Mexico Part II - Monarch Butterfly Reserve

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Monarch Butterflies en masse

The main reason for me to choose Mexico for this years Christmas trip is to visit a place called Monarch Butterfly Biosphere Reserve.

Monarch Butterfly Biosphere Reserve is a sanctuary in the highlands of Mexico protecting the winter habitat of the Monarch Butterfly. I have for years been thinking about visiting this place and now this year I have the chance to do so.

In the summer the Monarch Butterflies live in North and Central America as far north as southern Canada. These butterflies have a very interesting strategy in order to survive the winter - they migrate.

Each year somewhere between 50 and 100 million Monarch Butterflies live in a few forests in central Mexican highlands. Why they choose the highlands where the temperatures actually go down below freezing point at night is a mystery at least to me. I mean if I was a Monarch Butterfly I would rather spend winter at a beach in the Yucatan Peninsula. But I guess the butterflies have a reason for staying in the mountains.

The butterflies arrive in October and stay until March. When the weather is starting to get better the butterflies leave their winter habitat and start to fly north. In the middle of the summer the butterflies have come as far as the great lakes at the border between Canada and USA.

Soon after the butterflies arrive in southern Canada they start to fly down towards Mexico again. This is where it starts to get interesting. The butterflies that were in Mexico at New Year are at this point all dead. They have all died simply because their life span is too short to allow them to fly all the way from Mexico to Canada. So the butterflies that now start flying back south are the children or grandchildren of the butterflies who started the journey up north in March. Most of the butterflies who leave Canada in summer to migrate south will not make it to Mexico either for the reason given above.

It actually takes from three to five generations of butterflies to make the journey from Mexico up north in the USA and into Canada and then back to Mexico. It is amazing how these insects can find their way back to the same forests as their ancestors up to five generations back stayed in the winter. How they do it I don’t know. I guess they are genetically programmed to do this journey every year. Either that or they have some kind of trail of pheromones they follow.

To put things in perspective: I live in central Stockholm in Sweden. Drop me off 15 kilometres from Stockholm Central Station in any direction and I can’t find my way home. These insects, whose entire mass is less than 1% of my brain weighs, can find their way from Canada to Central Mexico without ever having been near the place. I find that amazing and I would believe one or two of you who reads this think so too.

In the winter the Monarch Butterflies mainly sit in large clusters half way up the trees. They sit in these groups to preserve energy and prevent heat loss. When the sun shines in daytime and the butterflies get a bit warmer some of them fly away and look for food. So in daytime there are hundreds or even thousands of butterflies flying in the air.

I visited Monarch Butterfly Biosphere Reserve in December which is the coldest time of the year. There were plenty of butterflies in the air in the afternoon but the vast majority was still sitting in the trees. I guess that in February and early March there are many more butterflies flying around. It would be interesting to go back there then and see that.

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