The annual migration of Monarch butterflies from Canada to Mexico and back again is as mysterious as it is beautiful.
Each autumn, the brilliant orange-and-black insects converge on fir and pine forests in Mexico’s Sierra Madre mountains, where they spend the winter in a kind of hibernation.
Scientists have only recently begun to tease out the complicated set of environmental cues that guide the insects’ long journeys. Now a new study suggests a surprising factor helps kickstart their northward travels each spring.
“What we’ve found is that coldness is the essential feature for flipping a butterfly’s orientation,” and starting its journey back to Canada as winter ebbs, said lead author Steven Reppert, a professor of neurobiology at the University of Massachusetts Medical School, whose work was published recently in the journal Current Biology.
When Monarchs head south each fall, leaving the eastern U.S., “they’re doing that because they are sensing that the days are getting shorter and winter is coming,” Reppert said. “The butterflies will die if the temperature goes below freezing. So it’s curious that they leave that area, migrate all this distance, and end up on top of a mountain range in a fairly cool environment.”
Temperatures in the oyamel forests where the butterflies roost each winter range from 0-to-15 degrees Celsius — about 32-59 degrees F.
To understand what turns the insects around when spring comes, Reppert and co-author Patrick Guerra, a postdoctoral fellow in his lab, trapped wild butterflies traveling south in autumn and exposed them to temperature and light levels that mimicked the environment at wintering grounds in Mexico.
Twenty-four days later, the researchers placed each butterfly in a plastic barrel “flight simulator” and tracked which direction the insect flew — in this case, north.
The researchers also subjected more butterflies to different combinations of light and temperature to confirm their hunch that temperature nudged the insects to change course.
Those results are surprising, experts said, because Reppert’s prior research showed that decreasing sunlight — not temperature — helps drive the butterflies’ journey south to Mexico in the fall.
“This is big because we have been trying to understand how the migration works going south, but then when you think about when they get to Mexico — how do they reverse it? That’s a question that’s been kicking around subliminally for a long time,” said Chip Taylor, an ecologist at the University of Kansas who directs the MonarchWatch monitoring program. “And what they seem to have is the answer.”
Karen Oberhauser, an ecologist at the University of Minnesota, said Reppert’s latest discovery “is a nice piece of the whole story of Monarch migration.”
It raises intriguing questions about how long and cool an overwintering period Monarchs require to trigger their northern migration, she said.
And it’s not clear whether Monarchs will be able to find the cool winter conditions they crave in Mexico as climate change intensifies.
“Anybody that has looked at what’s happening around the planet understands that it’s getting warmer,” Taylor said. “And if it becomes warmer [in Mexico], will Monarchs get the temperature cue they need to move again?”
The outlook for the butterflies is mixed. Oberhauser’s research suggests Monarchs are hardy insects, able to survive temperatures up to 40 degrees C (104 degrees F). The butterflies have also managed to thrive when transplanted to new environments outside their natural range, including Hawaii, Australia and the Caribbean. And then there’s the tiny but thriving band of Monarchs that live in Florida year-round — not migrating like most of their kin.
Climate-modeling work Oberhauser published in 2003 suggests that the Monarchs’ winter refuge in Mexico will stay cold in winter as the climate changes, but it will become wetter — increasing the chances of the cold, wet weather combination that is often lethal for the insects.
Further north along the Monarchs’ flyway in Minnesota, summers could become too hot for Monarchs, pushing them to fly longer distances to find refuge. And scientists aren’t sure how the insects’ food source, milkweed plants, will adapt to a warmer climate.
In the end, it’s the migration, not the Monarch, that scientists fear might disappear.
“The phenomenon is pretty remarkable,” Taylor said. “Tens of millions of butterflies go to a small area in Mexico. You can get up to 25 million butterflies per acre. . . . The thing we are all struggling with is how fragile is this migration itself — and if it is fragile, how can we keep it going?”
Data on the butterflies’ migration is limited, but fewer Monarchs have made the trip to Mexico in recent years. The number of butterflies that reached their traditional wintering grounds there fell 28 percent from 2011 to 2012, according to a report by the World Wildlife Fund and the Mexican government.
The butterflies covered just 2.89 hectares (7.14 acres) of forest last year, and an even smaller footprint this year, about 1 hectare (2.47 acres), Taylor said, suggesting the hot, dry spring and summer weather the butterflies encountered last year reduced the number that reached Mexican wintering grounds.
In the meantime, Reppert, who has spent the past decade studying every aspect of the Monarch, from discerning the factors that influence its migration to decoding its genetic code, said his next step will be attempting to identify proteins within the butterfly that act as temperature sensors.
“My main purpose is to understand the biology here, because it’s screaming to be understood,” he said. “It’s phenomenal.”
Climate Central, based Princeton, N.J., is an independent organization of scientists and journalists researching and reporting on the science and impacts of climate change.