In winter, the "brain shrinks" of the shrew — Reversible brain shrinkage indicates a path to neural regeneration

In winter, the "brain shrinks" of the shrew — Reversible brain shrinkage indicates a path to neural regeneration

2025年09月03日 00:46

1) "Shrink in Winter, Return in Spring" —— Unraveling a 70-Year Mystery with "Water"

The small mammal, the shrew, which survives the harsh winters of the Northern Hemisphere, adjusts the size of not only its body but also its brain according to the seasons. This phenomenon, known as the Dehnel's phenomenon, has been known for a long time, but how the brain shrinks without damage and then returns to its original size has been a mystery for many years. The latest international joint research repeatedly captured individuals in the wild during summer and winter, performed MRI scans, and tracked seasonal differences in the microstructure of the same individuals' brains. By combining microscopic-level cell analysis, they demonstrated that a **9% reduction in brain volume can be explained not by a decrease in cell number, but by a decrease in intracellular water (dehydration)**.


2) Cells Do Not Die; Rather, "Refilling" Occurs

Generally, cell dehydration tends to lead directly to damage or cell death. However, in shrews, the cells remain alive, and locally, there were findings of an increase in cell number. From the diffusion indices of MRI (increase in mean diffusivity and decrease in anisotropy), it can be inferred that water inside the cells decreases while relatively increasing outside the cells, indicating a reversal of water balance. As a result, the brain becomes "thinner and smaller," but it seems to switch to a winter energy-saving mode without critically impairing wiring or function.


3) "Priority Allocation" by Region: Neocortex and Cerebellum Exceptionally Protected

The shrinkage is not uniform. While many regions show a shift in water balance, the neocortex and cerebellum have relatively stable water exchange, and it appears that the "heating" of key areas involved in memory and motor control is not reduced. In terms of home energy management, it is like "keeping necessary rooms warm while saving energy in others." This uneven shrinkage is the key to allowing the shrew to continue hunting and exploring even with a smaller brain.


4) Molecular Clues: Aquaporin 4 (AQP4)

AQP4 is a water channel abundant in the astrocytes of the brain, controlling the rapid movement of water. In this analysis, AQP4 emerged as the central player in water transport associated with seasonal changes. Abnormalities in AQP4 have also been reported in human neurodegenerative diseases, and the idea of intervening in the "water pathway" could lead to future therapeutic strategies.


5) Hints for the Human Brain, but "Misreading" is Prohibited

The research team points out that the winter brain of the shrew appears to exhibit characteristics similar to the brain of human neurodegenerative diseases (such as volume reduction and water movement findings). The important point is that in shrews, these changes are reversible and recover in spring. Therefore, it is not a story of **"the human brain shrinking by 9% seasonally." Rather, if there is a physiological program to "shrink without breaking and then restore," identifying its switches and circuits** could lead to a way to prevent irreversible atrophy.


6) Why Shrink: The Triangle of Energy, Water, and Danger

Shrews have such a high metabolism that not eating for just a few hours can be fatal. In winter, food is scarce, and the brain is a voracious organ. Therefore, it is considered rational to **"reduce volume and water to lower overall maintenance costs."** However, water exchange is a fine line between brain edema and dehydration. Without precise control, it directly leads to functional failure. This "water removal" model highlights the delicate balance between energy conservation and neuroprotection.


7) Social Reactions: Surprise, Misunderstanding, and Expectations for Research (From SNS)

Following the report, three major reactions were observed on social media.

  • Surprise: "It's amazing that they can hunt normally even though their brain shrinks," "The plasticity of living creatures is beyond imagination." Many posts humorously self-deprecating with **"This is my brain in winter too."**

  • Misunderstanding and Anxiety: Confusion such as "Does the human brain shrink by 9% seasonally?" "Isn't dehydration dangerous?". As a fact-check, it was repeatedly shared that the subject is shrews, not humans, and that the shrinkage is a reversible and regulated physiological phenomenon.

  • Expectations for Application: "If we can intervene with AQP4, can we stop brain atrophy?" "Focusing on understanding the spring regrowth phase" and other expectations for medical applications. From neuroscience community accounts, explanations focusing on diffusion MRI (DMI)decipheringwater dynamics have been released. (Examples: explanatory articles introducing the research, thread posts, Threads' spread posts, etc.)


8) Limitations and Next Steps

  • Species Limitation: The observed mechanism was verified in shrews (mainly European species). It cannot be directly applied to mammals in general or humans.

  • Season and Environment: It is necessary to separate the effects of complex factors such as cold and food scarcity.

  • The Mystery of Regrowth: How does the spring "rewind" occur? Distinguishing the contributions of multi-stage processes such as neurogenesis, synaptic rewiring, and glial volume regulation is the focus going forward.

  • Intervention Possibility: It is unknown whether manipulating water transport pathways like AQP4 is effective in human neurodegenerative diseases. Determining the safety margin is essential.


9) What is "New" (The Essence of This Research)

  1. Tracking seasonal changes in the same individual with non-invasive MRI, it was shown that it can be explained by water movement rather than cell death.

  2. Asymmetry in water balance by region (neocortex and cerebellum are relatively stable) was depicted, suggesting a strategy for functional preservation.

  3. Attention has focused on molecular players like AQP4, providing a starting point for exploring the molecular blueprint of the regrowth phase.


10) Summary for Practitioners (Research, Medical, Science Communication)

  • For Researchers: Integrate DMI parametric maps (MD/FA, etc.) with immunostaining and transcriptome to clarify the causal path of water metabolism—cell volume—function.

  • For Medical Professionals: Reevaluate the possibility that "apparent atrophy" in human disease ≠ irreversible from the perspective of water metabolism. However, avoid shortcuts to clinical application.

  • For Public Relations and Journalists: Headlines like "The human brain does not shrink seasonally" / "Shrew's energy-saving strategy" are effective in preventing misunderstandings..

Mini Glossary

  • Dehnel's phenomenon: Seasonal adaptation where the brain, skull, and organs shrink in winter and reversibly return in spring.

  • Diffusion Microstructure Imaging (DMI): An MRI technique that estimates changes in cell size and water distribution from the diffusion behavior of water molecules.

  • Aquaporin 4 (AQP4): A water channel in the brain. Abundant in the end-feet of astrocytes, involved in rapid water movement.


    Reference Article

    MRI research has revealed that the rare seasonal brain shrinkage in shrews is caused by water loss rather than cell death.
    Source: https://phys.org/news/2025-08-rare-seasonal-brain-shrinkage-shrews.html

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