Chill turns monarchs north

Cold weather flips butterflies’ migratory path

Scientists have pinned down the answer to a long-standing butterfly mystery: what flips monarchs’ migratory compasses. A little cold weather may be all that’s needed. Just 24 days in a chilly lab incubator is enough to switch a butterfly’s flight orientation from south to north, researchers report online February 21 in Current Biology.

BUTTERFLY EFFECT Monarchs stop for nectar during their migration south. The butterflies don’t live long enough to complete a full round trip, making it a mystery as to how they know when to head north. Courtesy of Monarch Watch

HOMEWARD BOUND Cued after a cold winter, a tattered and torn monarch journeys back home in the spring. P.A. Guerra and S.M. Reppert/Current Biology 2013

“It’s pretty doggone cool,” says insect ecologist Orley “Chip” Taylor of the University of Kansas, who was not involved in the new research. But the finding is also disturbing, he says. “It suggests that as temperatures warm, monarchs may be in trouble.”

Each fall, in a massive monarch migration, millions of butterflies set off on a journey from their northern range to central Mexico to escape freezing winters. Nestled among Mexico’s Michoacán mountains, the butterflies cling to tree branches, huddled together in roosts to fend off the cold. “They sort of snuggle each other,” says study author Steven Reppert of the University of Massachusetts Medical School in Worcester. The snuggling creates a cozy microenvironment that buffers high and low temperatures.

Migrating to and overwintering in Mexico is tough work: The long journey strips the black-and-orange monarchs of their vibrant colors and tatters their wings. These old travelers are “battle worn,” Reppert says. “But they’re still troopers.” In the spring, when the monarchs begin their voyage back, he says, “even though their wings are beat up, they fly like crazy — moving ahead like nobody’s business.” The travel-weary butterflies are strong enough to start the journey north, find food and reproduce. Then their descendants finish the trip home.

Until now, researchers didn’t know what triggered the trek north. They suspected environmental factors such as temperature or changing day length could cue the monarchs. To find out, Reppert’s team studied southward-migrating monarchs captured in the eastern United States. The scientists housed one group of migrants in an incubator for 24 days and turned down the temperature to 4° Celsius during dark “night” periods and 11° C during light “day” periods — the average temperatures in the wintertime butterfly roosts. The team exposed a second group of butterflies to the same temperatures while also simulating the subtle increase in daylight that monarchs see over the winter while in Mexico.

Then the researchers took the two groups outside and tethered them one by one inside a flight simulator — a white plastic barrel that gauges flight bearings. In both experimental groups, the lab-wintered butterflies flew north. In fact, Reppert says, “the data were identical.”

Also, southern-migrating monarchs that had been captured in Texas and kept in the lab under fall conditions — with no pulse of nightly cold temperature — continued to head south when hooked up to the flight simulator.

“It’s astounding,” says ecologist Karen Oberhauser of the University of Minnesota, who supplied captured butterflies to Reppert’s team. She’s convinced that just 24 days of cold temperature is enough to switch butterflies’ flight direction from south to north. But she’s also curious about the effects of day length alone.

Reppert says the direction trigger might be modified by changes in day length, but “clearly coldness is the main factor.” Next, his team hopes to figure out exactly how the monarchs sense temperature.

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