Why do Monarch butterflies migrate so far? Monarch butterflies migrate for thousands of kilometers because they can’t survive the cold winters.
We know that many species of birds navigate when the weather gets too cold. We are familiar with seeing different bird species in the summer and the winter where we live and we know that birds generally fly south (north in the southern hemisphere) to escape the cold. We are less familiar with the butterflies that migrate every winter.
Monarch butterflies live in North America. They are probably one of the most recognizable species of butterfly in North America. They have a wingspan of roughly 10 cm, which makes it fairly large for a butterfly. They are a bright orange color with black marks on their wings. This bright orange color is to warn predators that they are poisonous and should be avoided. When animals use bright colors to signal to other animals that they are poisonous, it is called aposematism, which comes from Greek words that mean “away” and “signal”. Some animals make use of this and are just bright colors without being poisonous. Monarch butterflies carry a toxin that they get from their main food, milkweed. These toxins are called cardiac glycosides, and they make the monarchs taste very bad. A lot of animals will not be able to ingest them and will probably vomit. There are some birds that have evolved ways to eat them, but even those birds have to go without monarchs sometimes because the level of cardiac glycosides in their bodies builds up too high.
Monarchs are not the only butterflies that are poisonous. There are not many, but there are some. The thing that makes the Monarch butterfly special is its incredibly long migratory journey. Some of them migrate up to 5,000 km, depending on where they are located in North America. The most northerly Monarchs live up by the Great Lakes, just on the edge of Canada and they migrate all the way down to northern Mexico. The distance is pretty impressive, but that is not what makes this spectacular. The really impressive things are how they navigate to a place they have never been to before and how they live long enough to make the journey.
If you have ever driven a long way in your car, you know that navigating, unless you have Google Maps, is very difficult. Somehow, Monarch butterflies manage to successfully navigate their way for thousands of kilometers. Nobody knows exactly how they do it, but the most likely method is by using the sun to navigate. The butterflies only fly during the day, and they can use the angle of the sun compared with an internal body clock to keep going in a straight line. This has been tested by lepidopterists. On sunny days, Monarchs fly in a fairly straight line, and on cloudy days they tend to fly a random path. However, how they navigate is not as impressive as how they know where to go. As we will see, the butterflies that start the migration at the end of the summer are the great-great-great-great-grandkids of the Monarchs that made the journey the year before. Again, nobody knows the answer to this either. It seems that when the temperature drops to a certain point, they start to fly in the direction that keeps the temperature the same or warmer.
The really impressive thing is the way the Monarchs that do the migrating live longer than regular Monarch butterflies. During the summer months, adult Monarch butterflies live for 2 to 5 weeks. During that time, they eat, mate, and lay as many eggs as they can. Regular butterflies. Then there is a change. The butterflies that hatch in late August have several differences. The first is that they don’t become sexually mature straight away. The second is that they eat nectar instead of milkweed. This is a concentrated source of energy that they can store. The third is that they have a lower metabolic rate, which means that they can use more energy for flying. Once they hatch out of their chrysalis, they start to fly for Mexico. They stop along the way to eat and rest, but they generally just fly. These late summer Monarchs live for between 6 and 9 months! They spend the majority of their lives flying down to Mexico. Once there, they roost in trees, eat nectar, and stay warm. Only in the spring do they become sexually mature and they start mating. This is triggered by the temperature and the length of the day. Once they mate, they lay eggs, and these eggs are the generation that will start the journey back. Most of the long-lived Monarchs that flew all the way down will not leave Mexico. The next generation become butterflies and they start to fly up towards America, but they don’t fly it in one journey. They stop every now and then to lay eggs and each successive generation flies a little further up. Several generations will pass before they reach their summer haunts again. Several more generations live through the summer before the temperature drops and the whole cycle repeats. And this is what I learned today.
Sources
https://www.fs.usda.gov/wildflowers/pollinators/Monarch_Butterfly/migration/index.shtml
https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Monarch-Butterfly
https://en.wikipedia.org/wiki/Monarch_butterfly_migration
https://blog.3bee.com/en/because-butterflies-are-colourful
https://en.wikipedia.org/wiki/Monarch_butterfly
https://monarchjointventure.org/monarch-biology/monarch-migration
Photo by Tinthia Clemant: https://www.pexels.com/photo/monarch-butterfly-perching-on-red-flower-1557208/