|photo credit: jenn_jenn|
One of the most frequently asked questions we get in the Butterfly Center is: “How long does a butterfly live?” We answer, “Typically about two to four weeks” and then, nine times out of ten, the person asks “But then how do monarch butterflies fly all the way to Mexico?”
Good question – and the excuse for us to hold forth about the uniqueness of these butterflies, and the phenomenon of their yearly migration.
Admittedly monarchs are not the only butterflies that make mass migrations. Huge, unidirectional flights of sulphur butterflies, painted ladies, and snout butterflies have all been reported on numerous occasions and in various locations.
However, these migrations are different. They are often triggered when the butterflies run out of food plants for their caterpillars in an area (often after an exceptionally good season when there are lots of butterflies). The emerging adults then all fly to greener pastures, but the flights are not always in the same direction (one year they may fly east, the next west, or south, or whatever). Finally, these mass flights are a one-way trip for the migrating individuals; they do not fly back the way they came.
Conversely, the annual migration of monarchs is predictable, happening at the same time, triggered by the same conditions, and going in the same direction every year. Although most people know that monarchs make this annual long distance trip, many people are confused about exactly how far the butterflies go, how long it takes, and how many generations are involved.
In brief – only one generation makes the entire southward journey, and that same generation comes at least partway back north in the spring. Shorter summer generations don’t migrate south but continue moving northward to fill the entire northern range of this amazing butterfly.
Let’s look at a “case history” to explain in detail. As an example, we come upon a newly emerged monarch female in late August, up in northern Wisconsin. She senses that temperatures are dropping and that the days are getting shorter; she notices the declining angle of the sun. Instead of looking for a mate and preparing to lay eggs, as her mother did, she grabs a meal and starts to head southwest, guided by information in her genes telling her where to go. As she flies, she is joined by more and more monarchs headed in the same direction. After weeks of flying, with occasional fuel and rest stops along the way, they arrive in the fir and pine forests of Mexico’s Sierra Madre Oriental – about 4 hours drive west of Mexico City – sometime around November 1. The Day of the Dead is celebrated in Mexico on that date – and in communities near the monarch overwintering sites, some people think of the butterflies as the returning souls of their ancestors.
From November until about mid-February the butterflies settle down for a long winter’s nap at these pristine, high elevation sites, covering the boughs of the fir and pine trees. On sunny days some of them rouse themselves to fly down for a drink of water or a sip of nectar, but for the most part they just sleep – until the lengthening days and warming temperatures in February tell them it’s time to find a mate and to head back north. In the latter part of February and early March, they finally reach sexual maturity and engage in a frenzy of mating. Then, sometime around mid March, all the butterflies leave their protected winter shelters and head north. The males probably drop out before reaching the US border, but the females keep flying north, looking for newly sprouted milkweed plants growing in fields along the way. Once they’ve laid their eggs, they can finally lay themselves to rest – after a life of about nine months, unusually long for a butterfly.
But the saga continues. The returning migrants’ children hatch, become butterflies, and some continue to move northward, filling the eastern half of North America by mid-summer or so. These summer generations reproduce soon after emerging as butterflies, and their adult lifespans are much shorter, perhaps only a month or so. Then as summer draws to a close, the shortening days, cooler temperatures, and the sun’s declining angle will again signal to the emerging late summer generation that winter is approaching – and that it’s time to forego any sexual activity but instead head south.
Think about it: these new migrants are the grandchildren or great-grandchildren of the migrating generation of the previous year. Unlike migrating songbirds, where the new generation born in North America can follow their parents south to the tropics, these butterflies have no guides to take them to the sites where they return year after year. Somehow, the information telling them what to do and how to get to a distant place they have never seen before is encoded in their genes, and triggered by environmental conditions.
By the way, we have two populations of monarchs in North America. Monarch west of the Rockies behave similarly to their eastern brethren, but there, the fall generation heads to overwintering spots along the California coast. The community of Pacific Grove holds an annual Butterfly Parade in celebration of their winter residents. To learn more about western monarchs, check out the Xerces Society’s page on monarchs. Finally, not all monarchs migrate. Populations that live in areas without cold winters, such as Hawaii and Central America (and recently, Houston), do not migrate. Where temperatures are always balmy, and food for their caterpillars is available year-round, there is no need for the butterflies to undertake a long and arduous trip in order to survive.
Butterfly Sanctuary in Mexico
I’ve been fortunate enough to lead several trips for our museum members to the monarch overwintering sites in Mexico. Seeing millions upon millions of monarchs clustered on the trees like dead leaves, on warmer days filling the air like an orange snowstorm, is truly a magical experience. You can actually hear the whisper of wing beats with so many butterflies in the air. This part of Mexico is not often visited – it’s high (about 10,000 feet, enough to cause shortness of breath in us lowlanders) and cool. The vegetation and general aspect remind one more of the mountains in Colorado than of what we think as typical for Mexico. I recommend this as one of best of the “1000 places to visit before you die.”
It’s not all sweetness and light for the migrating butterflies. Many of them die along the way, or perish during the long months of hibernation. Some of their problems are human-induced; you can learn more about some of the problems monarchs face here.