Monarch butterflies are a familiar sight in the USA and southern Canada, but each winter they disappear. Unlike many species, they don’t die off –they migrate south, like birds, to hibernate in vast numbers in a few sites in the mountains of central Mexico, and to a lesser degree in sites in California. If you get a chance to witness them arriving or departing their southern home, don’t miss it. It’s one of the great natural history sights of the world.
The Monarch is an extremely handsome butterfly, up to 11 cm across, with its bright terracotta wings, pitch black stripes and white markings. Those markings are a warning – don’t eat me! I taste disgusting and might very well be poisonous. In the case of the Monarch this is true. The caterpillars eat varieties of milkweed that contain foul-tasting toxins which are stored in the body throughout the insect’s life. Once tasted, never forgotten. It is this quality that allows them to hibernate in the open in such vast numbers. If they were good to eat, they would be grazed to extinction by predators very quickly indeed.
The eggs are laid on the underside of the milkweed leaves. Once hatched, the caterpillars take about two weeks to turn into a pupae, and then another two weeks to hatch into the adult form. In this way, there can be many generations each year. It is estimated that in on average there are roughly 300 million individuals by the year’s end. This number is falling, however. Today, there are something like 33% fewer butterflies than when they were first counted. Sadly, those numbers continue to drop.
The autumn migration begins in the fall. All the butterflies from the central and western parts of the continent head south to congregate in the millions in just a few trees in central Mexico, some travelling many thousands of miles. They fill the sky as they arrive. As the weather cools, they become increasingly sluggish until at last they are all completely dormant. Wings closed, they hang from the conifers in such numbers that their weight bends the branches. They look like grey leaves. Only when spring warms the mountain air do they begin to come back to life.
Although the journey south is completed in a single insect’s life, the return north takes many generations to complete. With their rapid life cycle, they re-populate the entire continent as they travel. Their short lives mean that, unlike birds, no individual butterfly ever visits the wintering grounds twice. Indeed, the populations in any one place may well be many generations removed from those who made the original journey. And yet each year, they return to the same few trees that their great-great-great grandparents visited. No one teaches them, no one shows them how or where, and yet they complete the journey faithfully, without fail, year after year. How they do that, remains a mystery to this day.
If you want to see the Monarch Butterflies the next time you’re at the Rancho, just let us know and we’ll take care of all the details.