These Tiny Shrews Grow Bigger Snouts and Shrink Their Brains to Survive Harsh Winters, Study Suggests

Their snouts seem to elongate when the creatures are cold—a strange reversal of an already odd phenomenon. Scientists have long known that certain shrews, moles, weasels and other small mammals seasonally shrink their brains and other body parts, sometimes by more than 20 percent, during winter and regrow them in the spring. The adaptation, first described by Polish biologist August Dehnel in 1949 and thus called Dehnel’s phenomenon, likely helps the animals survive harsh, frigid conditions.

Now, in a study published in the Proceedings of the Royal Society B on August 19, researchers report that in addition to reducing their brains, long-clawed shrews also temporarily increase the size of their schnozzes. The findings mark the first known instance of what the researchers dub a “reverse Dehnel’s phenomenon” in shrews and deepen our understanding of this curious adaptation. Northeastern Asia is a hotspot for shrew diversity, but the species there remain “strikingly understudied,” the study authors write in the paper.

The seasonal size reductions have mainly been documented in small mammals in Europe. There are more than 385 known species of shrew worldwide. To address the knowledge gap, Yugo Ikeda, a mammalogist at Japan’s Toyo University, and colleagues examined 136 skulls of long-clawed shrews collected between 1948 and 1988 in Hokkaido, the northernmost of Japan’s main islands.

All specimens were housed at the Botanic Garden of Hokkaido University. The specimens covered each month of the animal’s short, less-than-two-year lifespan since the researchers couldn’t study live shrews. Examining 16 traits as the creatures aged revealed that the skulls went through the expected braincase reduction in winter and regrowth in the spring.

The snouts of shrews collected in the winter were 7 percent taller and 6 percent wider than those collected in the spring. The findings, Ikeda tells the outlet, were “completely unexpected and caught us all off guard in the best way possible.” Why would the creatures’ features grow Pinocchio-style in winter? The team has a few ideas.

A larger snout might widen dietary options in the winter by increasing bite force, allowing the animals to chow down on tougher prey. However, the jaw shape didn’t show a similar size change, the authors write, so there’s little direct evidence to support this hypothesis. Another reason might be that a bigger nose improves the shrews’ sense of smell, helping them find prey in the deep snow of Hokkaido winters.