See the related story: Final rule on monarch listing proposal deferred to 2030
I fell in love with West Texas while researching the monarch butterfly. In my day, which you might call the olden days, we didn’t study the monarch in school. When my niece asked for a story about the butterfly, I had to find a picture of it. At the first flash of those wings I was captivated. There seemed to be a meaning beyond their wondrous beauty.
During the pandemic I became an armchair explorer. Through books, films and scientific papers, I could follow the migration from eastern parts of the United States and Canada to isolated fir groves in Mexico’s Transvolcanic Belt. Wildlife photographers had illuminated stopping places along the trail. In an old magazine I came upon George D. Lepp’s breathtaking shot of monarchs densely clustered on an oyamel fir. Searching thousands of images, I found a rare view of monarchs flying past volcanic cones into the overwintering grounds. Placed side by side, the photographs were a window to a phenomenon that “dates back a million years,” according to neurobiologist Steven M. Reppert, whose research has focused on the monarch’s navigational abilities and unique circadian clock.
The terrain of Texas and New Mexico suggestsed the ancientness of the migration.
I decided my monarch would originate in a Rocky Mountain state and encounter glistening white sands, sky islands erupting from the desert, and a massive reef overlooking an evaporated sea. A paper by entomologist William H. Calvert validated this route: “Large numbers (hundreds to thousands) are traversing areas much further west than previously reported . … . . . If they continue to the south/southwest, they will soon strike the Davis Mountains or the mountains of Big Bend National Park. Following these mountain chains will take them rapidly into Coahuila, Mexico, where they are expected to continue on a south-southeasterly course.”
Oyamel, the sacred fir, creates a unique climate where North American monarchs can survive the winter. The treetops form a canopy that holds in heat and keeps out the rain and snow. For five months the monarchs cling to the firs, too cold to fly. While temperatures hover above lethal freezing, the trees radiate just enough warmth to keep the butterflies alive. Sexual maturity has been delayed to extend their lipid (fat) reserves. This is all part of nature’s plan. The monarchs must conserve enough energy to last the winter and begin the journey north.
Texas is crucial to the success of the migration. The population east of the Rockies, comprising 99% of the world’s monarchs, funnels through Texas in the fall. Tens of millions of monarchs feed on the late-blooming flowers. The sugary nectar is converted into lipids. Brower found that monarchs store moderate amounts of lipids until they reach Texas, where they “undergo a behavioral shift and spend more time nectaring.” Flowers will be scarce in the overwintering colonies, and they need large lipid reserves to survive.
To reach a tiny fir grove in a sea of volcanoes, the monarchs must navigate precisely. If at any point they have flown off course, they will miss the narrow entrance to the colonies. Southerly flight is sufficient until they funnel through Texas, states Reppert. “However, once the butterflies continue to approach their destination, how do they find their way to the same grove in Michoacán year after year?” This is an enduring mystery.
Monarchs born in late summer are the fifth generation of the year and the only ones to migrate. Weeks after emergence they embark, following the path of their ancestors. Precise information has been transmitted genetically. The purpose-driven monarch flies in the direction of its destination: south in the fall, north in the spring. Reppert states: “A time-compensated sun compass and an inclination-based magnetic compass … allow migrant monarchs to use reliable environmental cues, such as the sun’s position in the sky and the inclination angle of the Earth’s magnetic field, to fly in the appropriate migratory direction.” He postulates they also possess a “geomagnetic map sense” that guides them to the forest. The sense of smell, located in the antennae, may be a component of this map. Beyond Texas, the monarchs may smell the oyamel forests.
They can also sense the presence of water. Perhaps that is how they find Cuatro Ciénegas in Coahuila, one of the most spectacular stopping places. I had already collected George Grall’s aerial shot when I found NASA’s image of a deep gorge with a patch of blue at the bottom. The monarchs descend to a desert brimming with ponds, lagoons and marshes. There is no visual cue that pools of fish lie beneath the arid mountains on their path. Have they followed landmarks to this isolated basin, or have their ancestors left them a map to the best stopping places?
If landmarks serve as visual cues, they are absent when the monarchs approach the Transvolcanic Belt. To reach the colonies in the Monarch Butterfly Biosphere Reserve they must cross Sierra Gorda, a 900,000-acre region “crisscrossed by rivers and valleys that run in many directions. The region appears to lack the north-south ridge lines that would serve as physical cues,” according to NASA.
Near the overwintering grounds Calvert and Brower observed monarchs perched on high ridges. They hypothesized that the presence of these small groups “serves as a visual cue to attract more migrants” to the colonies. The entomologists also reported the astounding sight of towering columns of butterflies spiraling above the ridges. For this there is no photograph.
The wings are in near-perfect condition on arrival, Brower notes. The monarch is able to conserve energy and preserve its wings by soaring and gliding on favorable winds. A million microscopic scales, shingled and angled upward, “boost flight efficiency, generate thrust, and reduce drag,” explains Alan R. Masters. The black thorax, where the flight muscles are located, is also covered with scales. The scales insulate a cold-blooded creature who cannot fly when its flight muscles are below 55°F. According to Masters, “If too many scales are lost, the monarch loses some ability to thermoregulate.”
What makes the monarch butterfly an uplifting symbol? The migration saga throws light upon the infinite renewal of life. Through this story it is possible to see how natural forces work in synchrony to help the monarch complete its mission. The cycle culminates in a triumphal spring. In mid-March the monarchs fly north, reaching southern Texas first. Each female is carrying about 400 eggs. She must find enough milkweed to lay them singly, if possible. The caterpillars only eat milkweed, and they are voracious. Milkweed comes into bloom in Texas just in time to host the next generation. This ending to nature’s story has held me in a state of exaltation ever since.
I’ll never make it to the top of an 11,000-foot volcano, but one day I hope to see monarchs in the wild. I want to see a lush glen on a desert mountain; the Trans-Pecos thimblehead, offering a drop of nectar; Santa Elena Canyon in golden light and a river of stars in the dark skies of Big Bend National Park.
The Monarch’s Message is available in bookstores at Big Bend National Park and online at https://bbnha.org/shop-online/. Your purchase will support the park’s educational and interpretive missions. Please email the author with questions or comments: writings.lit@gmail.com.
