When Winter Hits, Tiny Shrews Survive by Decreasing Head Size

A common shrew, Sorex araneus, in the snow. Hanna Knutsson on Flickr

According to new research, shrews change their size in accordance with the season. The tiny gray mammals can lose 20 percent of their body size in the cold winter months, possibly to conserve energy.

Scientists have long known that shrews are bigger in the summer than in the winter, and any careful observer could spot the difference. But exactly why their size changed was unknown. Some ecologists thought that the larger animals died off in winter.

To learn more, scientists captured shrews in Germany using live traps, measuring them and repeating the exercise. They caught 100 live common shrews (Sorex araneus) during the spring and anesthetized them. They measured the shrews' overall sizes and used computer imaging and X-rays to look at the size of their skeletons and their brains. They released the shrews and recaptured them several times throughout the year, and found 37 of the same shrews multiple times. Of those 37 they saw a clear pattern: The shrews were getting smaller during the winter, and bigger during the summer.

"We don't know why for sure why this happens," Javier Lázaro, a Ph.D. student at the Max Planck Institute for Ornithology, told Newsweek. "We hypothesize that they have developed these shrinkings to face the winter months when they have less food." Maintaining body mass, especially brain mass, is calorically expensive, Lázaro explained; it means that you have to eat a lot of food to power your body, organs and brain. If you can't migrate to a habitat with better winter food options, it makes sense to simply change your body size. Previously, it had been thought that shrews looked bigger in the summer because the smaller ones had died off.

It's particularly surprising that as shrew skeletons shrink, their skulls shrink too—and so, too, does their brain case and brain. There's no research yet on whether it affects their intelligence.

A collection of shrew skulls. Left, top to bottom: greater white-toothed shrew, Trowbridge’s shrew, vagrant shrew, pygmy shrew. Right, top to bottom: Asian house, northern short-tailed shrew, southern short-tailed shrew. Austin George

The researchers also noticed that the length of the shrew's spine, spleen, liver, heart, fat and muscles all shrunk and regrew. In total, a winter shrew weighs only 6 grams, whereas a summer young shrew weighs about 8 grams, and a summer adult shrew weighs 14 to 15 grams. That means adult shrews nearly double in body mass during the warmer months.

Shrews are members of the family Soricidae, which eat insects and live almost everywhere in the world. They are some of the few terrestrial mammals able to echolocate, and this species is the first studied mammal to demonstrate an ability to change size back and forth after leaving the womb.

The research was published today in the journal Current Biology. Lázaro says the work demonstrates a very important evolutionary mechanism and could have wide-ranging implications, including for human medicine. "We are dealing with a system that loses in this case bone tissues and, in some still unknown mechanism," he says. "They are able to regenerate this tissue."