One morning you wake up and as you step out of your warm cozy bed you realize it is cold! It is the first day of the year that you turn the heat on in your house. You step outside with your jacket on and the smell of autumn is in the air and you notice it isn’t quite as light outside as it has been previous mornings: Winter Is Coming! A lot of northern dwelling animals already sensed this and started their migration south. Not all animals migrate, some, probably like you, will hunker down in the winter eating a little more and/or sleeping more, maybe even putting on a few extra pounds of fat to survive the winter… (or is that just my animal-like winter routine?). Some animals that do migrate south to escape the cold include sea turtles, whales, many birds, and the amazing little insect: the Monarch Butterfly (Danaus plexippus)!
Many of the beautiful butterflies you see flying around during the summer, or in your local butterfly garden, only live for 2-3 weeks. Even Monarchs born in the spring and summer only live for a total of 2 months from egg through butterfly. A Monarch will take up to 5 days to hatch from its egg. Then it spends the next 2 weeks chowing down on milkweed while it grows and accumulates fat. This fat will help it as it sheds its last caterpillar skin and forms a hard shell called a chrysalis. It spends another 2 weeks in its chrysalis as it turns into a butterfly, hatches out and spends another 2 weeks as a butterfly (1)!
Monarchs can sense when the seasons are changing though and the eggs they lay in late summer and early fall are different. They will develop into the great travelers that migrate to their southern homes and instead of only living for 2 weeks after hatching out as a butterfly they will live for 6-8 months (2)! The most well known butterfly migration is completed by the eastern Monarch butterflies. These butterflies live in locations east of the Rocky Mountains in both the United States and southern Canada. There are Monarchs that live west of the Rocky Mountains and while some of these also migrate to Mexico many will migrate to southern locations in California (3).
The late summer/early fall eggs that are laid will grow into adults that will have reproductive behavior suppressed (1-3). This means they will grow and age a lot slower than their spring/summer counterparts who mate within days of starting out their winged life as butterflies. This slower growth and aging allows these butterflies to migrate the thousands of miles to Mexico, some of them traveling all the way from southern Canada, a distance of over 2,800 miles (approximately 4,500 km) (2)! The temperatures in the mountains and trees of their winter home is low, but way warmer than the frozen world north. Since butterflies cannot control their body heat and keep themselves warm like we mammals do, they depend on the outside temperature so when it is cold outside, they are also cold. The lower temperature of their winter home keeps them in a low activity state and also keeps their aging to a slower pace. Man, if this worked on humans we all would be keeping that thermostat down. As spring comes and the temperature warms up so does the body temperature of the butterflies. Their aging catches up and they have mating on the mind. They start heading north and now, 6+ months after they were born they finally mate with their fellow winter friends and continue on northward. These overwintering butterflies will likely never see their summer homes again as they have now reached the end of their life and their children will carry on the northward journey home. These children however are back to the typical 2 week butterfly life span and also will likely never see their parent’s summer home. As only the butterfly stage can migrate, it can take up to 4 generations of butterflies to reach the summer homes of their great-great-grandparents. Of course the number of generations depends on how far north they must journey home. This is highly different from the fall generation of monarchs who complete the entire southern migration on their own!
Much of this information about migration has been collected through citizen science Monarch tagging programs. If you want to get involved with a monarch program in your area check out the Monarch Watch website to learn more (4). Although we knew Monarch butterflies migrated, it wasn’t until 1975 that a team of scientists led by Dr. Fred Urquhart discovered the location in Michoacán, Mexico that hundreds of thousands of Monarch butterflies cluster in forests creating a glow of red-orange trees (2). Of Course the locals in Michoacán already knew these Monarchs were spending winter with them, but it was news to the scientific community. It made breaking news and was subsequently published in National Geographic (5).
As many of us are currently at the point where we are ready for winter to end, to end the below freezing temperatures, and to see green instead of snow and the warmth of the sun warming up our bare arms, have faith. Monarchs usually begin their migration back to their northern homes in mid-March (2). According to them, we have almost made it out!
Citations:
- The National Wildlife Federation. Monarch Butterfly https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Monarch-Butterfly
- Reppert, S.M. and J.C. Rooder (2018). Demystifying Monarch Butterfly Migration. Current Biology 28: R1009-R1022.
- Reppert, S.M., P.A. Guerra, and C. Merlin (2016). Neurobiology of Monarch Butterfly Migration. The Annual Review of Entomology 61: 25-42.
- Monarch Watch https://monarchwatch.org/
- Urquhart, F.A. (1976) Found at Last: The Monarch’s winter Home. National Geographic 150: 161-173.
Courtney The Frogologist 
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