Image: W. Walas, Wikimedia Commons

Shrews cannot hibernate or migrate their way out of harsh winter conditions — so they shrink their skulls and brains to conserve energy. 

A recent study published in the journal Current Biology shows that individuals of the common shrew species Sorex araneus possess this unique talent — called the “Dehnel phenomenon”. Not only do they shrink their skulls, but they subsequently regrow them in spring. 

A team of researchers at the Max Planck Institute for Ornithology in Germany captured and measured the body mass of 37 shrews in summer, winter, and again in spring. The animals were trapped, X-rayed, and implanted with microchips before they were released back into the wild. Studies revealed that the animals’ brain case size decreased an average of 15% before winter and regrew nearly 10% in spring. 

Closer analysis reveals that there is some mass loss in other major organs as well as the spine. The shrew’s total body mass decreases by about 18% in autumn and rebounded nearly 84% in spring.

Image: James K. Lindsey, Wikimedia Commons

Red-toothed shrews are found in Britain, Europe and parts of Asia and rarely live longer than two years. From a small population that was studied throughout a two year period, researchers conclude the shrew’s heads shrink during consecutive winters, regrowing to nearly their original size during the second spring.

How and why the skull shrinks is uncertain, but it is presumed that because shrews have high metabolisms, their decline in size helps them survive times of food scarcity.

“Reducing head size—and thus brain size—might save energy disproportionally as the brain is energetically so expensive,” researcher Javier Lazaro stated in a press release.

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