Monarchs: Mexican Wintering Grounds and the Super Generation

The IMAX film Flight of the Butterflies highlights the dramatic discovery in 1975 of the Mexican wintering grounds of the eastern population of monarch butterflies. These special fir forests are a unique ecosystem, found on only a few isolated mountaintops in Central Mexico, and are vital to the survival of millions of monarch butterflies.

Where?
The Oyamel fir forests are found in the Transverse Neovolcanic Mountain range west of Mexico City. These firs grow at very high altitudes of 2400 to 3600 meters (approx. 8000 - 12000 feet), where temperatures are cool and humidity high. A total of 12 mountaintop sites in a region of about 4100 square miles have been identified as the wintering grounds for monarch butterflies, as shown on the map below. The Oyamel fir forests are an endangered ecosystem, with only 2% of the original forests now remaining. Fortunately, those forests are now recognized and protected from logging by the Mexican government, and organizations like Forests for Monarchs are working with local people to restore the original forests.

What?
Oyamel firs are the dominant tree atop these mountains. They are especially suited to the cool moist climate of the cloud cover of these mountain range and cannot live at lower elevations where temperatures are higher. They grow up to 160 feet tall, with trunks reaching diameters of over 6 feet. The Oyamel fir (Abies religiosa) has long been important in Mexico, even before the discovery of the monarch grounds. As the scientific name might suggest, these trees are also known as Sacred Firs and are used by local people in religious festivals, especially at Christmas.

Why?
Monarchs seek out these high cool forests as a place to rest throughout the winter. They gather in enormous numbers along the trunks and branches, packing in closely for warmth, and essentially shut down for a few months. The cloud cover provides moisture when areas at lower elevations are parched and dry, and the temperatures generally remain just above freezing - perfect for resting and waiting out the winter months. The main threat to their survival during this time are winter storms, which bring heavy rains and sub-freezing temperatures. Monarch butterflies cannot survive being frozen, and when their bodies are wet, they are much more susceptible to freeze. Monarchs who survive these storms usually do so by clustering together under the canopy for protection and warmth. Studies also show that butterflies clustered on the tree trunks of larger trees are more apt to survive, as they provide a slightly warmer micro-climate during the storms.

Who?
The monarch butterflies that overwinter in the Oyamel forests are known as the "Super Generation". Most generations of monarch butterflies live about 3 to 4 weeks as adult butterflies, mating and laying eggs quickly to produce the next generation. The generation that flies south and winters in Mexico lives up to 9 months. Monarchs that emerge from chrysalis in late summer or early fall lead very different lives than those that come before them. Their entire body chemistry differs, with their reproductive organs remaining inactive, quelling their need to mate. (This delay in development is known as diapause.) Instead, these butterflies begin their migration south though Texas to Mexico, feeding at every opportunity along the way to build up their lipid (fat) reserves for the long winter ahead. They fly purposefully and efficiently, soaring as high as a mile in the sky to take advantage of prevailing winds so they can use the least amount of energy possible in their journey.

When they arrive in the Oyamel fir forests, they begin to cluster on the trees, so thickly that the branches sometimes break from the weight. At this point, they enter torpor (similar to hibernation of warm-blooded creatures), slowing their heartbeats to almost nothing. This allows them to conserve energy and live entirely from the fat stores they built up along their journey south. They remain this way until the lengthening days and warmer temperatures of spring trigger a chemical change in their bodies. They begin to awake, leaving the trees and seeking out water sources like streams nearby. Their reproductive systems now activate, and many will now mate before begining the journey north again. This Super Generation generally reaches Texas in late spring, where they lay eggs on the milkweed once again available to them, and then die, leaving their future generations to continue the journey as far north as Canada.