Monarch Butterflies Migrate 3,000 Miles—Here’s How
The colorful insect’s migration across the North American continent is one of the greatest natural events on Earth.
Why won’t they make it all the way back? How do they know where to go in the first place?
How Does Monarch Butterfly Migration Work?
As the days get shorter and the temperatures drop off, monarchs begin to abandon breeding and feeding territories in search of a safe place to spend the winter.
For monarchs, that overwintering ground is found high up on just a few mountains in central Mexico. Once there, the monarchs huddle together by the millions on the branches of oyamel fir trees.
These trees, also known as sacred firs, create a microclimate that protects the insects, says Pablo Jaramillo-López, a research scientist at the National Autonomous University of Mexico.
“The tree canopy and ecosystem provide a blanket effect for the monarchs, so the temperatures don’t go too high or too low,” says López.
After waiting out the winter, these individuals head part of the way back north to warmer climes such as Texas, where they mate and lay eggs on milkweed plants. In just a few days, the eggs hatch into brilliantly striped caterpillars of black, gold, and white. These monarch larvae consume vast amounts of milkweed before forming a chrysalis and transforming into adult butterflies. (Read how the monarch’s decline is linked to milkweed.)
At this point in the cycle, the new butterflies take to the skies again and fly another few hundred miles north before finding another patch of milkweed and repeating the process.
It might take the monarchs as many as four to five generations to complete the journey all the way back up to Canada, says Sarina Jepsen, who directs the endangered species program for the Xerces Society, an international nonprofit focused on invertebrate conservation.
Then, when fall rolls around again, the monarchs make use of air currents to wing all the way back to Mexico—a seemingly impossible feat for such a delicate-looking insect.
“Interestingly, the waves of monarchs heading north will complete their entire life cycles in just five to seven weeks each,” says Jaramillo-López.
“But when fall rolls around again, a special ‘super generation’ of monarchs that can live up to eight months will make use of air currents to wing all the way back to Mexico—a seemingly impossible feat for such a delicate-looking insect.”
“This makes the migrating monarchs so unique as they are the same species but for some reason live much longer.”
Why Do Monarchs Migrate, and How Do They Know Where to Go?
This part is still very much up for debate, says Jepsen. But there are a couple of theories.
One explanation might be that when monarchs march north, they are essentially following the bloom of their primary food source—milkweed plants. And then when winter comes and those plants die back, the animals retreat to a place with conditions that protect them from the weather.
Another idea involves their immune systems. “Migration also allows monarchs to escape habitats where parasites have accumulated at the end of the summer,” says Sonia Altizer, an ecologist at the University of Georgia. (Read: “Imperiled Monarch Butterflies Get $3.2 Million From U.S. Government.”)
Altizer has found that adult butterflies infected with a protozoan parasite called Ophryocystis elektroscirrha, or OE, can’t fly as well in lab tests and travel shorter distances in the wild. (Read m
“We think that migration weeds out the most heavily infected monarchs, removing them from the populations,” says Altizer.
As for how the monarchs know where to go, a study published in 2016 suggests that the critters navigate based on their relative position to the sun. But scientists are still ironing out exactly how this works.
Why Are Monarch Butterflies Important?
While monarchs may seem small and insignificant, the creatures play a crucial role in the ecosystems they inhabit.
As adults, monarch butterflies visit countless numbers of wildflowers each year as they seek out nutrient-rich nectar. In doing so, the monarchs transfer pollen from one plant to another and assist in those species’ reproduction. (Learn the key to the monarch’s long-distance migration.)
And even though monarch caterpillars and adults are poisonous to most predators, thanks to toxins they acquire from milkweed, some animals are still able to stomach them. Orioles and grosbeaks in particular make a feast of monarchs over the winter, and ants, wasps, flies, and spiders have been known to prey on the caterpillars when they get the chance.
Are Monarch Butterflies Endangered?
The short answer is no. Monarch butterflies are actually quite common across the world, with populations occurring as far away as North Africa and New Zealand.
However, the subspecies known as Danaus plexippus plexippus is the only one that performs the great North American migration—and these butterflies are increasingly under threat. (See “Monarch Butterflies Hit New Low; ‘Worrisome’ Trend.”)
Jepsen says the Xerces Society has petitioned the U.S. Fish and Wildlife Service to declare the migratory subspecies of monarchs as threatened under the Endangered Species Act. That finding is currently under review.
What Threats Do Migrating Monarchs Face?
Because migratory monarchs have such a wide range, they also have many threats. These include declining milkweed populations across their range in the U.S. and Canada, parasites and diseases like those in Altizer’s research, and even a growing demand for avocados sourced from Mexico. (Read more about the world’s great migrations.)
The avocado problem is a tricky one, says Jaramillo-López, because the people who live near the monarch wintering grounds need good ways to make a living. Unfortunately, when native forests are replaced with avocado plantations, it can have a ripple effect on monarchs higher up in the mountains.
Mexico has designated almost 140,000 acres of forest as a protected area for wintering monarchs, but Jaramillo-López says much of this area is really just a buffer zone that prevents strong winds from damaging the core area where the monarchs congregate. So even if the area being razed for avocado cultivation is at a lower elevation than butterflies prefer, it can still affect the forests on which they rely.
According to Jepsen, people can also do damage to wild populations by trying to raise monarch eggs bought from commercial suppliers. There’s very little oversight, she says, which means that even if you’re trying to help monarchs, you might be inadvertently spreading disease. (See “How Your Backyard Can Save Butterflies.”)
“The best thing you can do,” says Jepsen, “is to provide habitat for monarchs and just let them do their thing.”
The monarch butterfly’s spots may be its superpower
An intriguing new study hints that the wing color patterns on a migrating monarch may help the insects fly better.
Butterflies congregate at their winter roost in Sierra Chincua, Mexico. The cool mountain climate slows their metabolism and saves energy.
Amazingly, some of the monarch’s continent-spanning magic may be owed to the size of its itty bitty wing spots, according to a study published today in PLOS One.
By studying photos of wing patterns of nearly 400 monarchs collected at various locations on their migration path, Davis and his co-authors discovered an interesting pattern—butterflies that make the journey all the way to Mexico tend to have white wing spots that are three percent larger than those of monarchs found in places such as Georgia or Minnesota. At the same time, the Mexican animals have three percent less black coloration on their wings than monarchs collected during earlier phases of the multi-generational migration. (Read how monarch butterflies migrate 3,000 miles.)
Now, here’s why this is interesting. Migrating monarchs soar at heights of up to 1,200 feet. As sunlight hits those wings, it heats them up, but unevenly. Black areas get hotter, while white areas stay cooler. The scientists believe that when these forces are alternated, as they are with a monarch’s white spots set against black bands on the wings’ edges, it seems to create micro-vortices of air that reduce drag—making flight more efficient.
Similar drag-reducing characteristics have been discovered in shark skin and the coloration of seabird wings. All of it adds up to some serious potential for the future of human technologies.
“If you want to develop drones that are flying for longer time and harnessing energy from sunlight, this is the best thing that we can look at,” says study co-author Mostafa Hassanalian, an associate professor of mechanical engineering at New Mexico Tech.
How the monarch butterfly got its spots
The second part of the study evaluated size differences in white spots between the monarch and six of its closest cousins in the genus Danaus.
When study co-author Christina Vu, then a student of Davis’, quantified the size of all those butterflies’ spots, she found the monarchs had by far the largest white markings. Next were the southern monarchs (D. erippus), which are semi-migratory, followed by the other five species—including the Jamaican monarch (D. cleophile), the solider or tropical queen (D. erisimus thetys), the queen (D. gilippus Berenice), and striated queen (D. g. strigose)—none of which migrate.
Indeed, it seems spot size—and, the scientists theorize, its ability to reduce drag—might be linked to migration itself.
“I think that the fall migration is already a major selective event each year,” says Davis. “It ensures that only the fittest individuals reach the finish line. The ones without any infections; or disease; or with the biggest, most robust wings.” (See National Geographic’s amazing photos of monarchs.) “And in this case, the best spot patterns,” he says.
The butterfly effect
“A butterfly can flap its wings in Peking and in New York, you get rain instead of sunshine,” quips Jeff Goldblum’s character in the original Jurassic Park.
From Havana to Donnie Darko, many films have introduced audiences to the so-called “butterfly effect,” or the general idea that small changes can sometimes create large consequences. And while many references misinterpret the idea originally laid out by MIT meteorology professor Edward Lorenz in 1963, it’s tempting to use the metaphor as a way to understand how a monarch’s wing patterns could be of any consequence.
After all, we’re talking about spots the size of a pencil eraser on an insect that weighs about as much as a kernel of corn.
“But the sheer distance that they have to travel, they’re spending 10 hours a day in the air, for 60 days in total, to get to their destination,” says Davis. (Read how monarchs may be doing better than we thought.)
So a minor difference—in this case, spot size—”is compounded on a daily basis. And so that could be the difference between life and death in during the migration.”
‘Revolutionary’ research
The next step would be to test whether such a small difference in coloration could have observable effects on drag, Michaël Nicolaï, a biologist at Ghent University in Belgium, said in an email.
For instance, the new research found just a three percent difference in coloration, whereas a study he conducted on seabirds revealed a 20 percent increase in efficiency for darkly colored wing feathers.
Nicolaï does agree that even tiny differences could be beneficial over such large distances, however, until real experimental measurements are provided, he will remain “very optimistic” but “not convinced.” Davis hopes his study inspires others to research that question.
“No one in the world of butterflies is looking at their wings in this way,” says Davis. “People are going to be looking at every other butterfly species out there now thinking, ‘Oh my God, now what does this color do for its flight?’” “I think this is going to be revolutionary.”