Monarch butterflies (Danaus plexippus) are notable for their long-distance migrations. The fact that an insect measuring a fifth of the weight of penny can fly a journey totaling up to 3,000 miles is amazing. What’s even more impressive is that the individuals that make the journey south have never been to their winter destination before.
Every year millions and millions of monarchs in North America, east of the Continental Divide, migrate to central Mexico to spend the winter months. These butterflies aren’t the same individuals that journeyed north earlier in the year. They’re the great, great grandchildren (grand-butterfly?) generation. The butterflies appear to fly on pure instinct, and the oyamel fir (Abies religiosa) forests in the states of Michoacan and Mexico are apparently an irresistible pull for the monarchs. Only monarchs born in the late summer or fall make the southward migration.
Larval monarchs feed on milkweed plants (Asclepias spp.) after they hatch from eggs. Milkweeds contain a noxious sap that makes monarchs, in turn, unpalatable to potential predators. Adult monarchs feed on nectar, but their high contrast coloration is a warning sign to potential predators of toxicity stemming from their larval days.
The range of oyamel firs in Mexico is extremely limited. The species is only found on 12 isolated mountaintops in Mexico’s Transverse Neovolcanic Belt. Oyamel forests are wet and cool and this environment helps slow the metabolism of monarchs so they can conserve food reserves throughout the winter to make the journey northward in the spring.
Milkweed population declines in North America and deforestation in the Mexican wintering grounds are leading to a decrease in monarch numbers. The 2008 designation of the Monarch Butterfly Biosphere Reserve in Mexico by the United Nations Educational, Scientific, and Cultural Organization (UNESCO) is an attempt to protect valuable monarch winter habitat from further destruction.
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