Up in the sky, at an altitude of 800-feet, the wind is in your face, and your journey (if successful); will allow for your species to continue to proliferate. As you fly over much of the North American continent beginning in Mexico, you, the Monarch Butterfly (Danaus plexippus), must pass through Southern California onwards to the Northern part of the United States. Though it is believed that the Monarch butterfly is indigenous to North America, there are various types of Monarch Butterfly that inhabit Central America, South America, and the Caribbean. Contrary to popular belief, the majority of the Danaus plexippus species do not travel long distances to migrate. Let’s look deeper at this species life cycle and history to get some insight as to how migratory paths and timing is determined.
Before we can truly understand the butterfly migration, further context is needed about their early stages of life. Monarch butterflies go through 3 essential steps in their life cycle: the egg stage, larva stage, and pupa stage. Monarch butterflies prefer a specific plant, the Milkweed, and;mating occurs in March or April. Female Monarchs are able to lay eggs in a single-file line which can weigh up to their own body weight approximately 0.75-grams. Females will lay eggs in the southern regions of the migration routes, either in southern California or in Mexico.The entire lifecycle of the Monarch butterfly will last anywhere between 2-6 weeks. As an egg hatches, what is known as an Instar (caterpillar) emerges; and it will then spend most of its time consuming surrounding vegetation, such as Milkweed, to store large amounts of fat in preparation of its transformation period. The caterpillar will then choose an area where it is safe for it to become a pupa by encasing itself in a chrysalis. It takes 8-15 days for maturation to occur and once an adult Monarch emerges from its pupa, it will then take 4-5 days for the adult to reach sexual maturity. However the late migrating generation will not reach sexual maturity until after overwintering (live through winter season). Once an adult, the monarchs begin to take flight northward bound.
There are essentially two main reasons as to why monarch butterflies migrate: temperature and suitable habitat. Since they are considered cold blooded insects, when temperatures drop below freezing, they enter into a hibernation state, resulting in prolonged times of no consumption. Upon waking in the spring, most of their primary food source (Milkweed) is scarce due to the sub-freezing temperatures. They begin their migration from the North to the warmer southern regions where food is more readily available. Mainly the native North American species of Monarch (D. plexippus) can tolerate the sub-freezing temperatures; this is a unique trait that they have gained through natural selection over centuries. The second reason for migration is the need to have suitable breeding grounds to help insure the survival of the larvae. Since the resources are once again scarce, if two adult monarchs were to breed in the colder regions, the offspring would not survive due to insufficient plant life caused by frostbite and desiccation. These Monarchs have a vital reason to find warmer climates, and it all results in their need to proliferate and conserve their population numbers.
As a native southern California resident, I have always heard about the fascinating migrations of the Monarch Butterfly. It is considered to be a special event for many butterfly watchers, and certain places hold events to watch the migration occur. But it appears that the need to migrate can only be found in the Monarchs that are indigenous to North America. Some species of Monarch further south of North America do not carry this innate trait, such as D. erippus. With there being a migratory and non-migratory population within the Monarch species, this suggests that both of these populations share a common ancestor. Scientists had taken DNA samples from both migratory and non-migratory populations and upon further investigation figured out that the North American Monarch had more of a basal lineage. This means that there was a divide of two distinct variant alleles within a population, derived from a common migratory ancestor, whereas the South American Monarch was mainly classified as an independent branch, free of any sign of branch splitting.
Though the species D.erripus does not carry the specific gene for long distance migrations, D. plexxibus , the population still is able to travel long distances if needed. These findings suggest that they both indeed share a common ancestor that had a trait for migratory tendencies, and we have not yet discovered another gene shared between both populations. As seen in our phylum tree, we can see that both populations share a common ancestor, shown by the node(intersecting space) they both share. Scientists also suggest that the North American Monarch (D. plexippus) is more inclined to migrate versus the South and Central Monarch Population (D. erippus ), due to pressures of natural selection. Those monarchs that migrate are able to survive and adapt to the colder temperatures of North America, whereas those species that could not conduct this are found mainly in South and Central America. Lastly, where the D.plexxibus and D.erripus populations differ is in their active and resting metabolism rates. Research suggests that the North American Monarch species has a significantly slower metabolism rate then their Non-migratory counterparts. With slower metabolism rates, it allows the migratory species to conserve greater energy when traveling long distances, giving them more of an advantage. It is due to natural selective pressures that we have two distinct populations of migratory and non-migratory Monarchs. Not only have both of these populations genes been conserved for migratory tendencies, but even their differences in metabolic rate is something to be marveled at.
The phenomena of the Monarch migration is in danger of disappearing due to the decline of the Monarch population. In the year of 2013, it was recorded as the lowest number of overwintering butterflies in North America and South America; this means that not many were able to enter a hibernation state while migrating up north, lower presence of monarchs. This decline can be due to multiple factors such as deforestation, drought, and reduction in preferred plants such as Milkweed. The greatest contributor to the decline of population is the destruction of the monarch resting sites. These rest sites are vital to provide food and shelter to the Monarchs during migration. Many of these sites range from as far North as San Francisco to Santa Barbara. People unknowingly destroy the monarchs’ home when they cut or trim eucalyptus trees, which are not native to California, but the Monarchs have adapted to use them as rest sites. There are currently talks within the state legislature about the western United States monarch were to go extinct, that efforts would be to introduce the eastern United States Monarch population to the western United States. The problem with this is that it is not known if the Eastern Monarchs are able to withstand long distance migrations due to their genome (DNA) not carrying genes to promote the necessary reduced metabolic rate or tolerance to colder temperatures as seen in their Western partners. Without any type of regulation for these rest sites, there will surely be a point past saving the Monarchs and it will lead them to extinction.
There are current efforts being conducted by both the citizens of this country along with possible laws being passed by California state government to help promote the conservation of these rest areas. You can help with this problem by: planting pollinator friendly plants, especially milkweed, also by getting involved with a local non-profit organization that supports restorations or clean-ups of local nature preserves and open space. You can also become aware of local rest sites for the Monarch migration and help make sure those areas are clean and protected. Tell your friends, neighbors, and loved ones about what you learned today, and spread the word to help save the Monarch butterflies! If you would like to take a more personal approach to help conserve the Monarch, there is a wonderful video by National Geographic that explains as to how you can create and manage your own Monarch Resting site. If you are interested in participating in any Monarch watches this coming spring, you can check out these locations: Goleta Monarch Butterfly Grove, Pismo Beach Butterfly Grove, Santa Cruz Monarch Butterfly Grove, Pacific Grove Butterfly Sanctuary, for great opportunities to get involved and witness this astounding natural phenomena of the Monarch Butterfly migration!