The Monarch Butterfly migration to mountains near Mexico City is one of the natural world’s greatest wonders. That five UF honors undergraduates observed it firsthand is even more remarkable! Dr. Emmel, along with Dr. Wilmott and Dr. Sourakov, taught “Insects and Plants,” an honors class this spring. As a surprise to all enrolled, Dr. Emmel generously invited his students to journey with him on his annual Hollbrok travel trip to observe the monarchs mate en masse at El Rosario and Sierra Chincua.
The monarch butterfly is the only butterfly to migrate in this way. Though there are resident populations throughout the United States, Canada, and Mexico, a large portion journeys back to Mexico every year during winter to mate.
The roots of this pattern began 40,000 years ago. The monarch originated in Mexico. However, after each period of deglaciation and resulting warmer temperatures, the monarch ventured further and further up the continent. To put it simply, they followed the milkweed. As milkweed, a monarch’s favorite food, began to grow in successively warmer climates, the monarch followed suit and settled in these areas as well. However, as these climates experience hotter temperatures than the ones traditionally conducive for mating, the monarch chooses to migrate to the mountains of Mexico, its ancestral home, to mate in order to survive.
Going to the mountains allows the monarchs to avoid competition for food, as the mountains (which reach 6 degrees below freezing during the winter) are the perfect baseline temperature for the monarch to survive. They lay dormant in the trees during the majority of their time there. However, as the mountains warm in late February, the monarchs “wake up” (fly from the trees where they laid dormant), mate, and fly north again.
The majority fly to the southern part of Texas for the spring milkweeds. They lay eggs there, as well as along the gulf coast. In fact, they come to Gainesville in late March! The generation of monarchs that reach Texas and the gulf coast have a 30 day life cycle (egg to death). They spawn the second generation, parts of which stay where born, while other parts move north (to Alabama, Georgia, etc). These monarchs, taking care only to lay one egg on any particular plant, mirror their parents, mating and dying in the span of 30 days. Their children in turn go further north in mid-May to early June, reaching as far as South Dakota, Idaho, and parts of Canada.
The fourth generation is the game changer. Dubbed the “Mensusalah generation” by scientists, these particular butterflies live for nine months due to an enzyme which decreases the fraying of their chromosomes and duly slows the aging process (Side note: this enzyme may one day be grafted to human genes!). This life span allows them to make an efficient journey back, always going to the same twelve colonies in the Mexican mountains surrounding Mexico City. These butterflies, guided by a cluster of six genes, have a sun compass that allows them to keep a constant orientation to this area of Mexico. The butterflies also recognize this area due to its status as a magnetic anomaly, allowing these mountains to respond to the magnetite in the butterfly’s wings. Though they take several different routes depending on their starting location, these butterflies, as they have for tens of thousands of years, nonetheless return in mass to this place.
MIGRATION IN THE MODERN DAY
Certain phenomena, like this migration, are too great in size to ignore. Even the ancient Teotihuacanians took notice. Murals along the Avenue of the Dead depict warriors chasing butterflies with spears. In the modern day, much of the media produced regarding the butterflies focuses on advocacy. “Butterfly Trees,” a documentary filmed by a crew traveling with the UF students, attempts to draw attention to the fact that the monarch butterfly migration is in danger of disappearing.
At first, this claim seems to be ludicrous. How could the monarch butterfly population be in danger? Numbers are in the millions! However, the passenger pigeon acts as a cautionary tale. Though incredibly numerous in 1900, they went extinct approximately 10 years later as a result of overhunting and climate change. The monarch migration is also in danger of disappearing. As temperatures become warmer each year, the monarch moves further up the mountain. Eventually, there will not be any further to go. Additionally, the monarch loses more of its food source with each successive year. As a result of more sophisticated pesticides and other chemical components, the amount of milkweed is precipitously low. Monsanto, for instance, produces a crystalline protein in its pesticides fatal to monarchs. Milkweed, which often grows among commercial crops, receives a dose of this protein and inadvertently kills the butterflies. Numbers could potentially drop so low in North America that the migration would cease to exist.
Though monarchs can survive in areas other than North America (populations in Hawaii, Fiji, and Australia exist as a result of milkweed spreading during the slave trade), the migration is nonetheless an important phenomenon to observe. The fact that millions of monarchs from different geographic areas intermingle in one place preserves a library of diversity in the monarch important to study. From a genetic perspective, they are fascinating, and the insights taken from study of their genes could radically alter ideas regarding aging and migratory gene coding.
Fortunately, the leaders of all three North American nations took preventative action last year. In 2014, President Nieto of Mexico, President Barack Obama of the United States and Prime Minster Stephen Harper of Canada agreed to cooperate in preserving the monarch migration. Obama, in particular, will set aside tracts of land in the Midwest solely for milkweed. However, the success of this effort also rests in the hands of citizen scientists. Researchers annually receive valuable aid from nonprofessionals of all ages who choose to tag and release monarchs. This facilitates more efficient tracking and counting of the monarchs as they complete the migration. With the combined efforts of research scientists, world leaders and everyday citizens, the monarch migration may weather climate change and last beyond this generation.