High up in the mountain forests of Mexico, one of nature’s greatest spectacles takes place.
Millions upon millions of monarch butterflies blanket the fir trees, huddling together as they wait out the winter. The sheer volume of butterflies is so dense that it causes branches to bend under the weight.
As the first signs of spring begin to appear, the air becomes filled with the gentle sound of countless beating wings. The time has come for the butterflies to leave these forests and begin one of the greatest migrations on Earth. A journey that will take several generations to complete.
The great migration of the monarch butterfly
Imagine spending the entirety of your childhood in a small town. Modern technology doesn’t exist in this fictional universe, and there is no mingling with nearby villages, so you have little information about the world outside the town boundaries.
Once you reach adulthood, you and your fellow townsfolk must travel to a specific patch of forest thousands of kilometres away. But neither you nor anyone else in your community has ever been to this place before. In fact, the last people to make this journey were your great-great-grandparents.
You also have no access to navigation tools like Google Maps or even a compass. It’s just you and your comrades on a monumental journey.
The chances of reaching your destination seem pretty slim, right? Well, not if you are a monarch butterfly.
During their annual migration, these insects accurately reach their overwintering sites thousands of kilometres away using some of the most incredible inbuilt navigation. They essentially have their own map and compass written into their genes, passed down from the generations that came before.
As autumn begins in the northeastern United States and Canada, millions of monarch butterflies leave their breeding ground, never to return to this site again.
They begin their journey south, setting a course for their overwintering sites up to 4,500 kilometres away on a few specific mountains in central Mexico.
Once they arrive, they wait out the winter huddled together on the branches of oyamel fir trees. Then, as spring approaches in late February, they set off north again. Only this time, they don’t make it back to their breeding grounds. Instead, they mate, lay their eggs on milkweed plants and eventually die partway into their northern migration.
In just a few days, the eggs hatch and the caterpillars dedicate their time to growing, which means eating plenty of milkweed. About two weeks later, they enter a chrysalis and eventually emerge as a fully formed monarch. This new generation then continues the journey that their parents began, but these monarchs only live up to six weeks.
The journey north takes up to five generations of butterflies to complete. It’s like an insect relay, with each generation passing the baton to the next.
As the summer draws to a close, and having reached their northern breeding grounds again, the females lay a very special egg. The butterflies that eventually emerge from these eggs are aptly named ‘super generation’ monarchs, which are larger and can live up to eight times longer than regular monarchs.
These powerful butterflies are capable of flying all the way back to Mexico in just a single generation.
How do monarch butterflies find their way?
Monarch butterflies navigate by orienting themselves based on their relative position to the sun. Research has also shown they use the Earth’s magnetic field as a compass to help guide them on cloudy days.
Their antenna also has an inbuilt clock that can tell the butterflies what time of day it is. So they know where they need to be relative to the sun at any time of day.
After the super monarchs emerge from their hibernation in the forests of Mexico, when their seasonal clock informs them it is time to leave, their inner compass flips direction, and they head north again.
The navigational skills the monarch uses to migrate are written into their genetics and passed down through generations. So from the moment they hatch, they know where they are and where they are going.
Monarchs also have a gene that helps to build efficient muscles to enable them to travel long distances. Scientists compared the genomes of migrating monarch butterflies with non-migratory relatives, such as the southern monarch butterfly.
They discovered a gene that reduces the energy expenditure in the muscles of migratory monarchs allowing them to fly more efficiently. So essentially, monarchs are designed to be ultramarathon runners, not sprinters.
Are monarch butterflies endangered?
Monarch butterflies, as a species, are fairly widespread. Only one subspecies, Danaus plexippus plexippus, is known to perform these great migrations.
These migrating monarchs are considered the most under threat as they rely on multiple locations stretching across North America.
Milkweed plants are an essential lifeline for migrating monarchs, who rely on this plant to lay their eggs. A decline of milkweed, alongside the use of pesticides and deforestation close to their overwintering sites, impacts the numbers of these butterflies.
The good news is that people are working to understand more about the monarch and protect important areas of habitat.
In 2008, the Monarch Butterfly Biosphere Reserve in Mexico was named a UNESCO World Heritage site to protect the forest habitat, where millions of butterflies return each winter.
These large-scale efforts will hopefully mean that the great migration of the monarch butterfly can continue to amaze future generations long into the future.
Further reading
- https://www.nationalgeographic.com/animals/article/monarch-butterfly-migration
- https://whc.unesco.org/en/list/1290/
- https://www.worldwildlife.org/stories/the-great-monarch-migration
- https://www.nationalgeographic.com/animals/article/141001-monarch-migration-genome-science
- https://www.nationalgeographic.com/animals/article/140624-monarch-butterfly-migration-magnetic-compass-insect-science
Header image © Isabelle OHara/Shutterstock
