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Butterflies by the million : the Monarchs of Michoacán

Tony Burton

Every winter, more than one hundred million monarch butterflies fly into Mexico from the U.S. and Canada. On arrival they congregate in a dozen localities high in the temperate pine and fir forests of the state of Michoacán. As a species, monarchs are native to North America, but they subsequently island-hopped their way around the world across the Pacific to Hawaii, Tonga, Samoa, Australia and New Zealand, and across the Atlantic to Europe. Most monarch butterflies never migrate, but a large part of the North American monarch population undertakes an annual, long-distance migration, a journey without parallel in the insect world.

Scientists are still unable to explain all the details of this enigmatic annual migration. How is it, for example, that the butterflies returning to Mexico, four or five generations down the line from those who flew north in April, manage to find exactly the same groves of trees as their ancestors, high in the Mexican mountains? Is their unexpectedly sophisticated navigational ability due to gravitational differences, an incredible innate accuracy in pinpointing position by measuring the angles of the sun's rays, or due to the effects of magnetism in underlying rocks?

The latter theory gained credence because the areas of Michoacán chosen by the butterflies for winter residence all lie along Mexico's Volcanic Axis, where various magnetic minerals are abundant. The exact sites where the butterflies overwinter were only found in the mid 1970's after a search lasting nearly forty years.

Surprisingly, their numbers are not severely depleted by predatory animals and birds during their three to four month somnolence on Mexican fir trees. In flight, the butterflies' bright orange and black wings act as a warning to birds. At rest, only the undersides of their wings are exposed; the resulting grey and white pattern is excellent camouflage so long as the butterflies remain on the trees. Furthermore, the monarch larvae feed on milkweed plants (Asclepias spp.), and thereby acquire the milkweed's cardenaloid poison. The resulting adult butterflies are sufficiently venomous to deter most would-be predators after their first mouthful.

The tagging of butterflies has proven that they make the 2500 kilometer trip each way at an impressive average speed of 20 km/h, with maximum speeds of up to 40 km/h. One third of their dry body weight is energy-giving fat but far from losing weight on their exhausting journey south, they actually appear to gain it!

The many mysteries about the monarchs only serve to heighten the pleasure visitors get from witnessing one of the most amazing natural spectacles to be seen anywhere on earth. Millions of orange butterflies, with black and white-spotted wings, whether flying overhead or, as on cooler days, clinging apparently lifeless to the grey-green fir trees in such numbers that the trees appear to be in blossom, are an absolutely unforgettable sight.

In September and October, as temperatures in the U.S.A. and Canada fall, and food supplies become scarce, the monarchs fly south, arriving en masse in the state of Michoacán towards the end of November. This migratory group then spends the winter in semi-dormancy on the pine and sacred fir (Abies religiosa) trees of the reserves. During February and March (the best months to see them), early spring sunlight penetrates the groves of fir trees, temperatures begin to rise and the forest floor slowly comes alive with new plant growth. The butterflies, having survived the worst weather, unfurl their wings and flutter about in search of food, water and a mate.

Soon (late March or early April), the butterflies begin to leave the reserves, flying back towards the north, the females laying hundreds of eggs a few days later in northern Mexico and the southern U.S.A. A new generation of butterflies soon emerges to fly still further north. This cycle is repeated several times over the summer. As summer draws to an end, the butterflies fly south to Mexico once more, emulating the migration of their forebears.

While the species as a whole is in no danger of extinction, the migratory group is under considerable threat from both climatic extremes and human activity. Unusually cold spells and hailstorms have sometimes caused the loss of millions of butterfly lives, though without, as yet, any discernible long-term effect on total numbers. Human activity has greatly reduced the area of the monarchs' natural habitats, in both California (for real estate developments) and in Michoacán (for timber and agricultural land).

The existing conservation program aims to provide alternative sources of revenue and employment for the local campesinos who depend on the land and forest for their livelihood. Modest entrance fees (about $2 US per person) help to fund development projects in the local communities.

How to get there:

The most accessible reserve open to the public is El Rosario, where there are dozens of souvenir stalls and rustic snack stands. Don't miss sampling the delicious hand-made blue-corn tortillas! Narrow trails within the sanctuary wind steeply several hundred meters uphill, reaching a maximum altitude of 3050 meters. This altitude can cause some shortage of breath and air temperatures are generally low, so bring a sweater.

Anyone driving their own vehicle to El Rosario is best advised to use the route via San Felipe on Highway 15 and Ocampo. The San Felipe-Ocampo junction is marked by a line of fruit and soft-drink stalls, many of which (in butterfly season) sell delicious granadas (pomegranates). Also at this junction there is an interesting sixteenth century church which, until as recently as 1995, had tombstones in its atrium, unusual in Mexico. Normally, the Spanish buried their dead as far away from the churchyard as possible, presumably to avoid the risk of disease.

It is only fourteen kilometers from San Felipe to Ocampo, from where any vehicle with adequate ground clearance, including the local taxis, can negotiate the fourteen kilometers of flagstone road to the sanctuary car-park. Hikers, however, may prefer to use the shorter, more direct, but steeper approach from Angangueo.

When to go:

The butterflies have become such a popular attraction that it is now definitely preferable to avoid going at a weekend when the reserves are at their busiest. It is far more pleasant to go during the week, to contemplate the marvels of nature in the company, not of thousands of people, but rather of millions of butterflies. Standing still for a few minutes in the quiet of the forest to get your breath back on the way up the trail, you will be just as surprised as I was when you realize that the gentle swishing sound you can hear when the birds aren't singing, isn't the sound of the wind blowing through the tree tops, or even the sound of your own heavy breathing, but actually the sound caused by millions of tiny wings beating as the butterflies flutter about in the sky.

Reference / Further Reading:

This article is an edited extract (with the author's permission) from Tony Burton's

"Western Mexico: A Traveller's Treasury"

(Perception Press, 2001). Copyright 2002 by Tony Burton. All rights reserved.

Published or Updated on: March 14, 2008 by Tony Burton © 2008
Contact Tony Burton

Author of Mexican Kaleidoscope: myths, mysteries and mystique (Sombrero Books, 2016),  Western Mexico, A Traveler's Treasury (4th edition, Sombrero Books, 2013) and "Lake Chapala Through the Ages; an anthology of travelers' tales" (Sombrero Books, 2008), available from all good book stores, and via his author's page at Amazon.com. Co-author of "Geo-Mexico, the geography and dynamics of modern Mexico" (Sombrero Books, 2010). His blog is at geo-mexico.com.

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