Each fall, millions of monarch butterflies leave their summer breeding grounds in the northeastern U.S. and Canada and travel upwards of 3,000 miles to reach overwintering grounds in southwestern Mexico.
But unlike birds or wildebeest that also embark on epic migrations, these individual butterflies will never return. (See National Geographic's amazing photos of monarchs.)
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.”