If there is such a thing as an enchanted forest, this special place, more than 10,000 feet above sea level in Central Mexico, must be it.
On a cold morning in late January, as one looks up into the 100-foot-tall Oyamel fir trees, there are huge clusters of what appear to be dead leaves. However, after closer inspection, one can see that they are in fact millions upon millions of monarch butterflies, motionless, waiting for the sun to heat them up so that they can come to life and start flying through the forest like countless fairies in an ethereal dream.
Monarchs are an iconic butterfly species found around the world. However, the Eastern North American subspecies is the only one to make an annual migration that can span thousands of miles. The migration ranges from the Sierra Madre mountains in Mexico to Southern Canada.
This amazing odyssey begins in the highlands of the trans-volcanic belt, west of Mexico City, where in 1986, the Mexican government designated approximately 140,000 acres as a “Monarch Butterfly Biosphere Reserve.” In 2008, it was officially designated by UNESCO as a World Heritage Site. The entire biosphere reserve consists of several smaller reserves of which a sanctuary called, El Rosario, is the best known and is the wintering ground for the largest number of butterflies in the world.
Though this migration has been occurring for centuries, up until the mid-1970s it was a mystery to scientists where the large numbers of monarchs in the northeast disappeared to in the fall of each year. In addition, it was just as mysterious to the people of Central Mexico as to where millions of monarchs suddenly appeared from.
It was a Canadian zoologist named Dr. Fred Urquhart who was able to finally solve that mystery by developing a special tag that could be stuck onto the monarch’s wings to provide a way to track their movements. By enlisting the assistance of thousands of volunteers across North America, he founded an organization called the Insect Migration Association that today is known as Monarch Watch. In 1975, using those tags and with the help of citizen scientists, Urquhart was finally able to discover the wintering grounds for the eastern population of North American monarchs. It was located in a secluded area in the mountains in Michoacán, Mexico. In August 1976, it was the cover story in the National Geographic Magazine, “Discovered: The Monarch’s Mexican Haven.”
Unlike the well-known migrations of animals such as the wildebeests in East Africa, the salmon in the Pacific Northwest, and a wide variety of birds around the world, the migration of the monarch is more complex. It will take three to five generations of monarchs to complete this epic journey with one special generation standing out against the others.
It begins with huge clusters of monarchs coming out of their winter rest as they blanket the fir trees in the Central Mexican Reserves where the numbers can reach millions of butterflies per acre. In the cold morning air, they remain dormant, clinging to the trees in such high densities that the weight has been known to break the branches. As the sun heats them up, they vibrate their wings, using them as solar panels to heat up their flight muscles until their activity reaches a crescendo where the skies are filled with hundreds of thousands of flying butterflies. There are so many that one can hear their wings buffeting the air as they erratically flutter all around.
For the several months that they winter in Mexico, their activity continues to increase as the days get longer and the heat of the sun grows stronger. By March, they have started to mate and begin their journey northward in search of the milkweed that they need to lay their eggs on. After approximately three weeks, their first stop is in southern Texas where the females can lay hundreds of eggs, one at a time, under separate milkweed leaves. Soon afterward, that generation dies.
Depending on the weather, the eggs that are laid take 5 to 10 days to hatch. The hatchling caterpillar will then feed voraciously on milkweed for approximately two weeks before metamorphosing into a chrysalis. Two weeks after that, the fully developed monarch butterfly emerges and continues the journey to the north. This new generation will live for an average of five weeks and will repeat the process of breeding and laying eggs somewhere in the central part of the United States just prior to dying. This process repeats itself, sometimes two or three more times for approximately four to five months, until the monarchs reach the northern range of the milkweed plant in the U.S. and southern Canada. It is when that last generation lays their eggs that something truly extraordinary happens.
Around August, the eggs laid by the last generation to travel northward begin to hatch. However, unlike the previous generations, in which the butterflies only lived about five weeks and it took several generations of them to migrate to the northernmost point, these hatchlings are part of a “super generation.” They are named the “Methusalahs,” after the biblical patriarch who was said to have lived 969 years and is the longest lived of all figures mentioned in the Bible. These monarchs will live an amazing eight to nine months and this single generation will make the entire journey of over 2,000 miles in one herculean effort to the highlands of Central Mexico where their great-great grandparents had spent the previous winter.
To put this truly extraordinary migration into perspective in relation to size and distance, a person weighing 150 pounds would have to circle the earth more than 10,000 times to travel as far as a monarch butterfly that journeys over 2,000 miles and weighs less than a gram! It is not only a mystery as to how this generation of Methusalahs comes to be, it is also a mystery how, never having been there, they know to migrate south for thousands of miles to a relatively tiny spot where their ancestors from generations ago had bred and begun this incredible cycle.
Several theories exist on how these amazing insects are able to navigate from so many areas in the northeast to the small biosphere in Mexico. It is believed that they initially use the position of the sun as a compass to guide them into Mexico. As to how they find their way to the same small reserves each year, monarch butterflies apparently have a magnetic detection mechanism. There are particles of magnetite found throughout their bodies which act as magnets that respond to magnetic fields. Due to large deposits of heavy metal such as iron ore that is created by the volcanic activity in the range on which the reserves are located, there is a magnetic disturbance and it appears that the butterflies can detect it and use it as a guidance system.
Triggered by shorter days and colder weather, the migration south from Canada and the northeastern U.S. begins in October with the first Methusalahs arriving in Mexico at the beginning of November. This coincides with the Mexican celebration of the “Day of the Dead,” where the local people believe that the monarchs represent the spirits of their departed ancestors and they rejoice in their return. This amazing generation of monarchs will blanket the trees of these Oyamel Fir Forests, where they will be in a relatively dormant state through December using the forest as a blanket to protect them from bitter cold and storms before slowly waking up and beginning the cycle all over again.
Though the numbers of migrating monarchs have decreased considerably from a high of close to a billion more than 20 years ago to less than 20 million in 2013, there is hope in the fact that they have since begun to rise with this year’s migration being the largest in 15 years, estimated to be close to 100 million.
Thanks to the efforts of the Mexican government to protect the forests where over 90 percent of the North American monarchs winter, combined with programs in the United States and Canada to plant milkweed and other nectar-providing plants necessary for their reproduction and refueling along the migration corridors, the hope is that this trend continues so that future generations can experience one of nature’s most amazing spectacles.