Steven M. Reppert, MD, professor of neurobiology, and Patrick Guerra, a postdoctoral fellow in Reppert's lab at the University of Massachusetts Medical School, reported the discovery of what triggers the annual return north by millions of monarch butterflies in Current Biology on Feb. 21, 2013.
The researchers captured a group of monarch butterflies just beginning the Mexico bound portion of their annual 2,000 mile migration and exposed them to the same temperatures that the butterflies experience in the parts of Mexico where they overwinter.
When the test butterflies were released after 24 days in temperature controlled captivity the butterflies headed north even tough it was December.
The monarch overwintering sites in Mexico are a relatively small region roughly 300 square miles in size atop frost covered mountains. The butterflies cluster together for warmth and are motivated to head north when the temperatures get close to the point that the insects would freeze.
The migration of monarch butterflies takes three generations to complete. The butterflies do not have aged guides to their wintering homes in Mexico nor do they have guides back north to the United States and Canada. Temperature and sun position guide the monarch butterflies on the annual migration.
The researchers note that climate change may alter the temperatures of the monarch butterfly's winter homes and alter the cycle of migration. Genetic adaptation to climate change may not be possible for monarch butterflies because the butterfly has a short life span.
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