1 Jemma Bauer Review BSPM 507 Migratory Monarch Butterfly Mechanisms Abstract Annual mass migration of monarch butterflies, Danaus plexippus, to and from wintering locations, has long been an awe-inspiring occurrence. Migratory monarchs breed all over the United States and Southern Canada before congregating by the millions in Michoacán, Mexico and along the California coast. How does each insect inherently know when and where they must travel? Furthermore, how do so many of these delicate organisms survive the ordeal? Prior research on the subject delved into the survival motivations of D. plexippus. Current research aims to shed light on how the butterflies perceive and adapt to environmental cues. By attempting to understand and better appreciate this complex lifecycle, hopefully we can support this unique migratory phenomenon before it unravels completely. Introduction Geographic Span: Monarch butterflies breed and forage throughout North America during the spring and summer. Thanks to the tagging research of Fred Urquhart and his team at the University of Toronto back in the 1930 s, the migratory pathway of monarch butterflies is well documented (Gibo, 1979). Monarchs are split into an eastern
2 migratory population and a smaller, less frequently studied, western migratory population. The eastern population inhabits most of the land east of the Rocky Mountain range (Redón-Salinas, 2014). This includes southern Canada, the East Coast, the Great Lakes region, the Midwest, and the South. Come early fall, eastern monarchs fly over 2,000 miles to their wintering site in the Oyamel fir forests of the volcanically formed highlands in central Mexico (Pence, 1998). Western migratory monarchs, on the other hand, inhabit land to the west of the Rocky Mountain range. They travel about 300 miles to hundreds of locations along the California coast to rest on eucalyptus, Monterey pine, and cypress trees for the winter (Redón-Salinas, 2014). Lifecycle: The average lifespan of a monarch butterfly, born into the spring or summer, lives a little over a month, at most (Frey and Stevens, 2010). D. plexippus commit this time to repopulate the continent after the mass departure the fall before. The last generation to emerge in late summer or early fall has a unique existence. These monarchs experience seasonal shifts that trigger the butterflies to postpone reproductive maturity and instead prepare for the long flight south (Brower et al., 2006). Reproductive diapause allows this generation to store nectar, collected along their journey south, as energy-packed lipids for winter survival (Brower et al., 1997). By mid-march, the migratory generation matures fully,
3 reproduces, and dies at an age anywhere from five to nine months old (Frey and Stevens, 2010). Advantageous Microclimates: The need to escape harsh winter weather seems to explain the motive behind annual relocation. However, in the past decades, researchers found that wintering sites of dense, old growth forests provide unseen benefits from environmental extremes. These thick ecosystems only occasionally reach freezing temperatures (Alcock 2001). When this happens, millions of butterflies die. However, it is believed that the benefits of these specific climates outweigh the risks. For example, Lincoln Brower s research team measured the difference between air and Oyamel tree trunk surface temperatures throughout the day and night in 2 different wintering reserves in Mexico. They found that on average, nighttime minimum tree trunk temperature was 1.4 Celsius warmer than surrounding air temperatures. Conversely, minimum trunk temperatures during daylight measured an average of 1.2 Celsius cooler than ambient air (Brower et al., 2009). Monarchs arrange themselves in heavy clusters that blanket tree surfaces. This congregation significantly increases survival totals after episodes of rain, hail, and snow. Researchers also hypothesized that this intense layering holds in a buffer of air between the butterflies wings and the bark of the tree. This helps maintain the climate control effect so that tree temperatures are mild in comparison to air temperatures (Brower et al., 2009).
4 Lipid stores, collected during the migration south, are the prime means of winter survival. Climactic and behavioral factors mentioned above combine to expend lipid reserves at a slow, steady the rate during these months. This is crucial, as monarchs depend on leftover lipids to kick-start the journey back north and subsequent mating in the spring (Brower et al., 1997). Depleting this supply before the end of the season ensures death. Discussion D. plexippus: The Original Sailplane: Dr. David Gibo, Professor Emeritus of Biology at the University of Toronto, explained that butterflies are the worst possible body form to attempt sustained flight with (NOVA 2009). Their wings supply low flight speed and cannot overcome winds head on. However, butterflies have a low body mass relative to their wing size, much like a glider (Gibo and Pallett 1979). Monarchs are opportunistic when traveling. They avoid migration when the wind is not directed in their favor. Due to their daytime activity, D. plexippus take advantage of both hot, rising air columns and lift surrounding landmark barriers, such as the Sierra Madre Mountains (Gibo and Pallett 1979). Catching obstacle and thermal updrafts allows monarch butterflies to escape the sticky flight boundary layer between the earth and atmosphere (Chapman et al. 2015). Original observations reported monarch butterflies reaching altitudes of at least 300 meters (Gibo and Pallett 1979). However, glider pilots from around the U.S. reported monarch sightings at altitudes between 600 and 1,250 meters high.
5 One pilot in St. Louis, Missouri even witnessed D. plexippus soaring into the bottom of clouds, due to stronger-than-average convection. This technique save significant energy, although catching lifts does require some work. Monarchs benefit from a glide ratio of 4:1. This means that if a butterfly reaches an altitude of 1,200 meters, it can soar a distance of 4,800 meters before engaging in powered flight again (Gibo 1980). Sun Compass: For the most part, monarch butterflies rely on sunlight to know both when to move and where to go (Gegear et al. 2010). Southwesterly migration in the fall is partly triggered by shortening daylight hours (Frey and Stevens 2010). Based on Drosophila gene expression as a jumping off point, most researchers assumed the circadian rhythm control center would be housed in the D. plexippus brain. However, Dr. Steven Reppert and his research team discovered that monarchs also rely on antennae to perceive, and thereby act based upon, celestial cues. The scientists cut off both antennal flagella of some migratory monarchs and left other migratory monarchs antennae uncompromised. They then exposed all subjects to simulated day and night photoperiods. Results showed that monarchs with full antennae correctly oriented their vector of travel to the southwest, while monarchs without antennal flagella flew just as well as the control group, but with no particular commitment to direction. It is also believed that one blue light photoreceptor protein, called CRY1, is the primary
6 acceptor of light and regulates circadian clock mechanisms in the antennae (Gegear et al. 2009). Later studies revealed that both antennae perceive time independently of each other. With one antenna on a migratory monarch painted black and the other painted with clear polish, the circadian clocks in D. plexippus were not in synch, resulting in inability to orient to the southwest. After this, researchers removed the black-painted antennae and left the clear-painted antennae intact. This restored the butterflies ability to locate and travel southwest (Gegear et al. 2012). In their natural environments, migratory monarchs rely on synchronized organs to perceive and react to constant changes in the angle of the sun. The D. plexippus sun compass really involves a cascade of receptors and neurons which stimulate muscular function. Earlier, Reppert s team found that antennae and brain tissue contained similar time-measuring proteins at similar concentrations (Gegear et al. 2009). It seems that antennae are responsible for data input, while the brain turns this information into motor output. Recently, by utilizing intracellular labeling and measuring differences in neuropil volume, researchers found a series of neurons carry out sensorimotor transformation (296) in the central complex region of migratory monarch butterfly brains (Asokaraj et al. 2013). Cold Cues: Researchers at the University of Massachusetts recently discovered that sun compass orientation is also controlled by temperature exposure. During fall,
7 migratory monarchs who are not exposed to temperatures below 15 Celsius never begin their journey south. On the other hand, overwintering butterflies that are exposed to temperatures less than 15 Celsius before mid-march fly northwards into certain death by frost (Guerra and Reppert 2013). Magnetic Compass: Celestial cues cannot be the sole mode of navigation for D. plexippus, because these butterflies still migrate in the right direction in overcast conditions (Gegear et al. 2014). Work at the University of Kansas, led by Chip Taylor, provided early evidence that magnetic fields influence monarch travel orientation. Monarchs in reproductive diapause flew to the southwest in typical magnetic fields, to the northeast in atypical magnetic fields, and lacked directional focus when free of magnetic forces. Back then, these researchers predicted that the thorax contained magnetosensors, as 65% of bodily magnetite concentrates here (Etheredge et al. 1999). It turns out, however, that monarch antennae house the magnetosensors (Gegear et al. 2014). Again, Reppert worked with a team to test magnetic perception based on light wavelengths between 380 and 420 nanometers. They employed a similar method as before, painting either both antennae black on some butterflies or both antennae clear on others. All monarchs were placed in a magnetic field that simulated spring conditions and influenced butterfly directionality north. Those with black antennae lacked vector focus while the control group had no difficulty finding north (Gegear et al, 2014). These researchers
8 proposed that antennae depend on ultraviolet-a and blue light radiation to navigate in spite of undetectable celestial cues. Whether or not this magnetic inclination is initiated by the same CRY1 cryptochrome receptors has yet to be determined. It is also believed that sensing magnetic forces also helps to fine tune the internal sun compass, making D. plexippus navigation astonishingly accurate (Chapman et al. 2015). Evolutionary Influence: As suspected, migratory monarchs have evolved independently of nonmigratory monarchs to sustain long distance flight. Migratory populations reduce wing stress by soaring within thermal currents, preserving healthy lipid storage, and postponing reproduction (Dockx 2012). Non-migratory populations, on the other hand, tend to have more collagen type IV alpha 1 production within the thorax than migratory monarchs (Altizer et al. 2014). It could be that short-distance monarchs need extra muscular reinforcement to protect their wings from year round foraging and more strenuous flight. This population also revealed much lower lipid preserves, due to constant energy output (Dockx 2012). In the end, migratory monarch butterflies maximize flight productivity while minimizing metabolic input. Recolonization:
9 Much of yesterday and today s research focuses on the eastern migratory population. It takes five generations to produce healthy numbers (Chapman et al. 2015). Two hypotheses exist to explain springtime repopulation patterns. The successive brood hypothesis suggested eastern monarchs leave Michoacán, breed along the Gulf of Mexico, and then die. The offspring develop and fly north where they continue breeding and migrating. The single sweep hypothesis suggested eastern monarchs also begin breeding along the Gulf, but then continue the journey north (possibly as far as Southern Canada) with their brood, laying eggs as they progress (Hobson et al. 2012). Evidence shows that the Midwest to Great Lakes region is the most productive breeding ground for monarchs. This area establishes as much as 88% of the population (Hobson et al. 2010). Recolonization depends on parental origins in Texas and Oklahoma (Flockhart et al, 2013; Baum and McCoshum 2014). Conservation: Back in 1986, the Mexican government established the Monarch Butterfly Special Biosphere Reserve in an effort to save the culturally celebrated phenomenon. However, economic pressures and illegal logging threaten this sanctuary daily (NOVA, 2009; Brower et al. 1997). Logging companies cleverly suggested that thinning Oyamel forests would benefit monarch butterflies in the end, by freeing light, space, and resources for flowering plants to grow. The flaw in this plan is that wintering monarchs do not need abundant food sources to survive
10 these months. In fact, gathering nectar during this time depletes lipid reserves so quickly, that nectaring butterflies die along migration in the spring (Brower et al. 1997). Ultimately, leaving pristine microclimates such as Oyamel forests untouched is the greatest security we can offer to monarch butterflies (Brower et al. 2009). Determining the birth origins sheds light on how vast this population is. In the future, efforts must consider protecting breeding grounds throughout North America, in addition to wintering reserves. Without healthy colonization, this spectacular cycle will never recover. Conclusion Recent research has uncovered some startling details about driving forces of monarch butterfly migration. It is important to keep in mind that much of the mechanism is still inexplicable. Migratory monarch butterfly preservation needs to expand, for the sake of understanding the biological underpinnings of this remarkable species in full.
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