AsianScientist (Oct. 23, 2026) –For tiny mammals, surviving winter is a difficult challenge. As temperatures fall and food becomes scarce, they must conserve energy while keeping their bodies warm. A strategy that helps the mammals is seasonal shrinking, where they shrink their bodies and organs by 20 percent during winter and regrow them in spring.
But the full story is more complicated.
According to new research led by Hokkaido University, the skull doesn’t simply shrink; it remodels itself, changing shape with the seasons. The findings of the study were reported in the Proceedings of the Royal Society B: Biological Sciences.
First identified in shrews about 70 years ago, this seasonal, reversible shrinkage is known as Dehnel’s phenomenon. The phenomenon has since been observed in several small mammals, including moles, voles and weasels. In winter, these animals can shrink their skulls as well as internal organs such as the heart, liver and kidneys. The changes are reversed when conditions improve in spring.
But scientists still don’t understand how it happens or how widespread the phenomenon is.
The study, led by Dr Satoshi Ohdachi from Hokkaido University, was built on the master’s thesis of Yukina Suwa – another author of the study; and was subsequently developed by Dr Yugo Ikeda of Toyo University in collaboration with Dr. Ohdachi.
To investigate how the skull changes through the year, the team examined 136 long-clawed shrew (Sorex unguiculatus) specimens from a collection at the University’s Botanic Garden.
The team measured body and skull weight, together with 14 cranial and mandibular traits. They also photographed the skulls from different angles and used morphometric analysis to track changes in their shape.
They found that the seasonal changes were more complex than expected. “We expected to find the classical Dehnel’s pattern, but we found something unexpected,” Ohdachi said.
“While the braincase shrinks in winter, the rostrum—the front part of the skull containing the nasal region—actually grows larger, and then shrinks back again in spring. Different parts of the skull are being remodeled in different, and sometimes opposite, directions depending on the season,” he added.
The researchers have termed this pattern the “reverse Dehnel’s phenomenon” because some parts of the skull, such as the nasal region, grow rather than shrink.
The reason behind the winter enlargement of the rostrum is not yet clear. The researchers speculate that changes in the nasal region could affect heat exchange and potentially help shrews retain heat around the brain during cold weather.
This possibility will need to be tested in future studies. The team plans to investigate the physiological, cellular and developmental changes underlying the Dehnel’s and reverse Dehnel’s phenomena. They also hope to determine whether similar changes occur in other shrew species in Hokkaido and elsewhere, with the hope that comparative studies will uncover general patterns.
Understanding how mammals can reversibly alter the size and shape of their tissues could eventually provide insights into tissue plasticity, neuroscience and biomedical research, the researchers stated.
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Source: Hokkaido University ; Image: Martin Koebsch/shutterstock
The study can be found at : Dehnel’s phenomenon is not a simple reduction of size: seasonal change in skull shape of the long-clawed shrew (Soricidae, Mammalia)
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