It's that time of year again when millions of migrants enter the southern United States from Mexico to a warm welcome. Monarch butterflies will soon start the second leg of their iconic migration having passed the winter in the Transvolcanic Mountains of Mexico.

But future generations of monarchs faced with changing climates may have a hard time finding their way home.

A monarch butterfly navigates using a sun compass in its mid-brain and circadian clocks in its antennae. But, until now, what makes a monarch reverse its direction has remained a mystery. New research shows that the chill at the start of spring triggers this switch.

Monarch butterflies, having flown south in the fall, reorient themselves and start flying north after they've been exposed to lower temperatures, according to the study published yesterday in Current Biology.

Steven Reppert and Patrick Guerra, neuroscientists at the University of Massachusetts, captured fall migrant monarchs and kept them under various light and temperature conditions.

After 24 days, they released the butterflies into a flight simulator. Monarchs maintained under fall temperature conditions continued due south. Monarchs that were subjected to temperatures similar to those in their overwintering grounds in Mexico of between 4 and 11 degrees Celsius reoriented themselves to fly north.

"How many days of the low temperature are needed or the actual temperatures themselves are just not known. All we know is that for 24 days, day and night, if we mimic temperatures in Mexico, on top of the mountains there, the butterflies then start traveling north," Reppert said.

A 'tremendous finding'
Orley Taylor, founder and director of the education outreach and research program at Monarch Watch, called that a "tremendous finding." A lot of species show reverse migration, but researchers don't know how they do that, said Taylor.

"In science, we have to go from signal to behavior. That's one of the beauties of this, that we have gone from signal to behavior," Taylor said.

The migration of the black-white-and-orange butterfly is unique in the lepidopteran world because its round trip is more like one undertaken by birds -- a journey in the fall from Canada and the northern United States to Southern California and Mexico. There, the monarchs spend the winter in "Goldilocks" microclimates -- low enough to keep their metabolic demands down but not so cold as to let them freeze.

In spring, they fly back into the southern United States, where they lay their eggs, which hatch into larvae that blossom into pupae. Later, a new generation of monarchs bursts forth and continues the journey north.

With temperature as the critical trigger for the monarch's northward journey, climate change could be a big spoilsport in its mass migration.

"What we've identified is a potential area of vulnerability. This area of temperature seems to be critical for their turnaround. If they don't have this cold-temperature environment, they seem to continue to go south," Reppert said.

Butterfly habitat at risk from logging
The authors point out that the monarchs fleeing the North American freeze end up in a delicately balanced Mexican microclimate where temperatures hover just above freezing.

Monarch butterfly watchers say that these microclimates are in jeopardy from logging and degradation in the Mexican forests.

"The butterflies, many of them, will seek out the darkest and coolest points of the forest, and that could allow them to reverse their compass and migrate north," said Taylor, who has watched the species' overwintering space shrink over the years from 7 hectares to less than 4 hectares.

The number of monarch butterflies has been falling in recent years for many reasons.

They have lost vast portions of their habitats to farms and development, drought has killed the milkweed they feed on, and a severe and sudden storm in 2002 killed close to 80 percent of the overwintering monarch population in Mexico.

"That was a very extreme and unusual weather event. It's usually the dry season; there aren't big storms there, but they just had a lot of precipitation. That was followed by cold temperatures, so that juxtaposition of precipitation and cold just killed all the butterflies," said Karen Oberhauser, a professor in the Department of Fisheries, Wildlife and Conservation Biology at the University of Minnesota. "Clearly, that kind of storm is predicted to be more common under climate change scenarios."

The discovery of a cold trigger also raises the possibility of an unseasonal cold spell in the north disrupting the whole migration south.

A different pattern from fall
The temperature trigger is unexpected.

Monarchs take their cue to start migrating in the fall from decreasing hours of sunlight. While studying monarch diapause, a hormonal change that keeps adult butterflies from producing eggs and sperm, Oberhauser found that they showed the greatest response when exposed to changes in both day length and temperature.

"Migratory organisms need to respond to conditions in one place that will predict conditions somewhere else," she said. From an evolutionary perspective, it makes the most sense for them to respond to something that is constant from year to year, she said.

"So it is surprising to me that they are responding to temperature and not day length, which is obviously a fixed cue and is not going to vary from year to year," she said.

Reppert and Guerra tested a group of monarchs by keeping the day length constant from the time they were captured and only decreasing temperature. The butterflies still flew north.

"Subtle changes in day length might still have some influence, but it doesn't seem to be anywhere near as important as temperature," said Reppert. He plans to conduct additional research by using molecular and genetic tools to uncover what proteins and temperature sensory mechanisms are at work.

The monarchs themselves are a hardy lot. As Oberhauser's research shows, they can withstand temperatures up to 40 degrees Celsius and can even weather summer storms by latching onto a plant. What is at stake is their annual cross-country passage.

"It would just be a shame to lose this migration," Taylor said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500