Colder temperatures and shortened daylight egg
caterpillar
pupa
butterfly
hours are indicators for many species of the shift from fall to winter and with the freeze of winter just arriving, some animals are hunkering down and staying local while others are headed to overwintering grounds in warmer climates. And while animals like squirrels and chipmunks have spent the fall stockpiling food to get through these colder months and many insects have found cover and gone dormant, monarch butterflies have been on a continent spanning journey that seems impossible for a creature so small.
What is migration and what purpose does it serve? Animal migration is the movement of animals from one region to another, which is often seasonal and over relatively long distances. It occurs in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects and crustaceans. Animal migration is primarily focused around finding food, livable conditions, and adequate breeding grounds. The monarch migration is truly one of a kind in the insect world, spanning distances of up to 3,000 miles each way, north and south. They travel 50100 miles a day and the whole journey takes only a couple months. If that weren’t amazing enough, it also takes several generations to do it, making monarch migration quite different from that of other wildlife. Monarchs begin their journey north in the springtime at their overwintering grounds in a rural, mountainous region of Michoacán, Mexico*. They make their way, flying, mating, laying eggs, and ultimately dying, passing the baton to the next generation. With an average two to six week life span, it can take four or five generations of monarchs to complete the journey north to their spring and summer grounds in various parts of the US (including Maine) and southern Canada. When the days get shorter and cooler in the fall, female monarch butterflies lay a special generation of eggs, known as a “super generation”. *This is the area of southern Mexico that monarch butterflies east of the rocky mountains go to. There are populations on the west of the rockies that overwinter in coastal California, as well as a small, non-migratory population in southern Florida.
These caterpillars will become butterflies able to fly to Mexico in a single generation, living about eight times longer than their parents. Hormones control most of a monarch’s actions in life, from metamorphasis (the transformation from caterpillar to butterfly), to mating, to migration. The super generation of butterflies manages to live an extra long life by producing less of a specific hormone; this essentially prevents them from aging and reproducing, keeping them in adolescence.
How does an insect with a brain the size of a poppy seed know to fly to a specific area hundreds of miles away? Put simply, monarch butterflies use a combination of the sun’s position in the sky and the time of day to orient themselves on their journey. Their antennae tell them the time of day while they use their eyes and the location of the sun to orient to the right direction as the sun move across the sky. They essentially have a compass in their minds! These are skills somehow written into every monarch’s genetics and scientists are still working to understand the intricacies and the mechanics of how they navigate. After a winter spent hibernating in Mexico, the shift to spring signals that it’s time for the super generation that had flown south to begin the trip back north. The hormone that had prevented them from aging and from reproducing becomes active and so they are able to reproduce before dying and passing the baton on to the next generations, who will continue the journey north.
Monarch butterflies hibernating in oyamel fir trees in Mexico. This specific forest type, with it’s elevation and moisture level, creates an ideal microclimate for monarchs, who can number in the tens of thousands per tree! The butterflies cluster together for warmth.
Monarch Annual Migration
Northern ran ge of m ilkwee No milkweed- no breeding area
Maine
Small non-migratory population
The Milkweed Connection
Monarch caterpillar on common milkweed (Asclepias syriaca)
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Monarch butterflies only lay their eggs on milkweed (Asclepias spp.) and monarch caterpillars can only eat native milkweeds. This symbiotic relationship formed through coevolution (when species reciprocally affect each other’s evolution) is essential to the survival of monarchs. They cannot live without milkweed and in turn, they are an important pollinator of milkweeds, helping this family of beautiful flowering perennials to spread and thrive.
Spring/summer
Monarch annual migration Winter