The common shrew looks unremarkable, a few grams of fur and teeth scurrying under leaf litter. Yet this tiny mammal performs one of the strangest tricks in biology, shrinking its own brain and skull in winter and then rebuilding them when conditions improve. That reversible transformation is now drawing serious attention from neuroscientists who hope the same mechanisms that let shrews regrow healthy brain tissue could one day inspire new ways to treat Alzheimer’s disease.
Instead of treating brain loss as permanent, the shrew treats it as a seasonal adjustment knob. I see in that strategy a radical challenge to long held assumptions about what mammalian brains can and cannot do, and a potential roadmap for therapies that repair, rather than merely slow, neurodegeneration.
The shrew that breaks the rules of mammal biology
Among small insectivores, the Eurasian common shrew, known to biologists as Sorex araneus, stands out for its extreme physiology. With a life expectancy of barely a year, the Eurasian animal burns energy at a furious pace and cannot hibernate or migrate to escape winter. Instead, it reduces the size of its body, including its skeleton and internal organs, to cut metabolic costs when food is scarce. Researchers first noticed that the skulls of these shrews were smaller in winter than in summer, a clue that something dramatic was happening inside their heads as well.
That seasonal remodeling is now known as Dehnel’s phenomenon, named for the Polish zoologist August Dehnel, who described these changes in shrews in 1949. Modern imaging and anatomical work has confirmed that the common shrew’s brain can shrink by a substantial fraction in the cold months and then expand again in spring, with the skull following suit. Recent coverage of this work has highlighted how Dehnel’s original observations have blossomed into a full research field, and how scientists now see this seasonal brain cycling as a potential window into human disease.
How a mammal shrinks and regrows its brain
For a long time, the obvious question hung over this phenomenon: how can a mammal lose so much brain volume and still function, let alone recover it later. Early work suggested that the shrew’s brain mass dropped in winter and then rebounded, but the underlying mechanism was unclear. Behavioral ecologist Nov Dechmann, at the Max Planck Institute of Anim, has described the common shrew as “a crazy animal” precisely because it appears to violate the usual rule that mammalian brains only shrink with age or injury, never in a controlled, reversible way. Reporting on Nov Dechmann’s work has emphasized how the skull and brain both contract in winter, then expand again when resources return.
More recent studies have begun to unpack the cellular details. An international team has shown that the seasonal changes in brain size represent one of the most drastic reversible transformations known in any mammal, affecting not just overall volume but also the architecture of different regions. Detailed anatomical analyses have documented that the shrew’s cortex, hippocampus and other structures undergo coordinated remodeling, with some areas shrinking more than others and then regaining tissue as conditions improve. A landmark study described profound seasonal changes in brain size and overall tissue shrinkage in winter, followed by regrowth, underscoring that this is not a minor tweak but a whole organ strategy.
Water, genes and evolution: the mechanics behind the trick
One of the most surprising findings to emerge recently is that the shrew’s winter brain shrinkage is not primarily driven by massive cell death. Instead, scientists have found that the cells themselves lose water, effectively dehydrating to reduce volume while preserving their basic structure. Cecilia Baldoni, a postdoctoral researcher at the Max Planck Institute of Animal Behavior in Germany, has explained that in the shrunken winter brains “the cells lost water” but the number of cells stayed roughly the same. Coverage of this work notes that Cecilia Baldoni and colleagues used this insight to argue that the shrew’s brain is not destroying and recreating neurons each year, but instead cycling between hydrated and dehydrated states.
Researchers at the Max Planck Society have framed this as a “water cure” for winter, showing that common shrews shrink their brains in cold seasons not by losing cells but by losing water, and that water is a key factor in Dehnel’s phenomenon. Their summary of the work stresses that common shrews are one of the few mammals known to undergo such extreme seasonal remodeling and that water balance appears central to the process. In a companion explanation, scientists describe how water behind Dehnel’s phenomenon allows the brain to contract without catastrophic damage, while a related overview from the same institute notes that common shrews are one of the few species where this seasonal brain shrinkage has been documented and that water is the main factor involved in the phenomenon.
From forest floor to MRI scanner
To move beyond skull measurements and dissections, scientists have turned to non invasive imaging. A recent project used MRI to follow individual shrews across seasons, tracking how their brains shrank and regrew in real time. The work showed that the volume changes were not uniform, with some regions, including those involved in sensory processing and memory, showing particularly marked fluctuations. Researchers involved in this project have argued that the shrew’s ability to repeatedly remodel its brain without obvious cognitive collapse could provide a model for how to protect or restore human brain tissue. A summary of the study notes that MRI scans captured the shrinkage and regrowth and that scientists see in this pattern a potential path to medical treatment.
Genetic work is beginning to catch up with the imaging. A remarkable new study highlighted by ScienceSphere25 has identified gene networks in the Eurasian common shrew that respond to environmental stress, including genes involved in calcium signaling and energy balance that enhance the hypothalamus’s response. Intriguingly, some of the same pathways are implicated in human conditions such as epilepsy, Alzheimer’s and synaptic function, suggesting that the shrew’s seasonal brain cycle taps into conserved molecular machinery. The description of this work emphasizes that Other genes linked to Alzheimer’s and synaptic function are active in the shrew’s hypothalamus as it adapts to seasonal stress, a tantalizing overlap for neurologists.
Why Alzheimer’s researchers are paying attention
For human medicine, the most provocative aspect of the shrew’s biology is not that its brain shrinks, but that it grows back. In Alzheimer’s disease, neurons and synapses are progressively lost, and current drugs largely aim to slow that decline rather than reverse it. The shrew shows that at least one mammal can orchestrate large scale, repeated changes in brain size and structure while maintaining basic function, and then restore much of what was lost. A recent overview of Dehnel’s phenomenon notes that scientists now explicitly see this tiny mammal’s seasonal brain regrowth as a potential key to Alzheimer therapies, precisely because it demonstrates that mammalian brain tissue can be remodeled on demand.
Research funders are starting to formalize that hope. A grant description focused on “Regrowing the brain” frames the shrew and similar species as models for understanding how organisms survive hard conditions, particularly winter, and how their brains adapt. The abstract notes that organisms need strategies to survive when conditions are hard and that for mammals winter is particularly difficult, then sets out to uncover the evolution and mechanisms of seasonal brain changes that could lead to future therapies. In that vision, the shrew’s biology becomes a blueprint for interventions that might one day help human brains repair themselves, as outlined in the Abstract describing how such work could lead to future therapies.
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