Scientists Discover That Shrews Shrink Their Heads in Winter

Nature is full of amazing animal adaptations. There are lizards who change color to blend in with their surroundings. There are multiple animals who can sleep through an entire winter without eating anything. And there are even fish that come equipped with built-in flashlights. And now, scientists with the Max Planck Institute for Ornithology in Germany have discovered a new adaptation which is just as interesting as those already mentioned.

The common shrew, which is a tiny insect-eating mammal that lives in parts of Europe and Asia, actually shrinks its skull and body as winter approaches. Then, when spring starts to arrive, it regrows itself.

This phenomenon has long been suspected, but this is the first study to finally prove that the phenomenon occurs. After catching, measuring, and taking X-rays of a number of wild shrews, researchers on this study found that, on average, shrews shrink their braincases by 15% when the weather gets colder. Some shrews’ brain size shrunk by up to 30%. Body mass also decreased significantly.

When spring arrives, the shrews regrow their braincases by an average of 9%, which means they don’t return to their original size. Since shrews only live a little longer than a year, they only go through this process once, and they never re-attain their previous size.

But what purpose does this shrinking serve?

Shrews don’t hibernate or migrate, so they must do something to deal with the decrease in their food supply in the colder months. Winter is a period of extreme food scarcity for these little animals, and the reduction in body mass reduces calorie requirements in them. They have high metabolisms and must constantly eat to maintain their weight. By reducing this weight, they don’t have to eat as much when there is little food available.

The brain is one of the body’s most energy-expensive organs, meaning it takes a lot of calories to keep it running. So, by reducing their brain size, the shrews are again reducing their need for food during the harsh winter months.

Now scientists know that the shrinking process definitely occurs, but they still aren’t sure how it happens. One theory is that the collagen that makes up the cranial sutures is reabsorbed by the body. More research will need to be done to figure out this remaining mystery.

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