Mothers with young cubs have remained active in recent winters, because warmer temperatures and decreased snowfall have made food resources such as nuts and berries available year-round, scientists with Spain's Brown Bear Foundation say.
It is not yet known how the new strategy may affect the bears' ability to survive and reproduce.
For some species, however, the impacts of disrupted hibernation go far beyond an annoying case of insomnia.
"The first thing to keep in mind is that hibernation isn't a solution to cold environments but to seasonally available resources," said Murray Humphries of McGill University in Montreal, Canada.
Temperature is often a cue to enter or emerge from hibernation. But a winter warm spell doesn't guarantee that food will be available.
For example, bats that take flight in January, as seen this year in Quebec, find a sky bereft of insects.
"They may be just exploring—or they may be starving," Humphries said.
Colder is Better
It might seem that milder winters would make life easier for animals living in cold environments. A longer growing season for plants can make more food available. And active animals require less energy when temperatures are warm.
During hibernation, however, the cold actually helps animals conserve energy.
"The colder a hibernator can get, the more energy it can save," Humphries said. "Without access to cold temperatures, hibernating species face a problem—running out of energy before spring,"
Gregory Florant of Colorado State University has studied the effects of temperature changes on hibernating ground squirrels.
In warmer conditions the squirrels use up stored fat more quickly. This in turn reduces their levels of the protein hormone leptin.
"If you have lower leptin levels, your body tells you that you don't have enough food," Florant said.
As a result, the animals wake up both early and hungry, with little remaining energy to see them through to their first meal.
An early wake-up might not be a problem if plenty of food is waiting just outside the den. But animals roused from hibernation by warmer temperatures in late winter or early spring may find nature's larder bare.
Timing is Everything
A similar problem is encountered by species that complete northward migrations too early.
There are now dozens of examples of species migrating earlier than in years past—and arriving to find an environment not yet prepared to accommodate them.
Far fewer studies have been done on changes in hibernation patterns. But the phenomenon of early arousal may be widespread.
Perhaps the best data come from Colorado.
Researchers with the Rocky Mountain Biological Laboratory have tracked the "first appearance" dates of migratory birds and hibernating mammals over a 30-year period.
In this region spring temperatures have increased. But so has winter snowfall.
Yellow-bellied marmots that used to emerge from hibernation in mid-May now appear in mid-April. Hungry for grass and other plants, the early-rising marmots often find their high meadows still covered by a thick layer of snow.
"When they get up early in years with a dense snowpack, they starve or are eaten by predators," said biologist Dan Blumstein of the University of California, Los Angeles.
[Blumstein has received funding from the National Geographic Society's Committee for Research and Exploration to study reproductive strategies of male yellow-bellied marmots. National Geographic News is part of the National Geographic Society.]
David Inouye of the University of Maryland published his first study on changes in marmot hibernation patterns in 2001. Since then, "the trends toward earlier emergence by marmots have continued," he said.
Strangely enough, golden-mantled ground squirrels and least chipmunks in the same region are actually hibernating longer.
"Chipmunks and ground squirrels may be using another cue," Inouye said, "perhaps the disappearance of snow from the burrows."
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