What Causes the Migration of the Monarch Butterfly?

Recently, I was able to visit the Ellwood Butterfly Preserve in Santa Barbara County.  As I watched, what seemed like bunches of leaves hanging off tall branches would suddenly explode into a flurry of colorful monarch butterflies, swooping and floating through the air.  The sight took my breath away.  For a while, I stood there in awe.  The butterflies had captured my complete attention.

Slowly, I became aware of a volunteer explaining the migratory patterns of the monarch butterfly.  For over one hundred years, the butterflies have been congregating in the preserve, and it is one of the few places outside Mexico where you can find monarch butterflies in the winter.  In late August and early September, monarch butterflies all over North America begin flying south, as one of the only migratory butterflies.

The butterflies who make the trip to Mexico and Ellwood Butterfly Preserve travel over 2,000 miles and live up to eight months.  In late February and early March, those same butterflies that migrated south begin migrating back north.  It then takes nearly five generations for the monarch butterfly to return to its winter home.

Initially, it makes no sense.  If monarch butterflies live for eight months, why does it take five generations for them to return to their winter habitats.  The answer is that the monarch butterflies who do not make the southern migration only live for an average of about five to eight weeks.

That blew my mind.  One generation lives for months and others for weeks?  I couldn’t understand how that worked.  While my curiosity did nothing to minimize the beauty around me, I did resolve to look into the existing research on monarch butterflies.  What I found was a biological system almost as elegant as the butterflies themselves.

Monarch butterflies have two sets of internal clocks that help direct migration.  The first regulates the juvenile hormone which is involved in sexual maturity.  As the days shorten, an unknown epigenetic mechanism is triggered and less juvenile hormone is released in the newest generation of monarch butterflies.  This arrests the development of the butterflies’ sexual organs, directing less energy into mating and allowing for the increased life span.  Secondly, there is a suite of approximately forty genes that regulate the migratory behavior.  These genes are thought to be regulated by day length as well.

Once the butterflies have traveled to their winter homes, they will remain sexually undeveloped until late January, when the levels of juvenile hormone begin to increase. At that point, the butterflies begin to mature. Simultaneously, they begin the flight back north.  The butterflies reach sexual maturity sometime in February, often when they have traveled as far north as central or northern Texas.  There, they reproduce and the next generation takes over the journey.

It never ceases to amaze me how so many species have evolved such complex, yet beautiful, mechanisms to ensure survival.

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