Bad news, bears. Hibernation is no longer the coolest thing animals do to survive the winter.

As cold weather approaches, tiny mole-like creatures known as red-toothed shrews will shrink their own heads, reducing their skull and brain mass by as much as 20 percent, according to new research published Monday in Current Biology. When warm weather returns, they will regrow the region nearly to its original size, giving new meaning to the phrase “spring ahead.”

Though it is not yet clear why the shrews go down a few sizes for the winter, the authors of the study speculate that the reduced head and brain size helps them conserve energy when resources are scarce.

“These tiny mammals cannot migrate long distances to avoid winter, nor can they enter any kind of energy-saving state” like hibernation, said Javier Lazaro, a doctoral student at the Max Planck Institute for Ornithology in Germany and an author of the study. They also have high metabolic rates and very little fat stored in their bodies. “Therefore, they starve within a few hours if they do not hunt constantly.”

The researchers say the shrinkage is a survival strategy.

“Brain tissue is energetically very expensive, so reducing overall brain size might decrease energy demands and thus food requirements,” he said.