Monarch Butterfly Migration May be Drastically Affected by Climate Change

During the fall months as leaves turn red and gold and as temperatures drop, millions of monarch butterflies flutter through the air, migrating from across the eastern United States toward the south. Escaping the frigid temperatures that herald the beginning of winter, they travel up to 2,000 miles in order to reach an overwintering site in a specific grove of fir trees in central Mexico. A new study, though, shows that an exposure to coldness in their overwintering site triggers their return north every spring. Without this exposure, though, these butterflies would continue flying south.

Published in Current Biology, these findings could help explain why monarch butterflies migrate such long distances to a relatively small region--only 300 square miles--atop frost-covered mountains. The monarchs usually arrive in November, congregating in tightly packed clusters in isolated areas amidst the coniferous forests. The trees and the clustering allow the butterflies to exist in a microenvironment that protects them against freezing temperatures while at the same time triggering their return north in the spring.

In order to find out what factors triggered this return, Patrick Guerra, Steven M. Repert and colleagues collected wild monarchs at the start of their migration in the fall and then subjected them to the same temperature and light levels that they would experience in their overwintering location in Mexico. When they were then studied in a flight simulator 24 days later, researchers found that, instead of resuming their southward journey, they butterflies headed north.

The findings showed that, in fact, temperature alone altered the flight direction of the monarch butterflies. If temperatures in our climate continue to warm, though, there could be major implications for the monarchs' migration patterns. It's possible that warmer temperatures could cause migration dates to shift, or even different migration patterns to emerge.

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