|Cold is Critical
A new study by researchers at the University of Massachusetts has shown that cold winter temperatures are needed to trigger spring migration.
"The temperature at the overwintering sites is a critical component for the completion of the migration cycle. Without this thermal stimulus, the annual migration cycle would be broken," says Dr. Steven Reppert co-author of the study.
The discovery solves one of the longstanding mysteries of the monarch migration.
Flight Direction Experiments
The researchers conducted controlled temperature experiments on monarchs migrating southward in the fall. Then they used a flight simulator to measure the effect on flight direction:
- Fall migrants after being subjected to cold treatment for 24 days oriented northward.
- Fall migrants not subjected to cold continued to orient southward.
The scientists tested other variables, such as day length. However, the results showed that changes in temperature alone altered the flight direction of the butterflies.
"Without exposure to coldness, the butterflies would continue to fly south; they would not be able to reverse their flight direction," the scientists noted.
Scientists have long understood that a delicate temperature balance is critical to monarch survival at the overwintering sites in Mexico: Temperatures must be cold enough to slow the butterflies' energy use, but not so cold as to freeze the monarchs. This new discovery adds another critical element:
"There appears to be a fine temperature balance at the overwintering sites that is critical for successful migration."