996). The eastern population overwinters on Oyamel Fir trees (Abies religiosa) in the Sierra Pelon Mountains as well as other high-altitude areas along what is known as the Transverse Neovolcanic Belt (Brower, 1996). The highest peaks that have these Oyamel forests are where the Monarch butterflies spend their winters. A total of 30 colonies of overwintering butterflies have been found on 9 different mountains but the number has since increased due to deforestation and climate change which I will discuss later (Brower, 1996). From November to March, the Monarch butterflies winter in these remote mountains and they cluster by the millions on the biggest Oyamel Fir trees. The biggest populations of
overwintering monarchs are concentrated in an area less than 800 square km specifically the Chincua, Chivati-Huacal, Herrada, Mil Cumbres, Palomas, and Pelon regions (Slayback, 2007). Throughout the winter the butterflies are mostly sedentary but they do occasionally fly in huge groups to nearby streams to drink water (Brower, 1996). As winter progresses, so does the frequency of mating throughout the Monarch colonies so that by the end of the winter months, nearly all the surviving butterflies have successfully mated (Brower, 1996). Mechanics of the Monarch Butterfly Migration The exact way that the Monarch butterfly migrates such huge distances without ever having been to their destination has been a mystery to science until less than a year ago. Monarch butterflies use their eyes and brains to process the position of the sun and polarized light to help them navigate during their migration (Reppert, 2010). This could be one of the main reasons as to why they don t fly at night during their migration. The main mystery of the monarch migration was how they still know which direction to fly, despite the shifting position of the sun (which they are using to navigate) throughout the day. Scientists did not have an answer to this question so they concluded that the butterflies must use some type of circadian clock that is located in their brains. Less than a year ago, Scientists in the Department of Neurobiology at the University of Massachusetts Medical School found out that the butterflies actually have a circadian clock that is located in their antennae (Reppert, 2010). Before this discovery, scientists did not think that antennae were sophisticated enough to possibly house the circadian clock of Monarch butterflies because the migration was such a huge undertaking. They had always just assumed that somewhere in the brain of the butterfly, the circadian clocks resided they just were
not sure where or even if that was really a mechanism that Monarchs used during their huge migration. It is now known that Monarch butterflies need to have their antennae in order to be able to properly orient themselves using the time-compensated clocks (Reppert, 2010). In experiments done by Steven Reppert and his fellow scientists, when their antennae were removed, the butterflies became disoriented. When the antennae are painted black to block light, the butterflies never oriented themselves correctly (Reppert, 2010). The antennae themselves still react to light even when they are not attached to the butterfly which shows that they function independently from the brain and thus have taken over the function of the brain clock that is found in most insects, but not the Monarch (Reppert, 2010). The antennae contain the timecompensated clocks with the same molecular complexity that is found in brain clocks, and are used as the primary means of timing in the sun compass orientation mechanism (Reppert, 2010). The use of a magnetic compass has also been theorized in how Monarch butterflies migrate. A lot of animals that migrate very long distances (birds, whales, etc.) use the Earth s magnetic field as a compass so that they can maintain a specific direction (Reppert, 2006). Monarch butterflies maintain a southwestwardly direction throughout the migration and magnetite particles (which could be used to detect magnetic fields) have been found in monarchs (Reppert, 2006). These findings have led to the notion that maybe Monarchs also use the earth s magnetic field to navigate. Tests have been done but they have produced contradictory results which makes the magnetic field theory, for the moment, improbable. Hormones also play a role in the Monarch butterfly migration, specifically Juvenile hormone. When migrating, both male and female Monarch butterflies enter a state of reproductive diapauses, which is triggered when JH (Juvenile Hormone) stops being synthesized
in their bodies (Herman, 2001). This same absence of JH also enables the migratory monarchs to live for 8-9 months while butterflies born in the summer months live less than 2 months (Herman, 2001). This increased longevity enables the butterflies to survive the winter and lay their eggs when the more favorable spring months come. Researchers found that when removing the corpora allatum, which produces JH in Monarch butterflies, butterflies who were born in the summer and not migratory, showed a 100% increase in longevity (Herman, 2001). Migratory butterflies that were injected with JH lost their longevity and died around the same time as their non-migratory brethren (Herman, 2001). This showed the researches that the longevity of the butterflies was solely dependent on the production of JH in the endocrine system. Wind currents and changes in the pressure of the atmosphere also play a big role in the migration of the Monarch butterflies. Monarch butterflies use thermals (the same way hawks do) to get a free ride, without using a lot of energy (Zhu, 2009). Monarch butterflies also orient themselves so that they can take advantage of wind currents which is another way that they conserve energy, thus making the migration a little easier for them (Reppert, 2010). Whether or not the antennae also play a role in the Monarch butterfly being able to detect these currents and atmospheric pressure changes is a mystery. Insect antennae have a number of different sensory organs like taste, smell, chemoreception, temperature detection, movement detection, and many others, (Reppert, 2010), so it would not be unusual to find that Monarch butterfly antennae serve multiple functions. Threats to the Monarch Butterfly and its Migration Monarch Butterflies, like most animals are now facing problems due to habitat destruction and other types of environmental disturbance. Over a billion monarch butterflies
spend each winter in the Monarch Butterfly Biosphere Reserve in which they cluster together through the harsh parts of the winter. This gathering, known as aggregation helps the butterflies stay warm during the night, cool during the day, and it also helps protect them from predators (Brower et al, 2008). This same gathering of millions of butterflies on just a few trees in a small area in the New Mexico Mountains has also led to the decrease in the numbers of monarch butterflies. This aggregation in such a highly localized way makes it so that even the smallest disturbance in these areas, affects literally millions of butterflies. In 1986 and the year 2000, presidential decrees granted protection to the overwintering forests of Monarch butterflies (Brower et al, 2008). The 1986 decree granted a little over 4200 ha around the butterfly overwintering sites as the core of butterfly wintering spots and in this area, logging was prohibited (Brower et al, 2008). An additional 11,000 ha were set aside as a buffer zone in which only a little logging could occur (Brower et al, 2008). But this did not help stop the destruction of the forest by local people who depend on the clearing of the forests for income. Monarch butterflies have stopped wintering in locations that they once wintered in, and they also have began to depart and break up sooner at the end of winter because of the disturbances in their environment (Brower, 2008). Monarch butterflies are also beginning to take winter in places that have less than ideal abiotic factors, which has led to the dying off of entire colonies (millions of butterflies) due to the harsh winter elements (Brower, 2008). Efforts are being made to help protect the few remaining Oyamel fir trees that are left in the areas that Monarch butterflies winter in. The sheer numbers of butterflies that take up residence on a single tree means that only the biggest and oldest Oyamel fir tree are used by the Monarchs because smaller trees cannot support the weight. Unfortunately, these big trees also
bring in the most profit to illegal loggers and more often than not cut down before their smaller counterparts. Monarch butterfly numbers are also being decreased by invasive plant species, particularly the Swallow-Worts of the genus Vincetoxicum (Casagrande & Dacey, 2007). These invasive plants are found throughout the north-eastern United States and southern Canada. These plants look similar to the Monarch butterflies normal host plant (Asclepias syriaca ) and also have similar chemical smells that Monarchs use to help them find the milkweed plants (DiTommaso & Losey, 2003). This has led to female Monarchs laying their eggs on these plants which cause a lower survival rate in the offspring as compared to caterpillars feeding on milkweed plants (Casagrande & Dacey, 2007). The main problem isn t the fact that female Monarchs are mistaking the Swallow-Wort for their normal host plant, the problem is that the Swallow-Wort are outcompeting the native milkweed plants that Monarchs depend on (DiTommaso & Losey, 2003). These highly aggressive invasive plants are displacing the native plant populations and are threatening no fewer than 54 different species of rare native plants, animals, insects and land snails (DiTommaso & Losey, 2003). It is not known exactly how huge of a negative impact the invasive Swallow-Wort will have on Monarch Butterfly populations, and it ultimately depends on the Monarchs ability to adapt to this invasive plant should it displace all the native milkweed (DiTommaso & Losey, 2003). Conclusion Monarch butterflies are perhaps one of the most spectacular of all the insects. They are beautiful, mysterious, delicate, and deceptively tough. The mechanics of their astounding migration, the challenges that they face, and how they overcome those challenges is something
that is truly amazing. Our current research has uncovered just how these insects do their migration, and also how they survive it, but there are still things we don t know. Further protection of their habitat as well as limiting the number of invasive species in the environment will help ensure that future generations can enjoy the spectacular odyssey that these creatures undertake.
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