A shrinking skull would be fatal to most animals, but not to this red-toothed shrew (Sorex araneus, pictured). The head of this tiny mammal shrinks by up to 20% every winter, and grows back at the onset of spring, according to a study published today in Current Biology. To document this phenomenon, the researchers used live traps to capture 12 shrews from June 2014 to October 2015. Then they anesthetized the animals before microchipping them for later identification and x-raying of their heads. At the beginning of winter, the shrews reduced the size of their skull by 15.3% on average, which regrew by an average 9.3% in spring. How this process works is not clear, but scientists think tissue within the fibrous joints connecting the skull bones might degrade and then regenerate. Several of the shrews’ major organs, including their brains, also lose mass during winter. All of this shrinkage may help the animals live through times of food shortage, the scientists say.