Monarch butterflies are well known for their ability to fly 2,000 miles south from North America to Mexico each fall and back again in the spring. Now, researchers reporting in Current Biology, a Cell Press publication, on February 21 have evidence to show that the butterflies would just keep on heading south if it weren't for the chilly weather.
The findings help to explain why the butterflies spend the winter on frosty mountaintops. They also imply that global climate change could profoundly influence the monarchs' migrations, the researchers say.
"The monarchs need the thermal microenvironment at the overwintering sites for the migration cycle to persist," said Steven Reppert of the University of Massachusetts Medical School. "Without that thermal stimulus, the annual migration cycle would be broken and we will have lost one of the most intriguing biological phenomena in the natural world."
Earlier work by Reppert's team found that monarchs rely on an internal compass and skylight cues to guide them in their long-distance travels south. Patrick Guerra and Reppert have now found that the butterflies rely on the same system for their return flight north.
They next wondered what triggers the switch in the butterflies' overall direction. To find out, they captured fall migrants at the start of their migration, subjecting them in the laboratory to the same changes in temperature and light that they would experience in the mountains of Mexico. Remarkably, after 24 days of such treatment, the butterflies headed north upon release.
Further study confirmed that the switch in flight direction depended only on the experience of cold, not on changes in day length. They also confirmed that butterflies brought in and protected from the cold kept right on flying south.
Given these discoveries and the recent availability of genetic and genomic tools for monarchs, Reppert said his team is now poised to understand much more about the biological processes underlying the iconic migration. And that could make all the difference for the monarchs' future.
"This increased understanding will help us protect the migration," Reppert said.
Current Biology, Guerra et al.: "Coldness Triggers Northward Flight in Re-migrant Monarch Butterflies."