Normally, if an animal’s brain shrinks in size, the results are not great. But for one species of animal, making their brain, skull, and some parts of their body physically shrink in sync with the seasons is a strategy they use to survive the harsh winters.

By following a number of common shrews over the course of multiple seasons, researchers from the Max Plank Institute for Ornithology have confirmed that as the year approaches winter, the shrew's head shrinks by up to 20 percent in size. When the seasons then begin to thaw and move into spring, the shrew’s head starts to grow again, reaching their maximum size in summer, before starting to shrink once more as autumn rolls in.

There have been previous suggestions that the shrews might shrink their heads in response to the season, with the ability named the "Dehnel phenomenon", but these were just general observations. This latest piece of research, however, took things one step further and fully documented the changing size of the mammals' heads over a period of a few seasons.

“We found that each shrew undergoes a dramatic decrease in braincase size from summer to winter,” says Javier Lazaro of Max Planck Institute for Ornithology in Germany, and co-author of the paper published in Current Biology. “Then, in spring, the braincase regrows, almost reaching the original size in the second summer.”

From the summer of 2014 to the autumn of 2015, researchers set traps to catch wild common shrews as they went about their business. When one was caught, the teeny animals were then anesthetized and X-rays were taken of its skull and body. After implanting a microchip, the critters were left to scurry away until the next season in which they were caught and the researchers could compare the measurements.

All adult shrews caught and measured showed the same pattern of growth and development. Not only were the animals growing and shrinking their heads, they were also losing mass in their organs and shortening their spines as the seasons fluctuated.

Why they are doing this is not well understood, but the researchers think they may have an idea. Shrews have massively high metabolisms and during winter they might struggle to find enough food to replenish the huge amounts of energy they are burning. It's possible that by shrinking their heads – and particularly the energy-sapping brain – they need less food to survive the leaner months.

The team now plan to look at exactly how the tiny creatures manage this shrinking trick, as well as how it might impact their intelligence.