This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Scientists had previously observed that some species of shrews undergo a form of biological plasticity in which their bodies, skulls, brains and some organs shrink in the winter, then regrow in the spring. Now, a new study in Japan, published in the Proceedings of the Royal Society B: Biological Sciences, has taken a closer look at this phenomenon and found that shrews' snouts exhibit an inverse effect, growing and then shrinking again.
In 1949, August Dehnel first described "Dehnel's phenomenon," a biological process in which certain small mammals—most notably the common shrew—shrink and regrow their skulls, brains and bodies to survive harsh winters by reducing energy requirements. Some studies suggested that winter brain shrinkage does not necessarily mean brain cells are dying off, but that cells might be shrinking and thus changing the water balance. Studies using CT scans had also hinted that skull changes are uneven across different regions.
The authors of the new study explain, "Recent evidence suggests that this phenomenon is not a simple, uniform reduction in size, but rather a sophisticated process of 'reorganization.' For instance, seasonal brain volume changes are now understood to involve non-degenerative processes, such as cellular shrinkage and water redistribution, rather than cell loss, with some regions like the olfactory bulb even exhibiting winter increases in cell populations. "Furthermore, high-resolution CT scans have identified that cranial shape changes involve distinct 'bending' effects and non-uniform structural transitions across regions rather than isometric scaling." To further determine how the shrews' brains were "reorganizing" in the winter, the team measured the skull and jaw dimensions of 136 long-clawed shrew skulls from a Hokkaido University museum collection. These Northeast Asian shrews had previously received little detailed study despite the harsh winters and high shrew diversity in the region.
While the team did not follow changes in live animals, it used specimens that covered every month of the animals' first year and their second year, for those that lived that long. The team found that the specimens showed the classic winter reduction and spring regrowth of the braincase. They also found that the braincase changed mostly by flattening in the winter, rather than shrinking equally in every direction.
More surprisingly, however, were corresponding changes in the shrews' snouts, which enlarged during the winter, then became smaller again in early spring. The team called this newly identified pattern the "reverse Dehnel's phenomenon." The team proposed three potential "non-exclusive hypotheses" for this seasonal snout expansion. They say expansion of the snout may widen dietary options during periods of resource scarcity by providing increased bite force, allowing for the processing of tougher prey.
However, there is little direct evidence for this hypothesis. A potentially more likely evolutionary purpose might be to enhance thermoregulatory heat exchange. The study authors say this is strongly supported by comparative anatomical and physiological evidence and that dense vascular networks in the snout may enable heat exchange and prevent brain cooling under subzero conditions, enhancing shrews' cognitive function despite reduced brain size.
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