Monarch Butterflies Won't Migrate North Unless They Can Chill Out First In Mexico
February 22, 2013

Monarch Butterflies Won’t Migrate North Unless They Can Chill Out First In Mexico

April Flowers for - Your Universe Online

Millions of monarch butterflies from across the eastern U.S. begin a southward migration each fall to escape the frigid temperatures of the northern boundary of their range. They travel up to 2,000 miles to reach an overwintering site in a very specific grove of fir trees in central Mexico.

A new study from the University of Massachusetts Medical School (UMass) suggests the butterflies require the exposure to coldness in the microenvironment of the overwintering site to trigger their return north every spring. Without the cold snap in Mexico, the monarch butterfly would continue flying south.

The findings of this study, published in a recent issue of Current Biology, explain why monarch butterflies travel such long distances to reach a small, 300 square mile region atop frost-covered mountains.

The butterflies reach their destination in November, congregating in tightly packed clusters in a few isolated locations in the high altitude coniferous forests. The forest cover and the clustering provide a microenvironment suited to protect them against environmental extremes. The temperatures remains low enough to slow metabolic demands, but not cold enough to cause freezing. This microenvironment ultimately triggers their return north in the spring.

The study also suggests that the butterflies may be influenced by - and vulnerable to — global climate change.

"The temperature of the microenvironment at the overwintering sites is a critical component for the completion of the migration cycle," said Steven M. Reppert, MD, professor of neurobiology at UMass Medical School. "Without this thermal stimulus, the annual migration cycle would be broken, and we could have lost one of the most intriguing biological phenomena in the world."

Each migratory journey is completed in a single year, but takes at least three generations of monarch butterflies to complete. The butterflies that return to Mexico each year have never been there before and have no relatives to follow. Scientists have been intrigued for generations with the biological and genetic mechanisms underlying their incredible journey.

Prior research by the same group found that the butterflies rely on a time-compensated sun compass to direct their migration south. The new study reveals that those same systems guide them north again each spring.

The sun compass alone isn't enough to explain what triggers the change in direction each spring, however.

Patrick Guerra, a postdoctoral fellow in Reppert's lab, collected wild monarchs at the start of their migration in the fall and subjected the monarchs to the same temperature and light levels they would experience in their overwintering ground in Mexico to understand why the change in direction happens. The monarchs were studied in a flight simulator 24 days later, and surprisingly, instead of resuming their southward journey, they headed north.

That changes in temperature alone altered the flight direction of the monarch butterflies was confirmed by further study. Butterflies subjected to cold headed north, while those protected from the cold would continue south.

Coupled with newly available genetic and genomic tools for monarchs, the findings of this study will lead to new insights about the biological processes underlying their remarkable migratory journey.

"The more we learn, the clearer it becomes that the monarch migration is a uniquely fragile biological process," said Reppert. "Understanding how it works means we'll be better able to protect this iconic system from external threats such as global warming."