Shrew skulls do not simply shrink for winter: the snout grows while the braincase flattens

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 ^[14][15]. That is the main limitation: the seasonal trajectory is inferred from a cross-section of dead specimens spanning four decades of collecting ^[16].

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 ^[11], 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 ^[12][13].

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 ^[17]. 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 ^[18]. Or it could improve olfaction for finding prey under snow ^[19]. Ikeda's framing is that skeletal plasticity "isn't just an all-or-nothing shrinking act" ^[20].

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 ^[21]; the lesser white-toothed shrew, from warmer regions, shows no seasonal cranial or body reduction at all ^[22]. The authors also propose comparative analysis across coexisting Sorex species in Hokkaido to separate local adaptation from phylogenetic constraint or plastic response ^[23], and Ikeda plans to examine how nasal tissues change across seasons ^[24].