Shrew skulls do not simply shrink for winter: the snout grows while the braincase flattens
A study of 136 museum skulls from Hokkaido found winter snouts about 7 percent taller and 6 percent wider even as the braincase shrank. The authors call it a reverse Dehnel's phenomenon.
The Scientist · Science desk
What happened
- Dehnel's phenomenon was first described by Polish zoologist August Dehnel in 1949, and involves certain small mammals, most notably the common shrew, shrinking and regrowing their skulls, brains and bodies to survive harsh winters by reducing energy requirements.
- The new findings were reported on August 19 in Proceedings of the Royal Society B.
- Dehnel's phenomenon involves winter reductions in body size and internal organ mass, and is thought to be an adaptation for surviving winter because paring back tissues cuts food energy costs when calories are scarce.
- The researchers measured the dimensions of 136 skulls from museum specimens of long-clawed shrews (Sorex unguiculatus), collected between 1948 and 1988 on the Japanese island of Hokkaido; the skulls came from a Hokkaido University museum collection.
- Some other animals such as weasels, voles and moles experience similar winter changes.
Why it matters
Researchers who measured 136 long-clawed shrew skulls from Hokkaido report that the animals' snouts enlarge over winter at the same time as the braincase shrinks, a pattern they call the "reverse Dehnel's phenomenon" [s1 c4][8][9]. The finding was published on August 19 in Proceedings of the Royal Society B [2]. It matters because the standard explanation for winter shrinkage in shrews is calorie economy, and a body part that gets bigger when food is scarce does not fit that account [3].
Dehnel's phenomenon, first described by the Polish zoologist August Dehnel in 1949, involves winter reductions in body size, skull, brain and internal organ mass, followed by regrowth in spring [1][3]. Similar seasonal changes are known in weasels, voles and moles [5]. Until now it was not established whether shrews outside Europe did the same thing, despite the severe snowy winters of Northeast Asia [6].
Yugo Ikeda, a mammalogist at Toyo University in Tokyo, and colleagues used museum specimens of Sorex unguiculatus collected on Hokkaido between 1948 and 1988, photographing each skull and using software to measure dozens of landmarks, then comparing dimensions against the season of collection [7][4]. The specimen series covered every month of the animals' first year, and the second year for individuals that lived that long, but no live animals were tracked [11][12]. That is the main limitation: the seasonal trajectory is inferred from a cross-section of dead specimens spanning four decades of collecting [23].
The braincase did contract as expected, and it did so mainly by flattening rather than by shrinking equally in all directions [8]. The snout went the other way: in winter months it was about 7 percent taller and 6 percent wider, reaching 2.43 and 2.19 millimetres, and was small again by spring [9][10]. In absolute terms that is roughly 0.16 millimetres of added height and 0.12 millimetres of added width [21], which is worth keeping in view when reading the percentages. Earlier high-resolution CT work had already indicated that cranial change in shrews involves bending and non-uniform regional transitions rather than isometric scaling, and that brain volume loss reflects cellular shrinkage and water redistribution rather than cell death, with the olfactory bulb showing winter increases in cell populations [20][22].
The authors offer three non-exclusive hypotheses for the snout expansion. A larger snout could raise bite force and widen the diet, though they note little direct evidence for this [13]. It could support heat exchange, with dense vascular networks warming inhaled air before it reaches a shrunken brain, which the authors say is strongly supported by comparative anatomical and physiological evidence [14]. Or it could improve olfaction for finding prey under snow [15]. Ikeda's framing is that skeletal plasticity "isn't just an all-or-nothing shrinking act" [16].
Two things would sharpen this. Jan R. E. Taylor of the University of Bialystok, who was not involved in the work, wants to know whether snout enlargement occurs in the milder parts of this species' range [17]; the lesser white-toothed shrew, from warmer regions, shows no seasonal cranial or body reduction at all [24]. The authors also propose comparative analysis across coexisting Sorex species in Hokkaido to separate local adaptation from phylogenetic constraint or plastic response [18], and Ikeda plans to examine how nasal tissues change across seasons [19].
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- [1]
Dehnel's phenomenon was first described by Polish zoologist August Dehnel in 1949, and involves certain small mammals, most notably the common shrew, shrinking and regrowing their skulls, brains and bodies to survive harsh winters by reducing energy requirements.
- [2]
The new findings were reported on August 19 in Proceedings of the Royal Society B.
- [3]
Dehnel's phenomenon involves winter reductions in body size and internal organ mass, and is thought to be an adaptation for surviving winter because paring back tissues cuts food energy costs when calories are scarce.
- [4]
The researchers measured the dimensions of 136 skulls from museum specimens of long-clawed shrews (Sorex unguiculatus), collected between 1948 and 1988 on the Japanese island of Hokkaido; the skulls came from a Hokkaido University museum collection.
- [5]
Some other animals such as weasels, voles and moles experience similar winter changes.
- [6]
Scientists did not know whether any shrews outside of Europe underwent these changes; Northeast Asia has extreme, snowy winters, and Northeast Asian shrews had previously received little detailed study despite the region's harsh winters and high shrew diversity.
- [7]
The work was led by Yugo Ikeda, a mammalogist at Toyo University in Tokyo, whose team photographed the skulls and used software to measure dozens of physical landmarks, then compared skull dimensions with the season in which the shrews were collected.
- [8]
The specimens showed the classic winter reduction and spring regrowth of the braincase, and the braincase changed mostly by flattening in winter rather than shrinking equally in every direction.
- [9]
During winter months the shrews' snouts were 7 percent taller and 6 percent wider, increasing to 2.43 and 2.19 millimetres respectively; once spring arrived the snout was small again.
- [10]
Ikeda refers to the discovery as a 'reverse Dehnel's phenomenon' and said the results were completely unexpected.
- [11]
The team used specimens covering every month of the animals' first year and their second year for those that lived that long.
- [12]
The team did not follow changes in live animals.
- [13]
One of three non-exclusive hypotheses offered is that snout expansion may widen dietary options during resource scarcity by increasing bite force for processing tougher prey, but there is little direct evidence for this.
- [14]
The authors say a thermoregulatory heat-exchange function is strongly supported by comparative anatomical and physiological evidence: dense vascular networks in the snout may warm inhaled air and prevent brain cooling under subzero conditions, supporting cognitive function despite reduced brain size.
- [15]
A third hypothesis is improved olfaction from greater nasal tissue surface area, allowing better prey detection in snowy conditions.
- [16]
Ikeda said the findings show that skeletal plasticity 'isn't just an all-or-nothing shrinking act.'
- [17]
Jan R. E. Taylor, an evolutionary ecologist at the University of Bialystok in Poland who was not involved in the research, is curious whether the snout enlargement is the same in more mild climates across the long-clawed shrew's range.
- [18]
The authors write that comparative analyses across coexisting species in Hokkaido, a diversity hotspot for Sorex, may clarify whether the reverse Dehnel's phenomenon reflects local adaptation, phylogenetic constraint or plastic response to environmental variation.
- [19]
Ikeda wants to study the shrews' behaviour and physiology, especially how tissues inside the nasal cavity change across seasons, and says tracking these changes in wild animals in the field would require substantial effort.
- [20]
High-resolution CT scans have identified that cranial shape changes in shrews involve distinct 'bending' effects and non-uniform structural transitions across regions rather than isometric scaling.
- [21]
The winter snout increase corresponds to roughly 0.16 mm in height and 0.12 mm in width.
- [22]
Seasonal brain volume changes are understood to involve non-degenerative processes such as cellular shrinkage and water redistribution rather than cell loss, with some regions such as the olfactory bulb exhibiting winter increases in cell populations.
- [23]
The specimen collection window spans 40 years.
- [24]
The lesser white-toothed shrew, found in warmer regions, shows no evidence of cranial or body reduction across seasons.
Sources
2 independent publishers whose own reporting we read for this story.
- phys.orgShrews exhibit reversible size changes in their skulls and noses to get through the winter
1 article · August 19, 2026
- sciencenews.orgShrews’ noses grow in winter as their brains shrink
1 article · August 18, 2026
Topics and entities
Topics
- Mammal cranial morphology
- Phenotypic plasticity
- Dehnel's phenomenon
Entities
- Hokkaido University
- Jan R. E. Taylor
- Long-clawed shrew
- Proceedings of the Royal Society B
- University of Bialystok