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Humans turn to heated blankets and fluffy socks to cope with winter, but long-clawed shrews have a very different adaptation: they increase the size of their snouts.

Researchers have previously discovered that certain small mammals, including the common shrew and European mole, reversibly shrink their body mass, skull size and brain mass in winter, with regrowth seen in the spring – a strategy that has been named Dehnel’s phenomenon.

Scientists now say they have documented similar changes in the long-clawed shrew, but have discovered the opposite trend occurs for the snout.

“This is precisely why we termed it the ‘reverse Dehnel’s phenomenon’,” said Dr Yugo Ikeda, first author of the research from Toyo University in Japan.

Writing in the journal Proceedings of the Royal Society B, Ikeda and colleagues describe how they examined 136 skulls from long-clawed shrews, collected in Hokkaido between 1948 and 1988 and now housed at the Botanical Garden of Hokkaido University in Japan.

“Measuring skull structures in live, wild shrews repeatedly without harming them is practically impossible,” said Ikeda.

The researchers note that wild, long-clawed shrews are born between spring and autumn and typically die before they reach their second winter. The specimens available covered each month of this lifespan, and were classed as belonging to either a sexually immature or sexually mature shrew.

Tracking 16 measurements by month, they found several changed by month or season – and not in a uniform way.

Among other findings, shrews collected over the winter season, between January and April, were found to have a brain case height (vertical skull measurement) 12% smaller than that of younger shrews collected from August to September in their first year of life. However, the height of the snout, also known as the rostrum, was 7% larger and its width was 6% greater in shrews collected in winter.

“This is a ‘true change’ where the bone structure of the rostrum itself physically alters its size,” said Ikeda. “What makes this particularly unique is that the rostrum expands while other parts of the skull shrink during winter.”

The team said these changes were reversible, with measurements including brain case height found to be larger in shrews that survived winter and were collected between August and November in their second year. By contrast, snout width – although not snout height – was smaller.

The researchers suggest a number of explanations for the seasonal change in snout size, including regulating body temperature.

“Expanding the nasal cavity creates more room for a rich vascular network,” said Ikeda. “When the shrew inhales freezing air, this network efficiently warms it before it reaches the delicate, winter-shrunken brain – helping maintain core cognitive functions.” The expansion of the nasal cavity, he noted, may also boost shrews’ ability to sniff out prey such as earthworms.

The researchers said a third possible explanation was mechanical reinforcement of the snout, allowing the shrew to process tougher prey when food is scarce – although they found little evidence to support this.

Ikeda said it was possible seasonal changes in snout size could occur in other shrews, adding that Hokkaido was home to three further species.

“We are very excited about investigating the presence, absence, and degree of both Dehnel’s and reverse Dehnel’s phenomena among these species in future research,” he said.